Avian responses to predator communities in fragmented, urbanizing landscapes
Laura J. Kearns, PhD
Advisor: Amanda Rodewald
Behavioral responses to predators during the breeding season can critically affect the nest success of songbirds. However, the ability of birds to modify behavior based upon perceived and actual predation risk at multiple spatial scales (e.g. local (within-site), site, and landscape) and in novel (e.g., urban) environments remains poorly understood. In this dissertation, I explored how information about predation risk influenced the nest-site selection and nestling provisioning behavior of two species of songbirds – northern cardinals (Cardinalis cardinalis) and Acadian flycatchers (Empidonax virescens), which are two relatively common forest songbirds of eastern North America with contrasting responses to urbanization. I studied the use of information regarding predation risk and behavioral responses of birds during the 2006-2010 breeding seasons at riparian forest sites within the urbanizing landscapes of central Ohio. Specifically, I investigated the following questions: 1) how do cardinals and flycatchers choose nest locations based on information about local-scale nest predator activity patterns, 2) do cardinals and flycatchers incorporate private (i.e. detectable information only known to the individual) and public (i.e. detectable information known to all individuals) information about predation risk in nest-site selection, and 3) are provisioning rates to nestlings adjusted relative to public information about site-level predation risk?
To assess the relationship between selection of nest locations and information regarding local-scale nest predator activity, I used cardinal and flycatcher nest location data collected from sites during the 2008-2010 breeding seasons. I created utilization distributions from mapped predator locations at each site and overlaid them with nest locations to determine the corresponding probability of predator activity. Initially, I predicted that nest survival would be negatively related to the level of predator activity in an area. Consequently, birds would avoid locating nests in areas of high nest predator use, particularly if the animal was a dominant nest predator. My results suggest that both species avoided nesting in areas of high nest predator activity, which was consistent with the finding that nest survival declined as predator activity increased at local spatial scales. These findings provide evidence that both cardinals and flycatchers use public information of nest predator activity at local spatial scales.
Because birds are not limited to public information about predators, I also studied the use of private information reflecting prior experience of individuals with predators (i.e., fate of previous nest attempts). I predicted that birds would use private information regarding previous nest fates in addition to public information about site level predation risk. From 2006-2010, I evaluated the changes in nest-site characteristics between successive nest attempts within each breeding season for both species. Cardinals appeared to use both public and private information when selecting nest sites, and seemed to rely heavily on actual predation risk, but the flycatcher used neither. The contrasting responses of the two species suggest differences in behavioral plasticity that may be related to their sensitivity to urban areas.
To evaluate how songbirds used site-level public information about predation risk in caring for young, I observed how cardinals provisioned nestlings at nests during the breeding seasons of 2008-2010. By using video cameras to document parental feeding rates, predator surveys to estimate perceived risk, and nest survival rates to calculate actual risk, I found no evidence that cardinals used information about either type of risk to make decisions about provisioning. Furthermore, provisioning rates were not related to nest fate. If provisioning rates do not influence predation, then cardinals may not need to adjust the frequency with which they feed young in response to predation risk.
In conclusion, the ways that birds used information about predation risk varied with species, type of behavior, and the scale of information. Cardinals incorporated local scale information about predator activity, previous nest fate, and at times, actual predation risk at the site scale, to modify nest-site selection. They demonstrated sensitivity to information at multiple scales and an apparent ability to adjust nesting behaviors in ways that may allow them to thrive in urban areas. On the other hand, flycatchers used only local-scale predator activity information in selecting nest-sites, were less responsive to site-scale information, but likely recognized and responded to predator information or other cues of habitat quality at the landscape level when making breeding decisions. Both songbird species exhibited more cautious breeding behaviors when faced with certain types of predation risk, but seemed sensitive to the scale of predator information in choosing to do so. Thus, differences in use of information about predation risk may reflect constraints on the relative behavioral flexibility of cardinals and flycatchers. Not only does this study reveal ways in which behavioral plasticity can vary between songbirds with different affinities for urbanizing landscapes, but also illuminates the importance of studying various scales and types of information in evaluating songbird responses to predators.