Removing Brown-Headed Cowbirds Increases Seasonal Fecundity and Population Growth in Song Sparrows

Parasitic Brown-headed Cowbirds may reduce the reproductive success of their songbird hosts and thus threaten rare and frequently parasitized host populations with extinction. The demographic impacts of cowbirds on hosts, however, have seldom been studied experimentally. We removed cowbirds to estimate how much they reduce the seasonal fecundity and population growth rates of a non-endangered cowbird host, the Song Sparrow. Over five control years, 61% of 515 sparrow nests were parasitized at three riparian study sites. When we removed female cowbirds from two of these sites, parasitism fell to 43% of 65 nests at Westham in 1996, to 30% of 117 nests at Westham in 1997, and 18% of 78 nests at Delta in 1998. The mean seasonal fecundity of sparrows increased from 1.67 fledglings per territory in controls to 3.40 fledglings per territory at Westham and from 1.23 to 3.16 fledglings per territory at Delta. At the third control site, Deas, seasonal fecundity averaged 2.07 fledglings per territory. The mean sizes of sparrow broods at fledging were increased in removal years from 0.53 to 0.95 young at Westham and from 0.47 to 1.15 young at Delta. Daily nest failure rates were reduced from 0.046 in control years to 0.030 during cowbird removals at Westham and from 0.049 to 0.035 at Delta. Survival over a 25-d nest cycle rose from an average of 0.32 in eight controls to 0.45 in three removals. Cowbird removals increased local population growth rates of song sparrows from well below replacement levels (λ = 0.70-0.87) to a stable condition (λ = 0.93-1.06). If cowbirds have similar or stronger effects on rarer and more vulnerable hosts, cowbird removal could improve the demographic performance of such hosts substantially.