Host-Egg Removal by Brown-Headed Cowbirds: A Test of the Host Incubation Limit Hypothesis

Abstract

-The incubation limit hypothesis (Davies and Brooke 1988) states that female brood parasites remove host eggs to reduce the parasitized clutch volume to a size that the host can incubate effectively, thereby ensuring that the parasite's eggs will hatch. We tested this hypothesis by adding freshly laid Brown-headed Cowbird (Molothrus ater) eggs to Yellow Warbler (Dendroica petechia) clutches and comparing hatching success in clutches from which one or no host eggs were removed (Treatments 1 and 2, respectively). Hatching success of cowbird eggs differed significantly between Treatments 1 and 2 in one of the three years of study. Warbler hatching success tended to be higher in control and in Treatment 1 clutches than in Treatment 2 clutches. Climatic conditions varied between years but did not influence cowbird hatching success. Cowbird eggs tended to hatch before warbler eggs whether or not a host egg was removed. Neither incubation period, hatching spread, nor hatching order was significantly influenced by egg removal. These results indicate that removal of a host egg is not necessary for successful hatching of cowbird eggs in Yellow Warbler clutches in all years, although it may increase hatching success in some years. Received 15 May 1996, accepted 18 October 1996. FEMALES OF MANY avian brood parasites remove host eggs from nests during the breeding season. These eggs usually are those of the host, and they are removed in association with the laying of the parasite's own egg (Sealy 1992). Egg removal reduces the number of host offspring that can be raised, and numerous workers have suggested that this behavior may increase the parasite's reproductive success (Davies and Brooke 1988, Rothstein 1990). Brown-headed Cowbird (Molothrus ater; hereafter cowbird) eggs have been found in the nests of more than 220 host species, and cowbird young have been raised successfully by at least 140 host species (Friedmann and Kiff 1985). Female cowbirds generally remove eggs from host nests (Friedmann 1963), but the number of eggs removed and the timing of egg removal vary among host species (e.g. Mayfield 1961, Zimmerman 1963, Sealy 1992). The fact that cowbirds make additional visits to nests to remove host eggs (Scott et al. 1992, Sealy 1994) and, in doing so, increase the risk of encountering hosts, suggests that egg removal is an important cowbird strategy. Sealy (1992) reviewed six hypotheses to explain the function of host-egg removal by female brood parasites and suggested that the incuba1 E-mail: dgmcmast@mb.sympatico.ca tion limit hypothesis (Davies and Brooke 1988) was the most deserving of further testing. The host incubation limit hypothesis proposes that the addition of a parasitic egg could increase clutch size above that which the host can incubate effectively (Hofslund 1957, Friedmann 1963, Post 1981, Fraga 1983, Kendra et al. 1988). Incubating females must use more energy to maintain egg temperature in large clutches (Biebach 1981, Moreno et al. 1991), but because the amount of heat energy that can be produced and transferred to the clutch is limited physiologically (T0ien 1989), egg temperatures may fall as clutch size increases (Batt and Cornwell 1972, Mertens 1977). Because most bird eggs must be maintained within a narrow range of temperatures for normal embryonic development (Webb 1987), extended cooling may reduce the hatching success of both host and parasitic eggs (Wyllie 1975, Davies and Brooke 1988). By removing a host egg, the female parasite may reduce the total clutch size to within the host's incubation limit, and possibly increase the probability that the parasite's own egg will hatch (Davies and Brooke 1988). Hofslund (1957) calculated that the addition of one cowbird egg to a Common Yellowthroat (Geothlypis trichas) clutch of four eggs increased the volume by 50%. Therefore, removing a host egg would appear to be especially important when cowbirds parasitize species whose eggs are smaller than cowbird eggs.