Egg crypsis and clutch survival in three species of blackbirds (Icteridae)

Abstract

For most open-nesting passerine birds, the primary function of egg pigmentation is crypsis. Interspecific variation for egg coloration is quite high, even among species that nest in the same locale. This variation might be maintained by selection to blend into species-specific nest cups, which vary in pattern because they are constructed from different materials. To test this hypothesis we photographed 30 clutches of Brewer's blackbirds (Euphagus cyanocephalus), 26 clutches of red-winged blackbirds (Agelaius phoeniceus), and 24 clutches of yellow-headed blackbirds (Xanthocephalus xanthocephalus), and quantified crypsis in terms of pattern (PD crypsis), brightness (MB crypsis) and brightness disparity (BD crypsis). The species were equally cryptic in terms of pattern crypsis, but differed significantly (P < 0·01) in MB and BD crypsis. Intraspecific variation in pattern crypsis was surprisingly high. Mean clutch pattern disparity was highly correlated with mean background pattern disparity (r = 0·999); this suggests selection for disparity matching, rather than for crypsis. We also monitored 25 clutches (7 Brewer's, 9 red-winged, and 9 yellow-headed blackbirds) for a 9-day period to determine if survival was related to crypsis. For Brewer's and yellow-headed blackbirds, successful nests were no more cryptic than failed nests. For red-winged blackbirds, there was a tendency (P = 0·1) for less cryptic clutches to have higher survival; in addition, successful red-winged blackbird clutches had significantly (P < 0·05) higher egg pattern disparity and background pattern disparity. Red-winged blackbird clutches are not concealed by overhanging vegetation, and high pattern disparity may enable them to blend into a large-scale heterogeneous background.