Expanded geographic extent of seasonal increases in partial hatching failure in Eastern Bluebirds over five decades

Background/Question/Methods

There are many documented ecological responses to anthropogenic climate change.  For example, clutch-initiation date in birds has advanced for many species.  Such phenological advances may be attempts to adaptively match breeding phases with temperature-mediated phenologies of prey organisms. To investigate the fitness consequences associated with clutch-initiation date, and more specifically changes in this relationship over time, we examined latitudinal and seasonal patterns in the probability of partial hatching failure in temperate-breeding Eastern Bluebirds using data from over 50,000 nest attempts collected by over 800 citizen science project participants from 1960 to 2011. We excluded clutches that completely failed to hatch and included nests for which clutch size, number of nestlings, and clutch initiation date were reported or could be accurately estimated. We used proc glimmix in SAS with a binomial error and logit link to model the probability of an egg not hatching based on the independent fixed-effect variables of year, day of breeding season, latitude, longitude, clutch size, as well as various interaction terms. We treated clutch size as a categorical variable so as not to impose an a priorirank.

Results/Conclusions

We found that 4-17% of eggs failed to hatch in a year.  The smallest and the largest clutches experienced the highest rates of partial hatching failure.  With slight increases over the years, about a third or more of clutches contained one or more unhatched eggs.  Historically, seasonal increases in incomplete hatching success occurred only at lower latitudes, whereas in recent decades, seasonal increases are apparent at all latitudes. The latitudinal, seasonal, and temporal trends were consistent with the idea that hatching failure increases under “late season” conditions, such as warmer temperatures.  Environmental factors affecting hatching may provide selection pressures for seasonal declines in clutch size, which are stronger than latitudinal trends in clutch size in this species.