Costs of multiple parasitism for an avian brood parasite, the Brown-headed Cowbird (Molothrus ater)

The Brown-headed Cowbird ( Molothrus ater (Boddaert, 1783)) is a generalist brood parasite that often lays into nests that contain conspecific eggs. Although it has often been assumed that this multiple parasitism reduces Cowbird survival, this has rarely been evaluated. We measured Cowbird survival in nests of the Blue-gray Gnatcatcher ( Polioptila caerulea (L., 1766)), Plumbeous Vireo ( Vireo plumbeus Coues, 1866), and Western Tanager ( Piranga ludoviciana (A. Wilson, 1811)) in New Mexico, USA. Our objectives were to measure the costs of intraspecific competition on Cowbird survival in multiply-parasitized nests, evaluate if these costs were related to host size, and to compare the costs of multiple parasitism relative to other mortality sources that occur over the entire nesting cycle. Intraspecific competition reduced Cowbird survival during the nestling period in nests of all three hosts, and was of particular importance in nests of the two smaller hosts. When all sources of egg mortality were considered, however, the costs of multiple parasitism were small compared with the large effects of predation and nest desertion. Given that multiple parasitism reduces Cowbird egg survival, it is unclear why Cowbirds multiply-parasitize. Possible explanations depend on an improved understanding of fecundity and level of host nest selectivity by female Cowbirds.

Effects of stage in incubation, time in season, and proportion of original clutch remaining on nest desertion by house sparrows, Passer domesticus

Life history theory predicts that individuals should maximize their fitness by balancing current investment in offspring versus future prospects for reproduction. Faced with reduction of their current clutch, birds should desert if the prospective opportunity would increase inclusive fitness more than continued investment in the reduced clutch. I studied nest desertion in response to clutch reduction by house sparrows (Passer domesticus) to determine if continuing investment in a reduced clutch differs based on proportion of original clutch remaining, stage in incubation, and ordinal date. Nests were reduced to two eggs early or late in incubation over two complete breeding seasons. Of 150 nests manipulated, 36 were deserted. Nests were more likely to be deserted when reduction occurred earlier in incubation, earlier in the season, and with a smaller proportion of original clutch remaining. This suggests that both time and brood size are used to assess the tradeoffs between current and future investment. However, near the end of the breeding season, the proportion of original clutch remaining and stage in incubation were less important, and low desertion was likely associated with a lack of re-nesting opportunities in the current season. Therefore, whether to desert or continue investing in a reduced clutch is a function of offspring reproductive value (RV) when there is opportunity for re-nesting in the same season. However, near the end of the season the decision is based on the residual reproductive value (RRV) of parents.