ABSTRACT
E. Duke Whedbee Jr. THE EFFECT OF BEHAVIORAL INTERACTIONS ON THE
STRUCTURE AND MAINTENANCE OF BLACKBIRD WINTER ROOSTS. (Under the direc-
tion of Andrew N. Ash) Department of Biology, December 1981. The purpose
of this study is to seek understanding of the factors controlling the
formation and maintenance of winter blackbird roosts in terms of behav-
ioral interactions that exist between the species involved.
The roost is composed of essentially four species; three of which
are native resident icterids, and one introduced sturnid. These species
are: the common grackle, Quiscalus quiscula Linnaeus; the redwinged
blackbird, Agelaius phoenicius Linnaeus; the bronzed-headed cowbird,
Molothrus ater Boddaert; and the European Starling, Sturnus vulgaris
Linnaeus.
Observations were made of the behavioral interactions that occur,
with ancillary data collected concerning roost population size, distri-
bution and composition during 1980 and 1981. Chi-Square analysis of the
results indicates that while each species possesses a common behavioral
catalogue, as described within the literature, each species also exhibits
a new behavior labeled "snub" which is passive in nature and thus is
highly important to the formation and maintenance of the roost.
A dominance hierarchy based upon the different roosting strategies
utilized by each species to gain and maintain a roost position was estab-
lished. This hierarchy indicates that accommodation is important in the
formation of the roost. The main effect of aggressive behavior is to
induce segregation of the species within the roost. Use of accommodative
behavior allows those species that are clearly subordinate in dominance
to still attain success in roosting and thus gain any benefit accruing
from roost participation at a minimal expenditure of energy.
THE EFFECT
OF BEHAVIORAL INTERACTIONS
ON THE STRUCTURE AND MAINTENANCE
OF BLACKBIRD WINTER ROOSTS
A Thesis
Presented to
the Faculty of the Department of Biology
East Carolina University
In Partial Fulfillment
of the Requirements for the Degree
Master of Science in Biology
by
E. Duke Whedbee Jr.
June 1983
THE EFFECT OF BEHAVIORAL INTERACTIONS ON
THE STRUCTURE AND MAINTENANCE OF BLACKBIRD WINTER ROOSTS
by
E. Duke Whedbee Jr.
APPROVED BY:
SUPERVISOR OF THESIS
Andrew N. Ash, Ph.D.
THESIS COMMITTEE
CHAIRMAN OF THE DEPARTMENT OF BIOLOGY
Char fes’ T. B1 and”,' f^lTrCT
DEAN OF THE GRADUATE SCHOOL
ACKNOWLEDGEMENTS
I would like to thank the members of my committee: Drs. Susan
McDaniel, Charles O'Rear, Everett Simpson and Robert Tacker for their aid
in producing this thesis. Each contributed in many ways, perhaps most
importantly, in making themselves available to me despite busy schedules.
I would especially like to thank my committee Chairman, Dr. Andrew
Ash for his patience, understanding and guidance in this study, and for
his friendship throughout my stay at East Carolina University.
In addition, I would like to express appreciation to Dr. Grant
Somes, Biostatician. School of Allied Health and Social Professions, East
Carolina University for his assistance in data analysis.
Finally, I would like to thank Brenda Elks who helped prepare this
manuscript. Laddie Crisp for help with the graphics, Bobbie Yokeley for
her drawing of the Snub Behavior, and the residents of Circle Drive, Rich
Square, North Carolina, for their patience.
Partial funding for this project was provided by East Carolina
University Research Committee Grant No. 93.
TABLE OF CONTENTS
PAGE
LIST OF TABLES v
LIST OF FIGURES vi
INTRODUCTION 1
LITERATURE REVIEW 2
The Nature of Blackbird Roosts 2
The Species Involved 3
Roost Structure 3
Behavior 3
Associated Problems 5
Agricultural 5
The Health Hazard 6
Aesthetics 6
MATERIALS AND METHODS 8
Site Description 8
Methodology 10
The Dai ly Routine 10
The Checklist and Code 11
RESULTS 13
Roost Population Estimate 13
Stratification and Segregation 15
Roost Composition 17
Behavior 17
Encounter Resolution 24
Roosting Strategy Success Index 26
i V
PAGE
Effects of Time and Temperature 29
DISCUSSION AND CONCLUSIONS 31
Grackles 31
Starlings 32
Redwings 32
Cowbirds 33
The Roost 33
Snub Behavior 37
Some Speculations on the Winter Roost 38
Literature Cited 40
LIST OF TABLES
PAGE
1. Bird and Tree Density 14
2. Species Composition of the Rich Square Roost 18
3. Roosting Success - Rich Square Roost 30
LIST OF FIGURES
1 PAGE10. .ThThee Rich Square Roost Study Site 92. Species Segregation within the Roost 163. The Snub Behavior ? 194. Frequencies of Active and Passive Behavior 215. Frequencies of Intraspecific Behavior 226. Frequencies of Interspecific Behavior 237. Frequencies of Encounter Resolutions 258. Frequencies of Intraspecific Encounter Resolutions 279. Frequencies of Interspecific Encounter Resolutions 28Competition Hypothesis 35
INTRODUCTION
Large concentrations of blackbirds have been roosting in southern
North America during winter since before the arrival of Man. In recent
years, however, due to the increase in human populations and the sub-
sequent modification of natural habitats, these roosts have become more
frequently associated with suburban areas. The resulting increased
interest has been focused on applied studies concerning the level of
damage, methods of control and eradication. Basic knowledge of the
mechanics of winter roosts is largely anecdotal and as a result, no
effective and feasible method of alleviating the problems posed for
farmers and suburbanites has been found.
