Are gardens an ecological trap for birds?

American kestrel

Birds living close to human activity get stressed out (photo credit: Boise State University)

Gardens are often thought of as havens for many bird species and many gardeners don’t just feed the birds but put up nest boxes to attract birds into their garden to breed. However an interesting study just released by Boise State University means it may not all be a bed of roses for birds in our gardens.

The study looked at American kestrels who are thought to be ‘human tolerant’ and are thought to benefit from human environments. Short grass makes seeing and catching food easier and the various sign posts and street furniture alongside roads makes ideal spotting platforms for them to hunt from. But rather than being a benefit to the birds the researchers say it could be detrimental to the birds – and possibly other urban birds – over the long-term as these apparent benefits are in fact just an ecological trap.

An ecological trap occurs when the attractiveness of poor-quality habitat is disproportionate to the value of the habitat for reproduction. In the case of the kestrels the research claim that the easier food availability attracts birds to urban areas but the extra stress that involves reduces their reproductive rates and long term survival.

The researchers, graduate student Erin Strasser, now with the Rocky Mountain Bird Observatory, and Julie Heath, a professor in the Boise State Department of Biological Sciences and Raptor Research Center, looked at kestrels living along Idahoe Interstate 84 and also in rural and urban areas including nest boxes in residents back gardens.

They took blood samples form the girds to monitor stress hormone levels and also monitored the levels of nest abandonment. They were helped by a history of monitoring the birds in the area since 1987 by Boise State University and the U.S. Geological Survey.

The results showed that female kestrels living close to human activity had high levels of corticosterone – the stress hormone of the birds. High levels can lead to behavioural and physiological changes. For instance it can raise the awareness levels of females but reduces their breeding activities. The males though had normal levels of the hormone. [pullquote]Birds evolved in an environment that was not dominated by humans.  In recent history, human roads and structures have left few areas untouched. We’re just starting to understand the real consequences[/pullquote]

The researchers think that females are more susceptible to high levels of stress hormone because they spend more time at the nest. With ambient human noise levels at high levels this makes it more difficult for the females to assess danger levels. This higher level of stress led to changes in parenting success with the birds giving poorer quality care to chicks and more likely to abandon a nest.  Kestrals living close to noisy roads or areas of high levels of disturbance were 10 times more likely to abandon a nest than birds nesting in quieter regions.

 “We hypothesized that this was a mechanism for how humans are impacting wildlife,” Heath said. “To birds, areas with  human activity may be perceived as a high-risk environment. Birds evolved in an environment that was not dominated by humans.  In recent history, human roads and structures have left few areas untouched. We’re just starting to understand the real consequences.”

 While the study concentrated on kestrels it does raise the question of whether the higher stress levels is species specific or if it affects all our urban bird species. As agricultural fields become barren for birds and they retreat to ‘sanctuaries’ of gardens for food will this impact on their breeding success as ambient noise levels in most gardens tend to be higher than in rural areas.

There has already been studies to show that birds change their song when living close to traffic or in an urban environment. Will we need to accept that birds that make their homes in our gardens will not just have different songs but also different reproductive rates than their rural cousins.


External sites:

Journal of Applied Ecology: Reproductive failure of a human-tolerant species, the American kestrel, is associated with stress and human disturbance.

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