First record of brown-headed cowbird egg in a lesser scaup nest
On 16 June 1999, I found a Lesser Scaup (Aythya affinis) nest parasitized by a Brown-headed Cowbird (Molothrus ater). On 22 June the female scaup had begun incubating the 10 scaup eggs but the cowbird egg was missing, presumably ejected by the female scaup.
On 16 June 1999, while searching for waterfowl nests near Erickson, Manitoba (50°30′ N, 99°55′ W) I found a Lesser Scaup (Aythya affinis) nest containing a Brown-headed Cowbird (Molothrus ater) egg and six Lesser Scaup eggs. This is the first published record of a Lesser Scaup nest containing a Brown-headed Cowbird egg (see reviews by Friedmann and Kiff 1985, Johnsgard 1997). I calculated the clutch initiation date to be 11 June based on candling of the six fresh scaup eggs in the nest at the time of discovery. On 22 June I checked the status of the nest and used a field candler to determine that the female scaup had begun incubation (Weller 1956). The nest contained 10 scaup eggs but the cowbird egg was missing. I looked around the nest for the cowbird egg but did not see it in the vicinity. It is likely that the relatively large size of the scaup's bill enabled it to grasp and eject the cowbird egg from the nest (Clayton and Moore 1997). On 26 June I found that the nest had been depredated. I do not know whether the parasitism event affected the eventual fate of the scaup nest, but avian predators might use cowbirds as a cue to find nests.
On the expected day of hatching (16 July), I recorded microhabitat and spatial location variables of the nest. The nest was located overwater in sedge (Carex spp.) vegetation. The vegetation near the nest was 0.7 m tall, formed a 35% canopy cover, and had a Visual Obstruction Reading of 6.6 (Robel et al. 1970). The nest was located in 0.1 m of water, 3 m from open water, 3 m from the nearest edge, and 7 m from the edge of the wetland.
While many warblers, vireos, phoebes, and sparrows are acceptor species of cowbird eggs, waterfowl are rejecters (Friedmann and Kiff 1985, Johnsgard 1997). Brown-headed Cowbirds have been documented as successfully parasitizing 144 of 220 species' nests in which their eggs have been observed (Friedmann and Kiff 1985, Johnsgard 1997). Lesser Scaup can now be added to the current list of 76 rejecter species to Brown-headed Cowbird parasitism. The Blue-winged Teal (Anas discors) is the only other anseriform documented to be a rejecter of Brown-headed Cowbird parasitism (Friedmann and Kiff 1985).
Koons, D.N. 2000. First record of brown-headed cowbird egg in a lesser scaup nest. Wilson Bulletin 112:554.