Reproductive Costs of Commuting Flights in Brown-Headed Cowbirds

Abstract

Brood parasitism by brown-headed cowbirds (Molothrus ater, hereafter "cowbird") has attracted much attention from conservation biologists as an edge effect over small spatial scales, reducing nesting success of songbirds near habitat boundaries. The mechanisms underlying this landscape-scale edge effect have not been studied, but this phenomenon has been attributed to the energetic costs of commuting by cowbirds-i.e., daily flights between breeding and feeding areas. We Studied cowbird egg-laying rates by counting ovarian follicles at a site in New Mexico, USA, where previous research had established the presence of short-distance commuting (SDC; approx 2 km) and long-distance commuting (LDC; approx 12 km) cowbirds. We hypothesized that LDC cowbirds would lay fewer eggs than SDC cowbirds and have more atretic yolky follicles (follicles degenerating prior to ovulation) due to the greater energetic demands of their commuting flights. In 1999-2000, we found that LDC cowbirds had half the number of post-ovulatory follicles corresponding to a 5-day period ((x) over bar = 1.8, n = 16) as SDC, cowbirds ((x) over bar = 3.5, n = 14; P = 0.008). Thus, the egg-laying rate of LDC cowbirds (0.36/day) was only half that of SDC cowbirds (0.7/day). Long-distance commuting cowbirds also had more atretic follicles ((x) over bar = 1.3, n = 16; P = 0.006) than SDC ((x) over bar = 0.2, n = 14), suggesting that the difference in egg-laying rates was due largely to energetic stress in cowbirds commuting longer distances. Wing length and body mass were smaller in LDC cowbirds than SDC cowbirds, suggesting that LDC cowbirds may have been subordinate individuals excluded from breeding habitat closer to feeding areas by intraspecific competition. These results suggest that management strategies, such as livestock removals, designed to reduce cowbird numbers at host nest sites may provide additional benefits to hosts by reducing the fecundity of individual cowbirds.