Geographical and seasonal gradients in hatching failure in Eastern Bluebirds Sialia sialis reinforce clutch size trends

Eggs untended during the laying phase can lose viability if exposed to high temperatures, such as those common at lower latitudes and late in the nesting season. The egg‐viability hypothesis states that constraints on viability during the laying phase could account for latitudinal and seasonal gradients in clutch size. We used 7 years’ worth of data collected by volunteers (The Birdhouse Network, co‐ordinated by the Cornell Laboratory of Ornithology) to look simultaneously at populations across the temperate breeding range of Eastern Bluebirds Sialia sialis in order to test three predictions of the egg‐viability hypothesis: the probability of hatching failure decreases at higher latitude and increases later in the season, and that these trends are strongest among large clutches. The overall average number of unhatched eggs relative to the total number of eggs laid was similar to that found by other studies (7.8%; range 6.8–8.9% annually; n = 32 567 eggs from 7231 nests from 530 study sites). Using generalized linear mixed models that controlled for the non‐independence of eggs within a clutch, we found that the ‘per‐egg’ probability of hatching failure was highest late in the season, highest at lower latitudes, and highest for both small (three‐egg) and large (six‐egg) clutches. The seasonal and geographical gradients in egg hatching failure reinforce documented seasonal and geographical trends in clutch size. Loss of egg viability prior to incubation currently provides the most parsimonious and consistent explanation of the observed patterns of hatching failure. However, alternative explanations for large‐scale patterns, particularly those not consistent with the egg‐viability hypothesis, warrant further research into other causes of hatching failure, such as microbes and infertility (related to extra‐pair mating). Our results demonstrate that investigating causes of variation in demography among local populations across a geographical gradient provides a potential means of identifying selection pressures on life‐history traits.