Effects of Urbanization on Birds in the Puget Sound Region

The Earth’s human population is growing rapidly and increasingly large proportion of this population is living in urban areas and urbanizing formerly undeveloped areas. Development greatly alters land cover by fragmenting, degrading, and converting native habitat allowing some wildlife species, often non-native, to flourish while many native species decline in abundance or suffer reduced productivity. Management for native wildlife communities in urbanizing areas has traditionally focused on the creation and maintenance of large, connected habitat reserves. However, this approach is both unrealistic and has little empirical basis in areas with moderate to high human populations and correspondingly high real estate values. The goals of my research are to determine (1) how the conservation value of habitat reserves is influenced by reserve size and landscape setting, (2) how development pattern influences the health of the in situ wildlife, and (3) how this information can be combined for a more realistic conservation plan. I focus on bird communities in 27 reserve and 36 development sites spanning the urbanization gradient in the Seattle, WA metropolitan area. Urbanization spatial pattern metrics such as urbanization intensity, mean urban patch size, and land cover contagion are quantified using satellite imagery. I quantify bird community composition and productivity through surveys, monitoring real and artificial nests, and netting. Because vegetation and resident mammals are likely to impact the bird community, I also quantify vegetation and small mammal abundance at each site.

Preliminary data analyses indicate that reserve size and landscape setting have an interactive effect on bird species richness. Small exurban sites and small urban sites have greater and lesser richness, respectively. Furthermore, richness does not appear to decline with size due to local extinction of native songbirds and American crows do not appear to greatly depress the reproductive success of native birds, as is generally believed. Instead, small mammal abundance (e.g. deer mouse) may be a better predictor of native bird community health.

Urbanization across the west has created a mosaic of habitats of different qualities for American crows. To understand the drivers of crow population increases, I am studying how juvenile dispersal interacts with local population dynamics. I have continued ongoing studies of crow reproduction and survival along a gradient of urbanization (from urban to suburban to ex-urban sites) by finding and monitoring nests of crows. In 2000 we monitored 57 nests, and caught and banded 93 crows. Nest success results from the year 2000 were similar to previous results: crows produced fewer young per nest in urban areas compared to suburban and ex-urban sites. Population projections suggest that observed reproduction and survival in urban habitat is insufficient to account for observed population increases.

To test whether crows from less-urbanized habitat produce young that then move into urban areas, we attached radios to 30 fledgling crows. I have continued tracking these radiotagged juveniles through the fall and winter. To date, 6 crows (20%) have died, 6 (20%) are still found in their natal areas, 7 (23%) have dispersed > 10 km from their natal areas, 6 (20%) are found irregularly, and 5 (17%) had transmitters that failed. Dispersal > 10 km has been from ex-urban to other ex-urban areas (n=1), from ex-urban or suburban to urban areas (n=3), and from urban to other urban areas (n=3), but never from urban to suburban or ex-urban areas. This gives preliminary support to the hypothesis that crows produced in suburban or ex-urban areas contribute to the growth of crow populations in urban areas.