This study examined winter roost formation and maintenance in terms
of behavioral interactions that take place between and among the species
involved. In addition, data on roost composition, roost structure, and
total population for 1 typical roost was gathered in order to give an
overall picture of the roost as a single entity.
Data collected were interpreted in terms of the evolutionary
significance for each of the component species in an attempt to gain some
insight into the selective forces behind the origins of winter roosts.
The data produced may enhance the search for control techniques and
provide better understanding of interspecific and intraspecific behavior-
al patterns within the roost.
LITERATURE REVIEW
The Nature of BIackbird Roosts
Each fall, millions of icterid and sturnid birds of various species
migrate southward across much of North America and join with resident
birds to form large congregations that roost in very high densities
(Meanley 1971). As many as 10 different species of birds can be part of
roosts depending upon the area in which the roost develops (Meanley et
al. 1975). Knowledge of roosts has been an anecdotal by-product of
research by government agencies examining damage caused by the birds
(P.A. Stewart personal communication).
The daily cycle begins approximately 1 hour before sunrise as the
roost breaks up into smaller foraging flocks that range widely in search
of food. By late afternoon, the foraging flocks begin to make their way
back to the roost site, often stopping at "staging areas" nearby. As
sunset approaches, the flocks stream back to the roost site, converging
in large circling masses over the roost area and finally dropping back
into the roost as night falls. This reformation continues for several
hours into the night before the roost settles down and the birds remain
relatively quiet until the following sunrise (Meanley et al. 1975,
Stewart 1975, 1977).
Information as to how the roost forms is almost nonexistent, but
P.A. Stewart (personal communication) believes that resident birds begin
to form roosts in late summer and are joined by migrants during their
fall arrival. He also believes that the common grackle. Qui seal us
quiscula initiates roost formation over much of the Southeastern U.S.,
3
with other species joining them later, but has no data to substantiate
this (Stewart 1977).
The Species Involved
Nationwide, at least 10 species can be associated with blackbird
roosts and an estimated 550 million birds in over 700 roost sites may be
involved (Meanley and Webb 1965). Of these species, 4 make up close to
80% of all roosts including those in Eastern North Carolina. The species
are; the common grackle; the redwinged blackbird, Agelaius phoenicieus;
the brown-headed cowbird, Molothrus ater; and the european starling,
Sturnus vulgaris. The first 3 species are native icterids and the last
is an introduced sturnid that has become part of the roost since its
initial introduction in the 1880's (Royall et al. 1975). Because of the
predominance of these species in most roosts, the resultant data should
have broad application.
Roost Structure
Information on the internal structure of winter blackbird roosts is
sketchy at best. Meanley and Webb (1965) observed some vertical
stratification among the species in roosts located in Arkansas.
Starlings were reported to occupy the highest branches, with
grackles and male redwings beneath them, the cowbirds and female
redwings occupying the lowest levels. Segregation was also noted
in foraging flocks and in lateral location within the roosts.
Behavior
All passerine species share a common background of behavior (Andrew
1961) and icterids, in particular, have certain common behaviors that
4
allow interaction (Nero 1963, Wiens 1965). Territorial behavior can
range from nesting territories to the immediate area in close proximity
to an individual (termed "individual space") (Hinde 1966, Wilson 1975).
It was a basic assumption of my study that obtaining and defending a
perch or roost space could be construed as a form of territorial
behavior.
The basic repitoire of agonistic behaviors of icterid species has
been studied in terms of individual species (Nero 1956, 1963, Ficken
1963) and interspecific interactions (Crombie 1947, Hinde 1966), giving
necessary basic data.
There are 4 basic behavior patterns which differ only in the degree
of intensity of the individuals reaction to intrusion. The terms are
those used by Wiens (1965) in his study of territorial aggression between
and among grackles and redwings in a Wisconsin marsh during breeding
season, and are listed in the order of increasing intensity:
1. "Tail flicking" - The bird is observed repeatedly flicking its
tail upwards in an erratic manner.
2. "Bill tilt" - The displaying bird bobs its head and points its
bill in a vertical direction.
