Urban Birds Not As Successful at Breeding

A new six-year study compared breeding Acadian Flycatchers in urban and rural forests. Flycatcher pairs at rural sites produced an average of two young per season, compared with one in urban areas. Rural birds started nesting earlier in the breeding season and were more likely to try again in the case of nest failure. Urban birds were less likely to return to the same nesting site in subsequent years.

So what causes the lower breeding rate in urban areas? Apparently, it is not a result of predation. While urban areas have more predators, such as cats and raccoons, predation rates were similar for both urban and rural breeding birds.

Instead, it appears that birds find urban sites less attractive. Birds nesting at urban sites were slightly smaller than their rural counterparts. They may be less fit birds that were forced to take lower-quality breeding sites as the stronger birds filled higher-quality sites outside of cities. As a result, they start breeding later and give up more quickly if something goes wrong.

One remaining question is why the urban sites seem less attractive to Acadian Flycatchers. The research team is currently investigating several hypotheses. One is that urban sites have a larger population of invasive plants, which change the character of the forest understory. A second is that urban nests are more likely to fall victim to nest parasites like Brown-headed Cowbirds. Noise pollution, light pollution, and the smaller size of urban forests may also play a role.

As our world becomes increasingly urbanized, wildlife conservation will depend more and more on the availability of useful habitat within cities. Studies like this one help define how to maintain and improve sites to give breeding birds the best chance possible.