Courting male ruff sandpipers (Calidris pugnax): do males vary their tactics in the absence of one morph?

Abstract

The ruff sandpiper (Calidris pugnax) is a medium sized wading bird native to Eurasia that displays strong sexual dimorphism in plumage. Unique among shorebirds, the ruff has a lek mating system where breeders gather on an arena and males display with the purpose of enticing females to mate. Within ruff populations there are three distinct genetic morphs that specify differences in male mating behavior and plumages. During migration, Independents and Satellites grow ornamental mating plumage, a head tuft and ruff around the neck, which is showcased during ritualized displays. Independents display ‘Resident’ behavior, each defending a small court on the lek, whereas Satellites move among courts and co-display with the Resident. Faeders, the third and rarest morph, are smaller in size and resemble females in appearance, lacking ornamental plumage. These are heritable, permanent genetic morphs: the Satellite and Faeder morphs are determined by variants of an autosomal chromosomal inversion. I observed behavior on the lek of captive bred ruff sandpipers at Sylvan Heights Bird Park (SHBP) in Scotland Neck, NC. This colony presently lacks Satellite males providing an opportunity to observe males for potentially compensatory behavior in the absence of this morph. Anecdotally, Faeder males have been observed to display in captivity under some circumstances, and an Independent male was observed co-displaying at SHBP in a previous year. To further explore this behavior, observations were made both in person and using video of ruffs before and during the mating season. Behaviors categorized as aggressive and display were recorded, as well as proximity to other males. These observations were then quantified using standardized methodologies to characterize the behavior of individual males and identify instances of co-display behavior by Independents and the Faeder. Observations of this colony from the 2020-2022 breeding seasons captured co-display behavior by the Faeder and at least one Independent. Observations from the 2023 breeding season did not. Failure to observe co-display behavior on 2023 may have been influenced by subdividing the males among two aviaries, thereby reducing the density of males. It is conceivable that previous peculiar observations of co-displaying by Independents and the Faeder were due to both the absence of Satellites in conjunction with limiting space for all males to have courts. Data from the 2023 season did reveal a significant relation between male level of aggression and rate of display. Neither of these parameters were significantly related to male mass, body condition (mass/wing chord), or age. Future studies should examine the possible role social environment plays in the behaviors of a species with a permanent genetic morphs. Such investigations are important for understanding the interplay between gene expression under different social contexts.