Degree Name

Master of Science (MS)

Semester of Degree Completion

1995

Thesis Director

Eric K. Bollinger

Abstract

Obligate brood parasites, like the Brown-headed Cowbird (Molothrus ater), lay their eggs in the nests of host species, usually to the detriment of the host's reproductive effort. In addition, Brown-headed Cowbirds often remove one or more host eggs near the time of parasitism. Although several hypotheses exist, the adaptive significance of egg removal has not been clearly established. Peer and Bollinger (in press) proposed the host incubation efficiency hypothesis which states that the number and size of host eggs influences the incubation efficiency of a parasitic egg. Thus, host egg removal by cowbirds should increase the parasitic egg's chance of hatching in a host's nest or reduce its incubation length, especially if the host eggs are larger. They tested this hypothesis using a large host, the Common Grackle (Quiscalus quiscula), and found that cowbird eggs had a greater chance of hatching and a shorter incubation length in smaller clutches. Thus, for cowbirds parasitizing larger host species, egg removal was adaptive.

However, Brown-headed Cowbirds currently parasitize species noticeably smaller than grackles. Therefore, I tested the host incubation efficiency hypothesis to determine whether egg removal confers an advantage in hatching efficiency and incubation length for medium and small host species. Red-winged Blackbirds (Agelaius phoenicus) and Chipping Sparrows (Spizella passerina) served as host species. Either a House Sparrow or a Brown-headed Cowbird egg was placed in each host nest. In some nests, a host egg was removed as well (addition/removal nests), whereas in others no host egg was removed (addition nests). Overall, the addition/removal treatment did not decrease incubation length or increase the incubation efficiency of parasitic eggs compared to the addition treatment. However, in addition/removal nests of Red-winged Blackbirds, parasitic eggs were more likely to hatch and have a shorter incubation length in smaller clutches. In addition nests, parasitic eggs hatched sooner in smaller clutches, although hatching efficiency was not dependent upon clutch size.

Parasitic eggs always hatched successfully in Chipping Sparrow nests and host eggs were often inefficiently incubated. The addition/removal treatment did not increase hatching efficiency or shorten incubation length for parasitic eggs compared to the addition treatment. Clutch size did not affect hatching efficiency or incubation length in either treatment. Overall, parasitic eggs hatched earlier and were less likely to be inefficiently incubated in Chipping Sparrow nests compared to Red-winged Blackbird nests.

These results provide support for the incubation efficiency hypothesis of host egg removal by Brown-headed Cowbirds. Although Red-winged Blackbird nests with smaller clutches had shorter incubation lengths and increased hatching efficiency for parasitic eggs, the addition/removal treatment did not significantly decrease the incubation length of parasitic eggs. Secondly, although parasitic eggs in Chipping Sparrow nests had shorter incubation periods than Red-winged Blackbird nests and caused the inefficient incubation of host eggs, addition/removal and addition treatments did not differ.

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