3. "Song spread" - Most typical icterid agonistic behavioral
pattern. The bird ruffs out its feathers, spreads its wings horizontally
and faces the intruder with an open bill.
4. "Diving" - Actual physical flight at an adversary.
All of these behaviors were observed in preliminary studies of
blackbird roosts among and between all species involved. It was clear
5
from the beginning, however, that "bill tipping" and "tail flicking" were
merely precursors to the "song spread" in those cases where a focal
individual (individual bird being observed) is reacting strongly to an
intruder. Furthermore, the "diving" behavior was not observed in focal
individuals at all since all focal individuals were occupying a perch at
the time of observation. All intruder birds could be said to have
exhibited the "diving" behavior by the very nature of their approach.
For these reasons and the discovery of a new behavioral pattern to be
discussed later, observations of "song spread" were considered as the
only major type of aggressive or "active" agonistic behavio)^.
Associated Problems
Although not directly related to this study, some examination of the
problems associated with blackbird roosts may help shed some light on the
necessity for better understanding of roost dynamics beyond the simple
curiosity surrounding interspecific interactions.
Agricultural. In those areas of the United States where the matura-
tion of grain crops coincides with winter roost formation, crop depreda-
tion has long been considered the main source of concern. Several
studies have been conducted to establish the degree of crop depredation
(Dolbeer et al. 1978) and to devise means of control of blackbird numbers
in affected areas (U.S. Fish and Wildlife Service 1976). There is
still some controversy over how much damage is caused by blackbirds
to crops such as rice in the South-Central U.S. (Woronecki et al.
1981), but control and prevention remain the major focal point of
various governmental agencies.
6
An additional facet of agricultural damage is the damage that can
occur to the roost trees themselves. Earl Raye, Chief Forester for Union
Camp Corporation timber interests in Hertford Co., N.C. (personal
communication) indicated that pine trees below 30 years of age could be
severely damaged by the high concentrations of fecal material which are
characteristic of blackbird roosts. My own observations confirm this in
several roost areas including the area in which this study was done,
where at least 80% of the trees are now dead or dying. Such losses can
be devastating to the landowner.
The Health Hazard. Blackbird roosts that occur in close proximation
to cities and towns can cause direct health problems. Roosts that occur
near airports can endanger aircraft landing and taking off (Caslick and
Meanley 1966) by flying over the runways, especially in the case of jet
aircraft.
The large amount of fecal material produced by roosting birds that
occupy roosts can initiate blooms of the soil fungus. Histoplasma
capsulatum which can cause infections in humans ranging from skin and eye
irritation to serious infection of internal organs such as the lungs and
liver (Chin et al. 1970). It is this danger that has produced the most
alarm among residents of areas close to roosts more than any other
consideration because of the danger to area children.
Aesthetics. While this consideration is entirely subjective I can
state from personal experience that the smell resulting from the
droppings and the noise the birds make must be included in any examina-
tion of the problems connected with blackbird roosts. Overall the
7
quality of life enjoyed by residents near roosts is reduced and the
property value of homes as well. While this problem is the least serious
in nature, it is the most obvious and to the landowners the most
economically detrimental. Long before any considerations of the health
hazards occur, the residents are faced with the very loss of the use of
the outdoor areas around their homes. As one homeowner put it to me,
"It's as if we were under seige!"
MATERIALS AND METHODS
Site Description
All of the observations in this study were made in the roost that is
located within the corporate limits of the town of Rich Square,
Northampton County, North Carolina. This roost has been in existence
since 1975 despite periodic efforts by the 1200 people of Rich Square to
drive them off.
The study site was a 7 hectare monoculture stand of loblolly pine,
Pinus taeda. The trees averaged 5 to 7 meters in height and density, and
distribution was relatively uniform (Figure 1). The western border of
the pine stand was a cultivated field, with the eastern edge parralleling
U.S. highway 158. On the north, the roost area abuts an open field and a
small strip of mixed hardwoods and older pines further separate it from
an automobile dealership. The southern edge of the site consisted of
older mixed hardwoods and pines which are interspersed among the homes of
the Circle Drive subdivision.
At the time of this study, only about 5.8 hectares of the roost site
were in actual use by the birds. The easternmost section of the pines
had been used in previous years but most of these trees were dead or
dying when I first visited the roost in December of 1979.
W
Field 9
S N
E
O
Water Tower
Mixed
Pines / Hardwoods
QUADRATS
QUADRAT WITH
BIRD COUNT
US 158
BLACKBIRD ROOST,RICH SQUARE N.C.
Methodology
Preliminary observations showed that it was possible to observe
birds within the roost quite easily as they did not flush once they had
settled in. The main problems involved observation of their behavior.
For this reason all observations were made at sunset and involved the
return of foraging flocks. The morning outflights took place before
daylight and observations were impossible. Weather conditions
limited the duration of daily observations.
