Spatial patterns of shiny cowbird brood parasitism on chestnut-capped blackbirds

Abstract

Shiny cowbirds,Molothrus bonariensisparasitized a high frequency (average 48%) of nests in five colonies of chestnut-capped blackbirds,Agelaius ruficapillusin Argentina. Two distinct egg morphs occurred, spotted and immaculate white eggs, as well as a few very lightly spotted intermediate eggs. The clear differences between morphs, combined with considerable variation in spotting pattern between spotted morph eggs, made it possible to visually match eggs into groups that were probably laid by single females. Eggs attributed to a single female were more similar in size and shape than eggs attributed to different females. Using egg dimensions and colour patterns to infer spatial patterns of laying by individual females indicated that: (1) individual brood parasites often laid several eggs in the same colony, (2) females also laid eggs in more than one colony, and (3) several females laid eggs in each colony, and often in the same host nests, ruling out the notion that parasites defend exclusive territories with respect to host nests. Multiple cowbird eggs per host nest invariably resulted from several females laying in the same host nest: egg morphs and visual comparisons of spotting patterns at 14 nests indicated that individual females never laid more than a single egg in the same host nest. Experimental parasitism of nests with spotted and white morph eggs, combined with observations of naturally parasitized nests, demonstrated that chestnut-capped blackbirds accept all morphs of cowbird eggs. Since other important host species reject white eggs, however, the adaptive maintenance of the white morph is difficult to explain.