Local Settlement in Woodland Birds in Fragmented Habitat: Effects of Natal Territory Location and Timing of Fledging

Factors such as early fledging and natal territory location have been shown to influence dispersal and settlement success of woodland birds. Early fledging allows for earlier dispersal, increasing the chances of an individual locating good quality habitat. However, for birds in fragmented woodland, the advantages of early dispersal may be modified by natal territory location in relation to the availability of suitable habitat in the surrounding landscape. Connecting habitat corridors may promote dispersal, and connectivity in landscapes is usually considered as positive, but this may not always be the case. In landscapes where habitat is highly fragmented, corridors may promote departure, but leaves dispersers with little chance of success. An individual-based Spatially Explicit Population Model was used to investigate the effects of timing of fledging, natal territory location and proximity to potential dispersal corridors on local settlement rates. Modelling was based on both hypothetical and real woodlands and used marsh tit as an example woodland bird. For each modelled scenario, the number of young that settled was recorded for each territory, the overall results being expressed as the mean percentage settlement rate per territory. Territory location and edge effects were both found to influence dispersal and settlement rate. Fledging early clearly demonstrated a general advantage for local settlement success. However, territory location, in relation to the likelihood of dispersing out of the wood was found to interact with fledging order. Fledglings from internal territories had an advantage over those from edge territories and local settlement rate could be reduced by a location favourable to emigration. In general, the effect of exits was to reduce the settlement rate of early fledging young while that of later young tended to increase. In highly fragmented woodland, fledging late from a well-connected edge territory would appear to be the worst case scenario.