Genetic evidence for the effect of a postglacial population expansion on the phylogeography of a North American songbird

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Abstract
During the last 30 years, there have been marked declines in the populations of many British songbirds breeding on farmland, while two of their main predators, sparrowhawk (Accipiter nisus) and magpie (Pica pica), have spread back into areas from which they had disappeared. The causes of the songbird declines remain unclear but given the coincidence in timing, it might appear that increased predation could be responsible. Although many studies have failed to find links between changes in the populations of breeding songbirds and mortality from avian predators, previous work has, with few exceptions, involved only short–term studies on small spatial scales. Here we use large–scale, long–term data from a national bird census scheme to examine whether magpies and sparrowhawks could have depressed the rates of year–to–year population change in 23 songbird species. Our results indicate that magpies and sparrowhawks are unlikely to have caused the songbird declines because patterns of year–to–year population change did not differ between sites with and without these predators.