Background/Question/Methods Habitat fragmentation and increased urbanization are threats to biodiversity and animal populations around the world. Accompanying these general threats, bird species populations have been declining dramatically over the past few decades, and any effects of urbanization on avian annual survival will have important implications for their future population trends. The influence of urbanization is of particular concern for non-migrant songbird species that undergo the largest declines in survival in the winter. Predation risk and food scarcity are strong determinants of adult survival in the winter as non-migrant or resident species encounter harsh conditions. These risks can be mitigated by forming mixed-species flocks that aid in both predator evasion and food detection as species are able to understand and respond to heterospecific behavior. However, we are lacking an understanding of how land-use changes along an urban-to-rural gradient influence mixed-species flock structureand diversity. Here, we conduct a study on the structure, diversity, and dissemination of information on predation risk of mixed-species flocks along an urbanization gradient in the greater Milwaukee, WI area. Understanding how the dissemination of social information in mixed-species flocks changes across this urban-to-rural gradient will inform conservation efforts that can increase over-winter survival for resident bird species. Results/Conclusions The structure of social networks for winter bird mixed-species flocks changed along an urbanization gradient involving 12 sites across rural, suburban, and urban environments. Black-capped Chickadees (Poecile atricapillus) were a central species in rural environments and lost their central position in urban landscapes. However, the Black-capped Chickadee closeness centrality was highest in urban and suburban environments. This suggests that when chickadees were present in urban and suburban sites other species were close by, even if Black-capped Chickadees had less of an influence over the flock. Additionally, there was some evidence that Dark-eyed Juncos (Junco hyemalis) assumed a more central role within flocks in more urban environments. Shifting responses of bird mixed-species flock structure and diversity to land-use changes such as urbanization reinforce the need to understand how information is passed through flocks and the implications for fitness. Moreover, understanding how land-use changes impact flocking dynamics for winter resident songbirds will aid in their conservation and mitigate widespread population declines across North America.
