Source-sink population dynamics may complicate the interpretation of long-term census data

We examined the reproductive success and long-term population dynamics of Neotropical migrant birds in the fragmented landscapes of Illinois. Our primary objective was to assess whether annual variation in abundances and persistence of populations within woodlots was related to variation in reproductive success. Nesting success of migrants was so low in the woodlots where we sampled that it is unlikely that these populations are self-sustaining. Moreover, populations frequently disappear from and recolonize these woodlots. These data suggest that the source-sink metaphor for population dynamics within spatially structured populations is applicable to migratory birds in Illinois. The scale of this dynamic is unknown, but may be regional (i.e., midwestern U.S.). An important implication of this result is that census data must be interpreted carefully. For one woodlot, we detected no systematic trend in the overall abundances of migratory birds over much of the 20th century. Yet the viability of populations within that woodlot likely decreased dramatically over that time owing to increasing rates of nest predation and brood parasitism. Lack of data on dispersal is a major gap in understanding the population dynamics of Neotropical migrants and prescribing effective conservation measures.