ASSESSING GENERALIZED EGG MIMICRY: A QUANTITATIVE COMPARISON OF EGGS OF BROWN-HEADED COWBIRDS AND GRASSLAND PASSERINES

Perceived similarity in appearance of eggs of the Brown-headed Cowbird (Molothius ater) and several grassland passerine host, has been suggested to represent a form of generalized mimicry evolved to impede egg discrimination We tested this hypothesis by quantitatively comparing foul parameters (ground color, maculation color, distribution of maculation and maculation density) among cowbird eggs and those of six grassland passerines known to preferentially accept cowbird eggs over non-mimetic blue cup Cowbird eggs did not significantly differ in three of four parameters (mean ground color, and color and density of maculation) from the average grassland passerine egg measured lot this community, in southern Saskatchewan. when all grassland passerine eggs were pooled Cowbird eggs sufficiently overlapped grassland passerine eggs in these three parameters that 88% of cowbird eggs measured were statistically indistinguishable from among all grassland passerine eggs The frequency at which cowbird eggs were misclassified as eggs of each grassland passerine ranged flout 8 to 48%, which suggests that a single cowbud egg is capable of mimicking the eggs of more than one species These results support the hypothesis that cowbirds have evolved a generalized egg appearance that mimics the eggs of multiple grassland passerine hosts within a simile community Received 31 October 2008 Accepted 23 October 2009