Clothing and protection from the odor and feces were necessary in th
form of rain suits and hat at all times. A respirator capable of
filtering particles 1 micron or larger removed the odor and provided
protection from possible infection by Histoplasma capsulatum microspores.
Boots were also of help within the roost since the concentration of
droppings reached depths of 15cm on the ground and rainfall caused
collection in depressions and ditches to even greater depths.
During preliminary observations, binoculars and a spotting scope
were tested but it was soon apparent that naked eye observation was the
most effective since the birds would allow a close approach if the
observer remained relatively still once in location.
The Daily Routine
The average day of observation began around 10:00 am with the
delineation of a 10 meter square quadrat, the site of all observations
for that day. The quadrat was then explored and all dead birds and roost
trees within the boundaries counted. If time allowed, other quadrats
were delineated for future use.
The quadrat was reentered about an hour before sunset. Observations
of the various behaviors and interactions were recorded as long as light
permitted accuracy. Observation focused on a perching bird until 15
seconds elapsed or a behavioral interaction occurred. When light no
longer allowed accurate observation of behavior, estimates of bird
density were made from randomly selected trees within the quadrat to
establish some estimate of average birds per tree. The process was
repeated each observation day in a randomly selected quadrat.
The Checklist and Code
To insure that the maximum number of observations were recorded each
day with accuracy and consistancy, a standardized data recording sheet,
or checklist (Lehner 1979) was developed.
A standardized alphanumeric code was designed for use on the
checklist and its compatibility with Statistical Analysis System ensured.
The resulting system maximized efficiency of observation.
For each observation, it was necessary to record certain information
to achieve the type and quality of data which would enable me to address
the question I sought to answer:
1. Date - A 5 number code, indicating the day, month, and year the
observation was made.
2. Time - A 4 number code the time of the observation in.
3. Species - A 2 digit code, the first digit representing the focal
species and the second representing the intruder.
4. Behavior - A single digit representing the behavioral pattern
observed.
5. Resolution - A single digit was allotted to indicate the
reso1lu0t.ioTnheof the encounter between the observed birds.6. Quadrat - A 2 digit field was reserved to indicate the quadratin which the observation was made.7. Number of Trees - The 3 digits of this field were used torecord the number of trees within the quadrat.8. Dead Birds - The number of dead birds observed within thequadrat were noted in this 3 number field.9. Temperature - A 2 digit space was utilized to record thetemperature (°C) at the time of observation.checklist also provided a section for comment in the event
of any unusual phenomenon.
Using this code, a typical observation would be recorded in the
following manner:
(1) (2) (3) (4) (5) (6) (7) (8) (9)
101180 1610 13 1 1 06 C71 014 21
A literal translation of the code for this observation would be:
"On October 11, 1980, at 4:10 p.m., a common grackle was observed
interacting with a redwinged blackbird, using the song-spread behavior
pattern and drove it off. The observation occurred in quadrat 6, which
contained 71 trees and 14 dead birds. The temperature was 21°C."
Once the code was familiar to the observer, data could be recorded
almost as fast as events occurred.
RESULTS
Data were collected at the Rich Square roost from November 2, 1980
to January 31, 1981; 111 observations were recorded. A total of 33
quadrats were observed; in 12 of these tree density and bird density were
measured. Analysis was for the most part carried out using the
Statistical Analysis System through the Triangle Universities Computer
Center, with some simple statistics done by hand or on a TSR-80 personal
computer.
Roost structure and maintenance are described separately.
Roost Population Estimate
Quadrat data produced estimates of average density of trees in the
roost and the average number of birds utilizing each tree (Table 1). The
average number of birds per quadrat was derived and number in the entire
5.8 hectares being used by the birds extrapolated. The estimated
population was 4,279,240 birds.
The U.S. Fish and Wildlife Service, using a rate of flow technique,
in which the number of birds moving into the roost area at sunset was
estimated from a small visual sample from one observation point, was
3,000,000 birds. Because of the highly subjective nature of this
technique, the estimate taken by me using the quadrat method contains a
higher level of inherent accuracy and precision and is, in my opinion,
closer to the true numbers. While the quadrat method does require
additional equipment and that the observer actually enter the roost area,
I feel the results are worth the additional effort in many cases in order
to arrive at more accurate figures to use as a basis of future control
Table 1. Bird and Tree Density
Estimated
Quad # # Trees/Quad Birds/Tree
1 75 no
2 104 120
3 69 90
4 57 130
5 76 120
6 77 100
7 67 85
8 49 100
9 56 120
10 82 100
11 71 90
12 57 100
X 70 X 105.4
policies and decisions.
The major problem with the quadrat method may be the assumption of
uniformity of distribution of birds within the roost. In the case of the
Rich Square roost, the uniformity of the vegetation lends validity to
that assumption. In roosts without such uniformity, the accuracy of
estimates may be less reliable, but still better than that obtained by
the rate of flow technique.
