Clutch size varies more with longitude than latitude in North American breeding Tyrannus (kingbirds)

The driver(s) of geographic variation of clutch size in birds is a long-standing question. The main pattern—increase of clutch size with latitude in the northern hemisphere—has been attributed to latitudinal increases in parental feeding ability, reduced threats of nest predation to the north, and latitudinal differences in stage- or age-specific mortality. Longitudinal variation has been largely ignored. I use 2,874 museum egg sets for 6 species of North American breeding Tyrannus (kingbirds) to describe geographic variation in breeding date and clutch size. Significant intra- and interspecific differences in date varied directly with latitude, and to a lesser extent with longitude and elevation. Species differences were negligible after accounting for geography. Although clutch size tended to increase with latitude in low latitude species, most intra- and interspecific clutch size differences were associated with longitude. Western breeding Tyrannus species produced larger clutches than the mainly eastern breeding T. tyrannus; the latter also produced larger clutches in the west. Clutch size also declined with breeding date and elevation. Interspecific clutch size differences were larger than for breeding date after accounting for geography due principally to the large clutch size of T. forficatus. The westward increase of clutch size cannot be ascribed to either the likelihood of nest failure or selection for greater mobility at fledging as the predicted association of smaller clutches with frequent nest failure was not supported by literature review and Tyrannus nestlings all exhibit similar growth despite differences in clutch size. Demographic compensation remains a possible explanation, but a paucity of adult survival estimates precludes a definitive conclusion. Instead, at present it appears that small clutch size occurs where breeding season precipitation is frequent and high, suggesting that high day-to-day stochastic variation in insect food supplies for these aerial foragers favors small clutch size and iteroparity.