A PAN-AMERICAN VIEW OF THE INFLUENCES OF ECOLOGY AND PHYLOGENY ON CLUTCH SIZE AND THE CLUTCH SIZE-LAYDATE REGRESSION IN TACHYCINETA SWALLOWS.

RINGELMAN, K.; WINKLER, D.; HUSSEL, D.; CLARK, B.; LOMBARDO, M.; DUNN, P.; LEONARD , M.; DAWSON, R.; SHUTLER, D.; MASSONI, V.; QUIROGA, M.

Veracrus, Mexico

Congreso; IV North American Ornithological Conference; 2006

American ornithologist Union

Using more than 11,000 data points from almost 30 sites across the Americas, we analyzed how clutch size and the clutch size-laydate regression varies with geographyand phylogeny in four Tachycineta species. Following Young’s study of Troglodytes we hypothesized that clutch size should decline more steeply with laydate in the shorterbreeding seasons of temperate latitudes, and we explored Young’s hypothesis that differences in the clutch size-laydate regression could in part be responsible forlatitudinal variation in clutch size. We tested the effects of centered laydate, distance from the Equator, longitude, elevation, and distance to the nearest ocean, togetherwith their interactions, on clutch size. Our analyses indicate that distance from the Equator, longitude, and elevation are all important geographic determinants of clutchsize, though none of these main effects is as strong as that of laydate. The clutch size-laydate regression is strongly responsive to both elevation and distance from theEquator, and, as predicted by Young’s hypothesis, it grows steeper with distance from the Equator. We will evaluate the relative effects of different aspects of geographyon this regression, and the power of the regression to explain geographic patterns in clutch size.