Stratification and Segregation
Meanley and Webb (1965) described vertical stratification in
Arkansas roosts, but I found no evidence of it in the Rich Square
roost. The most obvious reason for this is that the Rich Square
roost exhibited no vegetational stratification which probably
precluded stratification by the birds themselves.
Horizontal segregation was indicated by the presence or absence of
species in each quadrat (Fig. 2). Grackles were found in every quadrat
and starlings in most. Redwings and cowbirds were apparently segregated
in peripheral areas, though the degree of segregation is not clear due to
the coarseness of the sample.
Meanley and Webb (1965) also reported that redwings were
segregated by sex, but the scope of my study did not allow determi-
nation of sex. It is obvious that there are two distinct concen-
trations of redwings in the Rich Square Roost.
Figure 2 Species Segregation within the Roost
November 1, 1980 - June 31, 1981
w
Starlings,Cowbirds, Redwings Field
I J I 1,1 s N
j,v,v Starlings, C owbirds
E
o
Wcter lower
Mixed
Pines/ Hardwoods
BIRD COU^fr
US 158
ROOST COMPOSITION
Roost Composition
Estimates of species composition for the Rich Square roost were
obtained by examining the behavioral data collected. Each behavioral
observation contained the identification of 2 birds by species, providin
1554 identifications (Table 2).
Meanley (1971) indicated that roost composition varied greatly, but
that roosts examined in the White River area of Arkansas were 60-80%
grackles, compared to 73% grackles in this study. Roost composition may
differ with habitat. For example, one might expect to find a higher
percentage of redwings in a roost adjacent to a marsh.
Behavior
Four behaviors were recorded during the study, including 1 not
previously described. The 3 reported behaviors were those previously
discussed and differ in the degree of aggression involved. Both "bill-
tip" and "tail-flick" were considered transitional since they only
occasionally occurred in conjunction and prior to subsequent
interactions. The most frequent aggressive behavior was "wing-spread"
and involved direct conf'^ontation between focal individuals and
intruders. This interaction resulted either in the fleeing of the focal
bird or of the intruder.
The other observed behavior was not aggressive but consisted of a
ritualized accommodation between the focal individual and the intruder.
I call this behavior "snub" since it involved acceptance of the intruder
on the perch and redirection of both birds' attention to other
approaching birds (Fig. 3).
Table 2. Species Composition of the Rich Square Roost
SPECIES FREQUENCY^ PERCENT POPULATION ESTIMATE
Crackles 1143 73.55 3,147,381
Starlings 286 18.40 787,380
Redwings 77 4.95 211,822
Cowbirds 48 3.10 132,657
Total 1554 100.00 4,279,240
^ Based upon observations of proprietors and intruders for 111
observations of behavior.
Figure 3. The Snub Behavior
19
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?/^&i \ !Lv .i-W
•? / /I •f'-;y
\
-
V rfiv
Reasons for this behavior not being reported are self-evident.
Since this behavior is passive and occurs within a matrix of highly
visible aggressive activities, the eye is not drawn to it and
considerable field time is required before the absence of movement
becomes clearly recognizable. In turn, sufficient time must pass before
the novelty of the new behavior wears off and the observations are more
objective. Otherwise, the observer finds himself looking for the new
behavior exclusively rather than for behavior that occurs in a focal
animal .
The discovery of "snub" behavior did much to clarify interactions
involved in the formation and maintenance of roosts. Prior to noting
this behavior, the only response possible to a focal bird was aggression,
that is, each encounter could only result in the direct conflict between
2 individuals. The inclusion of the "snub" behavior into the behavioral
repitoire allowed 2 options instead of 1. A focal individual could
actively defend its perch or passively accommodate the intruder. As a
result, the behavioral data in this study were categorized as either
active or passive.
Basic frequencies of active and passive behavior (Fig. 4) according
to the focal species indicates that there is no significant difference in
how each of the species reacts to intruders or any other species (Chi-
Square analysis: P>.05).
However, when intraspecific and interspecific interactions (Figs. 5,
6) are analyzed, there are significant differences in how the birds react
to an intruder of the same or different species. In all species, there
Figure 4. Frequencies of Active and Passive Behavior
%
100
90
80
5 0,0 5 0.0
CRACKLES STARLINGS REDWINGS COWBIRDS ALL SPECIES
•/, %
Signilicant ll'lilülilill'ill'i'
P < .05 Freq.
ACTIVE PASSIVE
Figure 5. Frequencies of Intraspecific Behavior
62.5
54.5
50 50
ET
m
iS'Üri:*
l'i'"
39
CRACKLES STARLINGS REDWINGS COWDIRDS ALL
SIgnIlicant %
DHference
Freq.
ACTIVE PASSIVE
Figure 6. Frequencies of Interspecific Behavior
%
100
1 00
90
80 ' ?í :'vii¡
76 f11:,1v 7 5;, •!1,,'
f'
1. .'4:'
70 ?i-iiliii
? 'ï'V ? 1 i
60 fífel
?
