Site-Specific Impacts of Urbanisation on Annual Survival of a Forest Bird

Habitat changes associated with urbanization have major and complex effects on wildlife. In birds, urban populations often have lower reproductive success but are able to maintain similar or higher densities than non-urban populations. One explanation proposed for this paradox is that higher survival of birds in cities may compensate for lower reproduction. We use a 9-year dataset and Cormack-Jolly-Seber models to compare annual variation in apparent survival probabilities of adult great tits (Parus major) at two forests and two urban sites located in Hungary. Our analyses tested the effects of sex, age, year, population density on apparent survival, after correcting for the probability of detection. Apparent survival of great tits varied between 0.122 and 0.736, with study site and year having the greatest influence. Unexpectedly, urbanization did not have a consistent effect: the sites with the lowest and highest estimates of survival were both urban habitats. Survival probabilities at the two forest sites were similar to each other but were ~0.15 lower than survival in the best urban site and ~0.1. higher than survival in the worst urban site. Survival probabilities exhibited marked inter-annual variation in all sites, although temporal patterns were not consistent among sites suggesting the variation was not driven by inter-annual variation in regional scale factors. Survival probabilities decreased with bird age at both urban sites in most years, but such patterns were not detected at forest sites. Our results demonstrate that the impacts of aging on avian survival rates can diverge between urban and forest habitats, and that the demographic factors regulating urban populations can vary between locations. Age-specific variation should be taken into account in urban ecology and further exploration of the factors driving the heterogeneity will help inform conservation of biodiversity along rural-urban gradients.