A model of the dynamics of cowbirds and their host communities

Abstract

Few studies have examined the potential consequences of cowbird (Molothrus spp.) parasitism on entire avian communities. Because cowbirds are host generalists, an obvious opportunity exists for such community-level effects. We developed a model to predict how cowbird abundance affects and is affected by the relative abundances of different types of host species. Toward this end, we divided the passerine hosts of cowbirds into three categories, each having different population dynamic properties: (1) ejectors of cowbird eggs; (2) extinction-prone acceptors of cowbird eggs (species that decline in abundance in response to high levels of cowbird parasitism); and (3) insensitive acceptors of cowbird eggs (species that maintain their abundance even at high levels of cowbird parasitism). Ejectors are sinks for cowbird eggs and thus indirectly benefit extinction-prone hosts. Conversely, insensitive acceptors can raise cowbird young without a concomitant decrease in their own abundance; as such, they indirectly harm extinction-prone species. Although cowbird abundance is determined by the abundance of both ejectors and insensitive acceptors, the reverse is generally not true (i.e. their abundance is independent of cowbird abundance). The mathematical model of cowbird/host community dynamics we present consists of two ordinary differential equations that incorporate the above assumptions about the different classes of hosts and the manner in which they interact with cowbirds. The model predicts that extinction-prone species will have a higher potential to persist when one or more of the following exist: (1) ejectors are relatively more abundant than insensitive acceptors; (2) ejectors are abundant relative to extinction-prone carrying capacity; (3) maximum potential cowbird per-capita population growth rate is small; and (4) the potential effect of cowbirds on extinction-prone population growth rate is small. Extinction-prone species will decline or go extinct in reverse situations.