,'!;ïlilí ’1I
57 1 ill
!,
If; É’i
50 i
: 11.1
i| '• 'I ' 42 9
40 ?Slii'l.
' •' ''it îï#
»?
30 - i''-'.!'.ii'iü!
• !'? ? 2 4
'
¡í '''
20
1 0 - 72 54
lií*
CRACKLES® STARLINGS® a aREDWINGS COWBIRDS all
a
Slgnilicant
Diflerence % %
Fren i Freq
il''!'
ACTIVE PASSIVE
ro
CO
was an interspecific increase in aggressive response and in the case of
the redwings there was no passive response to interspecific intruders.
While some samples are small, the trend is noted for all species.
Encounter Resolution
Each focal bird responded to an intruder actively or passively.
There were also several possible resolutions to these interactions.
Success or failure of a bird in maintaining its position or in
gaining a position within the roost can be viewed as a form of dominance.
Those species least successful in gaining or maintaining a perch were
those which expend the most energy to gain whatever advantage in survival
the roost offers. Even the least successful species must gain an
advantage sufficient to offset this expenditure of energy or the roost no
longer is of a positive benefit.
When a focal individual responds to an intruder aggressively, the
resolution is either dislodgement of the perching bird (or proprietor) or
the failure of the intruder's attempt to gain a perch. If the proprietor
responds passively, then both birds are able to share a perch at little
or no cost in energy. Thus there is great selective advantage in the
passive behavior.
These 3 possible resolutions (proprietor wins, intruder wins, and
accommodation) can be used to indicate relative dominance. Figure 7
shows the frequencies of encounter resolutions by species and a pooled
frequency for all species. Chi-Square analysis indicated that there is a
significant difference in how each species resolves encounters (P<.05).
Figure 7 Frequencies of Encounter Resolutions
%
1 OOh
90 -
80 -
70 •
60 ?
50
43.69
3 3.3 3 3.3 3 3.3
CRACKLES STARLINGS REDWINGS COWBIRDS
I’/c .I
Freri, Freq.
INTRUDER ACCOMMO- PROPRIETOR
WINS DATION WINS
ro
on
Crackles and starlings resolve encounters similarly, with few
intruders successful and most interaction resolutions split evenly
between proprietor wins and accommodation. Cowbirds exhibit a larger
percentage of encounters resulting in intruder wins and redwings are
split evenly.
A different picture is produced when the encounter resolutions are
broken down into intraspecific and interspecific interactions (Figs. 8,9).
All species showed increases in intruder wins and decreases in
accommodation interspecifically. Cowbirds exhibit no intruder wins
intraspecifically and redwings, no interspecific accommodation.
Roosting Strategy Success Index
Accommodation, while more important to intraspecific interactions,
does occur between species. This allows different species to intermingle
within the roost, but the frequencies of intruder wins and proprietor
wins is such that there is a better chance for an individual to gain a
roost perch if it interacts with conspecifics. Success in interspecific
interactions is modified by the relative dominance of each species as
well. The term dominance has many definitions and as such can be
detrimental to the understanding of roosting interactions. For this
reason, I have chosen to abandon the term "dominance" in favor of
"roosting strategy success." How the species interact interspecifically
can be considered a measure of the -success of the choice of enconter
resolution used by each of the species.
Figure 8. Frequencies of Intraspecific Encounter Resolutions
% %
Freq.
intruder ACCOMMO- PROPRIETOR
WINS DATION WINS
ro
Figure 9. Frequencies of Interspecific Encounter Resolutions
%
100^
90
BO
75
70
60
CRACKLES STARLINGS REDWINGS COWBIRDS
% •/. %
Freq.
INTRUDER ACCOMMO- PROPRIETOR
WINS DATION WINS
ro
CO
The simplest measurement of roosting success is a simple frequency
based on wins and losses between birds of a particular species, but since
each species has a different level of the ability to accommodate other
species this can change the success of that species' roosting strategy.
For example, a species which seldom retains its perch against intruders
of a particular species in aggressive encounters would be considered to
have a low index of roosting strategy success in relation to that
species. However, if that same species exhibits a higher than expected
frequency of accommodation with the intruder, then its actual success in
gaining a perch within the roost may be higher overall than indicated by
a simple comparison of wins and losses. Therefore, in order to produce a
true picture of roosting success, the accommodation factor must be
included in any algorithm used. For the purpose of discerning the
relative success for each species, the following equation has been used:
Success ^ #wins ^ 112 (#Accommodations) ?- #losses
Total # Interactions
I chose to halve the number of accommodations since each accommoda-
tion is both a win for each bird and yet the inclusion of other species
in the area reduces the chances of success for the other members of the
proprietor species (increased aggression and decreased accommodation is
typical of interspecific interactions), thus accommodation cannot be
considéred a clear "win". A relative frequency is generated by division
using total interactions as a denominator (Table 3.)
