The use of GIS in estimating spatial variation in habitat quality:: a case study of lay-date in the Great Tit Parus major

Finding the most biologically meaningful scale at which to describe environmental variation is a persistent problem in ecology. Most studies of forest passerines are conducted at the scale of the habitat or woodland and do not account for environmental variation between individual breeding sites. Here we employ two GIS models, and four spatial scales, to describe environmental variation among 4683 Great Tit Parus major breeding sites occupied over a 32-year period in Wytham Wood, Oxford, UK, and use these data to help explain variation in an environmentally sensitive trait, first egg date. Model 1 used Thiessen polygons to generate individual spatial scales for each breeding pair, while Model 2 used a range of predetermined radial scales around each breeding site. Environmental variables included local altitude and aspect estimated from a Digital Terrain Model, and the number of Oak Quercus robur trees around each nest-site, used here as a surrogate for local food (caterpillar) availability. In a Linear Mixed Model, Model 2 explained the most variation in lay-date at a scale of 25 m from each nestbox. However, the model that returned the lowest Akaike Information Criterion (AIC) contained environmental variables from Model 2, but measured at different fixed spatial scales of between 25 and 75 m. Results from this final model showed that birds breeding in low-altitude, north-facing and Oak-rich areas bred earlier than those in high-altitude, south-facing and Oak-poor areas, at radial scales of 25, 75 and 75 m from each nestbox, respectively. In addition, birds in interior sites bred earlier than those nearer the woodland edge, although this edge effect was only apparent on north-facing slopes. Thus, the current study demonstrates that a range of GIS models can be effectively used to scale and describe environmental variation between individual breeding sites, and that fine-scale topographic variation, food availability and edge proximity can affect the breeding date of Great Tits.