The impact of urbanization on daily feeding activity of songbirds: a test of foraging theory

Background/Question/Methods

Foraging theory explains the survival of small birds during cold winters as a balancing act between the risks of predation and starvation, which leads to tradeoffs that impact daily patterns of foraging activity. The less predictable food sources become, the more birds must forage earlier in the day to hedge their bets against the possibility of not being able to feed later. The greater the danger of being depredated, the more time birds spend in hiding or being vigilant, leading to low levels of foraging most of the time and a spike of activity at the end of the day. Urban areas have more predictable food resources and lower predation rates than rural areas, providing a previously unexploited opportunity to test predictions of foraging theory. We marked 500 house sparrows (Passer domesticus) and house finches (Carpodacus mexicanus) with radio frequency identification (RFID) tags at urban and rural sites in east-central Illinois from April through November of 2011. At each site, we installed bird feeders equipped with RFID readers to monitor the date and time of visits from all birds marked with RFID tags. Data were collected from the readers from November 2011 to February 2012.

Results/Conclusions

Foraging behavior differed dramatically between urban and rural sites, even after correcting for potentially confounding patterns caused by dominance hierarchies. Feeding activity steadily increased throughout the day in rural sites and peaked in the afternoon, whereas in urban areas, foraging occurred at high levels throughout the day. Theory predicts that rural sites should have peaks of feeding activity in the morning and afternoon due to unreliable food sources and high predation pressure, respectively. Perhaps the presence of study feeders at rural sites was sufficient to mask the effects of less reliable food sources elsewhere across the landscape or food resources in general may not have been as rare and unpredictable as expected. The peak of activity in the afternoon supports the prediction that predation in rural areas alters daily patterns of foraging. In urban sites, the high rates of feeding throughout the day conform to the prediction that predictable food resources and low levels of predation should reduce peaks of foraging activity early and late in the day. Ultimately, the differences in foraging activity between urban and rural areas highlight the fact that urbanization affects the fundamental forces influencing animal behavior.