Effects of Time and Temperature
Chi-Square analysis indicated that neither time nor temperature affected
the freouency of behaviors or resolutions for any of the species.
Table 3. Roosting Success - Rich Square Roost
SPECIES ROOSTING STRATEGY RANK TOTAL INTERSPECIFIC
SUCCESS INDEX INTERACTIONS
Grackles 0.5634 1 126
Star!ings 0.0000 2 25
Redwings^ -0.5000 4 8
Cowbirds -0.3750 3 8
DISCUSSION AND CONCLUSIONS
The structure and maintenance of winter blackbird roosts are the
direct result of the roosting strategies used by the species involved.
Consequently, examination must proceed from the specific to the
conceptual, a natural and logical route to understanding. For this
reason; discussion of behavior of each species, the roost in general,
accommodation and how it relates to roost maintenance, and possible
directions for future research are included here.
Crackles
The grackle was the most numerous (73.55%) and successful species
studied. The grackle is found in all parts of the roost with no apparent
segregation from the other species, and provides a background upon which
all behavioral interactions and encounter resolutions involving other
species must be considered.
Stewart (1975), working with roosts of 15,000 birds or less,
concluded that the grackles functioned as the nucleus of the roosts and
referred to them as the "leader species" in roost formation. The data
from this study leads me to conclude that this roost is a "grackle roost"
which is being utilized by the other species to varying degrees. How
much this situation changes in roosts where grackles are not so over-
whelmingly superior in numbers must be left to further study, but I
believe that changes in numbers alone will increase other species'
chances of competing successfully only slightly and any roost containing
sizable numbers of grackles will remain essentially a "grackle roost".
Starlings
Starlings were the second largest group within the roost and were
also second ranked in roosting strategy success. It should be noted that
the starling is an introduced species that initiates roosts in Europe
(Stewart, 1975), and has successfully invaded the grackle roost as an
integral part of the roost structure. That an exotic species, moreover
one that is of a different family, has been able to establish itself so
successfully may indicate that the roost is essentially a response to
basic behavioral and ecological factors. It also suggests that the
starling possesses an extraordinarily high adaptability and enough common
ancestry with the native icterid species to allow it to become so fully
integrated.
Redwings
Redwings were the third largest group of birds within the roost and
the lowest ranked species in terms of roosting success. Of all species
in the roost, they are clearly the most aggressive. Redwings do not
exhibit any accommodation interspecifically which compounds the detrimen-
tal effects of aggression. Redwings exhibited a relatively large
percentage of intruder wins in interspecific encounters and since there
was no interspecific accommodation to mollify the intensity of that
effect, the result was a segregation of redwings into pockets within the
roost. In other words, a redwings' best chance of obtaining a perch is
interacting as often as possible with conspecifics, more so than in the
case of the other species which do have some chance of accommodation.
Obviously, redwings will be more successful in those roosts where they
predominate numerically or in pure redwing roosts which may be the
evolutionary direction for this species in the future.
Cowbirds
Cowbirds were the least abundant species and the third most
successful species. Cowbirds were second only to grackles in accommoda-
tion during interspecific encounters. This relatively high rate was the
main reason for being more successful than redwings in roosting strategy.
At the same time cowbirds fared poorly in aggressive encounters, losing
all interspecific aggressive encounters.
Comparison of redwing and cowbird strategies would indicate the
great importance of accommodation to a species' success in roosting.
Aggression alone as exhibited by the redwing cannot guarantee roosting
success nor can numerical superiority.
The Roost
Winter blackbird roosts are the result of a combination of behavioral
characteristics of the species involved. Stewart (1975), in speculating
on the development of roosts, noted that there seemed to be conflicting
forces in operation that led to roost formation, which he called
"communal" and "noncommunal". I believe that the newly discovered "snub"
behavior is the major mechanism of the communal force. Without
accommodation, any congregation of birds would be in constant turmoil
with rates of perch aquisition and loss being equal. It is only when
accommodation occurs that eventual stabilization can occur. Accommoda-
tion allows birds of low relative dominance to become part of a roost as
exhibited by cowbirds and that low levels of accommodation can reduce the
success of an inherently more dominant species such as the redwings.
The control and activation of communal response is seasonal and
involves hormones which stimulate territorial aggression and the related
behavior patterns that predominate during the breeding season (Hinde
1966). The long term survival value of the winter roost is unknown. B.
Meanley (personal communication) stated that there is a general assump-
tion that low temperature and the relatively benign environment within a
roost is the major cause of roost evolution. The fact that time and
temperature had no significant effect on behavior in this study, while
not conclusive, does indicate that we should consider other factors as
possible selective forces. One possibility is the role of winter roosts
in reducing competition for food. Birds with similar feeding habits
dispersing radially from several points within an area are more likely to
come into direct competition for food even in areas where food is not a
limiting factor (Fig. 10). On the other hand, birds radiating from a
central point may face less competition.
