A classic life history pattern revisited: Day length and the latitudinal gradient in avian clutch size

The latitudinal gradient in avian clutch size (LGCS) is one of the most striking patterns in Ecology. Assumptions regarding the apparent consistency of the pattern across taxa are based primarily on either comparisons of species or family means for clutch size across large geographical scales or a handful of excellent detailed studies of within-species variation in clutch size across latitudes. Since Lack (1947), no one has compiled, for a variety of unrelated species, a summary regarding the generality of the latitudinal gradient in clutch size within species. Here I present the results of my work using linear regression and ordinal and multinomial logistic regression to evaluate the relationship of clutch size to latitude across populations for 21 North American birds. My results confirm that the trend is widespread and suggest that previous use of linear regression statistics may have underestimated the generality of the trend and masked variation in how changing frequencies of particular clutch sizes create the pattern. In addition to examining the universality of the LGCS, I evaluated the explanatory power of the original mechanism proposed to explain it---that latitudinal gradients in day length during the breeding season provide breeding birds at high latitudes with more time in a day to forage for food for young. To test this hypothesis, I performed a comparative study of resource acquisition and daily chick-feeding duration in two populations of tree swallows breeding across a large latitudinal gradient within the temperate zone. My results show strong support for the day length hypothesis and suggest that gradients day length may have important consequences for other life history characters besides clutch size and for many aspects of birds' breeding behavior and physiology.