Do female Eastern Bluebirds trade-off egg size with clutch size?

Abstract

Egg size and clutch size are traits that show a fundamental trade-off in life history theory. Producing a larger number of offspring can come at the cost of being able to invest in those offspring. The degree of maternal flexibility in these traits remains poorly understood. As part of a long-term study, clutch data have been collected for the past six years for Eastern Bluebirds Sialia sialis breeding at East Carolina University’s West Research Campus. In 2014, a spatial cluster of 2-egg clutches coincident with the occurrence of a late prescribed burn was observed. I measured eggs in these clutches from standardized clutch photographs taken in the field. The dimensions of eggs in these small clutches were larger than average, suggesting the mothers may have compensated for number by investing more in each egg. The additional observation that some of the 2014 2-egg clutches had brittle, misshapen eggshells led us to hypothesize that these mothers were resource limited, possibly lacking calcium due to the depletion of arthropod prey caused by the late burn. If calcium limits shell formation, then we might expect a negative relationship between clutch size and eggshell thickness more generally. We tested the prediction that shell thickness should be inversely correlated with clutch size. Based on six years’ nest summary data, I examined the relationship between clutch size and egg size, and determined that small clutches occurred near areas of prescribed burn. I measured eggshell thickness of egg specimens laid by different females in clutches of different sizes in different years. I found that egg size was significantly negatively related to clutch size and burn status, providing evidence of a trade-off between egg size and clutch size. Furthermore, clutch size was significantly negatively related to whether an area was burned during the previous winter. However, no significant relationship was found between eggshell thickness and clutch size. My results show that Eastern Bluebirds have an adaptive response in a life history trait in relation to prescribed burns. Further research should explore the effects of female age and condition in moderating the trade-off between egg size and clutch size.