Another consideration is survival rates within the roost. The
number of dead birds in each quadrat was noted and these data indicate
that the monthly mortality rate within the roost was 0.08%. This does
not account for birds that died away from the roost or were hidden by the
droppings but does indicate a low death rate within the roost itself.
It is probably safe to assume that the roost is of value to the
birds that utilize it, if for no other reason than the fact that an
introduced species (starlings) quickly adapted to the roost structure.
Figure 10. The Competition Hypothesis
f
I
\
35
MANY SMALL ROOSTS
SINGLE LARGE ROOST
What has become apparent is the mechanism of roost formation. It is
known that the roost begins to form near sundown but that the birds do
not settle down until fairly late in the evening. It is possible to
picture the act of stabilization occurring in concert with the increase
in accommodation that results from the growing pockets of segregated
species. The essence of roost formation is the growth of these pockets
of calm in which the associated species finally gain some measure of
insularity from the dominance of the grackles.
The interactions recorded within this study involve the establish-
ment or retention of roosting perches, and may be thought of as a form of
territoriality. Huxley (1934), in his description of the "elastic disc"
and Krebs (1971), in his concept of the "invincible center" proposed that
an individual, barring sickness or gross differences in size, is usually
undefeatable by conspecifics at the center of its own territory. In the
case of blackbird roosts, since each bird is defending a very small area
indeed, one could assume that any proprietor would have a higher degree
of success than any intruder.
In the Rich Square roost, the proprietor wins greatly outnumber
intruder wins in intraspecific encounters indicating that
proprietorianism was operative. Thus even in the case of the grackles,
it is probably the accommodation behavior that allows the roost to
stabilize for without it intruders would continue to lose in their
attempts to gain a perch from conspecifics.
Snub Behavior
Without some form of accommodation, the roost remains chaotic and
unstable, with half of the birds on perches and the other half attempting
to join them. It is possible for all birds to eventually gain a perch
without accommodation, but it requires more space with larger distances
between individuals. Generally, winter roosts do no occupy all of the
apparently suitable habitat in an area, indicating some advantage in
individual proximity.
The origins of "snub" behavior are not known but there are several
obvious possibilities that should be considered. There is a similarity
between the general posture of bird exhibiting "snub" behavior and one
exhibiting bill tipping, except for the motion of the head. It is
possible that "snub" is at least in part a modification of this low
intensity aggressive display through the processes of habituation or
displacement.
Another possibility is what I call the individual stimulus horizon.
Each proprietor is constantly bombarded with a rich and varied selection
of visual stimuli provided by the many intruders that are moving above
them. There must be conflict within the proprietor as to which stimuli
should be responded to. This conflict could lead to a paralysis of the
proprietor's response or in concentration on the most intense stimulus to
the exclusion or ignorance of lesser stimuli. There may also be a level
or angle below which stimuli are ignored in relation to those stimuli
that are occurring above the "horizon". This would allow those intruders
which are less stimulating to the proprietor to approach the proprietor
closely and actually land on the perch. Once the intruder has landed,
the original proprietor may respond by continuing to concentrate on more
intense visual stimuli above him while responding to the perching
intruder with a modification of bill tipping ("snub") which allows the
intruder to concentrate on the overhead stimulus.
Habituation to intruder stimuli might also play a role in accommoda-
tion but since there seems to be no change in the frequency of accommoda-
tion over time, it is not likely that it does. Undoubtedly, more
research into the nature of accommodation is necessary in order to gain a
better understanding of the nature and origins of this behavior.
Some Speculations on the Winter Roost
The fact that 4 species of birds can exist in close proximity for a
season, have developed compatible behavioral repitoires and developed a
means of accommodation that permits them to overcome their natural
tendencies towards aggression warrants continued research into the
nature of blackbird winter roosts. Perhaps more important to man is the
increasing conflict that occurs when roosts coincide with human
habitation. The chances of disease as well as aesthetic and ecological
damage to areas valuable to humans only enhances the rationale for such
research. To date, we have no data from which to develop control methods
to reduce detrimental interactions between these birds and man. As a
result, all methods for control have tended to be stopgap or more
detrimental than the roosts themselves. We can no longer assume
understanding of winter roost dynamics or the reasons that the roosts
exist at all. This particular study has raised more questions than it
has answered, but we do know something more about how the roost forms and
is held together. New avenues for research exist for the future. It is
likely that a simple, successful, and cost effective means of control can
be found with continued effort. To destroy the birds or the habitat is
not the answer. To decrease the advantages gained by the birds, at least
in areas close to human habitation, or the manipulation of behavioral
patterns become more viable alternatives. If we can begin to learn more
about the ecological and evolutionary facets of the problems in the
process, so much the better.
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