Shiny cowbirds follow the ‘shotgun’ strategy of brood parasitism

Abstract

Brood parasitic cowbirds,Molothrusspp., should lay their eggs in host nests at a time that maximizes both the probability of acceptance by the host and the chances of survival for the parasitic egg and young. Moreover, because multiple parasitism of a host nest is likely to result in low cowbird success, cowbirds are expected to avoid it by defending nests or laying territories, avoiding already parasitized nests or by destroying previously laid cowbird eggs. Shiny cowbird,M.bonariensisegg-laying behaviour was studied in house wren,Troglodytes aedonnests, to determine whether cowbirds monitor host nests to lay their eggs in synchronization with the host's laying period, and whether cowbirds avoid multiple parasitism. Shiny cowbirds monitored nests of house wrens during early and late nest building and during egg-laying stages. Each female cowbird monitored several wren nests over several days, and each wren nest was monitored by several cowbirds. Although to some extent shiny cowbirds synchronized egg laying with their host, they did it in a fairly sloppy manner. Of 185 cowbird eggs laid in 40 nests, 47% were laid in coincidence with the wrens’ laying period, but 35% were laid before the wrens started laying, and 18% were laid after wrens had finished laying and started incubation. Cowbirds also laid 82 eggs in 20 nests that were abandoned by wrens before laying. Multiple parasitism was common and cowbirds did not avoid multiply parasitized nests. There was no evidence that cowbirds removed previously laid eggs, and only six of 150 cowbird eggs were destroyed in nests. Therefore, shiny cowbirds seem to follow a ‘shotgun’ strategy; i.e. they rely more on high fecundity and chance than on the precise placement of each egg. This strategy is possible with house wrens because wrens do not reject cowbird eggs.