North Central Research Station

Songbird Nesting Success and Habitat Use: An Urban-Rural Comparison

Although urbanization is fast converting natural habitats into “built” environments on a large scale, little is known about the effects of urbanization on songbirds. Small-scale nesting and abundance studies have been conducted on individual songbird species, but no study has compared the habitat use and nesting success of an entire songbird community between comparable urban and rural settings in a replicated fashion.

The Study

We examined the influences of urbanization on nesting success of songbirds that nest in floodplain field habitats in both urban and rural landscapes. We tested whether nest predation by enemies such as snakes, raccoons, and housecats was greater in urban settings than in rural ones. We also examined whether parasitism by the brown-headed cowbird, a bird that lays its eggs in other birds' nests at the expense of the “host” species, differed between urban and rural settings.

Results

After three years of study, we found that birds such as northern cardinal and indigo bunting nesting in the urban landscape were parasitized by Brown-headed Cowbirds 2.5-to-12 times more frequently than in the comparable rural landscape.

Figure 1. Percent of nests parasitized by Brown-headed Cowbird according to landscape type

We found that Northern Cardinals, a year-round resident, started nesting in the urban landscape on average one week earlier than cardinals in the rural landscape. However, Indigo Bunting and Yellow-breasted Chat, which are long-distance migrants that arrive in May, started nesting at about the same time in both landscapes.

Although there were no differences in nest predation between the two landscapes, some songbirds, including Blue-winged Warbler, White-eyed Vireo, and Field Sparrow were rarely or never detected in bird counts in the urban landscape, whereas they were common in the same type of field habitats in the rural landscape. However, Brown-headed Cowbirds, the brood parasite, were more common in the urban landscape.

We conclude that floodplain fields of similar size and structure may contain very different songbird communities, because some songbird species appeared to avoid otherwise apparently suitable habitats in the urban setting. The species common to both urban and rural landscapes (Northern Cardinal, Indigo Bunting, and Yellow-breasted Chat) were parasitized much more heavily in the urban landscape, perhaps because of the greater abundance of Brown-headed Cowbirds. Because cowbirds feed heavily at bird feeders and forage in short-grass habitats such as lawns, urbanized settings may constitute cowbird-rich habitats that are not beneficial to songbirds during the portion of the breeding season in which cowbirds are active. However, nest predation was similar in the two landscapes, and songbirds in both landscapes successfully fledged host young.

Conservation implications

Because of their proximity to a large number of people, urban habitats provide areas where many people can enjoy birds. These habitats may not be as suitable for some songbirds, however, as habitats in less developed settings. Future research will investigate the potential impacts of these differences on the viability of songbird populations.