Temporal consistency in fine-scale habitat relationships of woodland birds during a period of habitat deterioration

Temporal consistency of models of animal-habitat relationships is important because it is widely assumed that such models have long-term relevance for conservation management. Habitat associations can potentially shift over time due to changes in factors such as habitat quality, predation risk and intra-specific competition. Relationships between vegetation attributes and bird distribution were examined in rotationally harvested woodland in two breeding seasons separated by 16 years. Over this period habitat suitability deteriorated for understory bird species as a result of increased browsing pressure by deer. Generalised additive models were constructed for eight species and three groups of songbirds (all species, migrants, hole-nesters), with choice of predictors and the assumed form of underlying relationships informed by cross-validation deviance. Three species differed strongly between years in their responses to age of vegetation regrowth following harvesting; in two of these cases the differences were probably a consequence of deer browsing altering relationships between age of regrowth and vegetation structure. Canopy height was generally the single best predictor and for six of the 11 response variables, height was the best predictor in both years. Density of shrub layer foliage was frequently the second best predictor in both years. Multi-variable models in most cases selected canopy height as the first variable. Floristic variables (i.e. ones measuring aspects of tree or shrub species composition) featured in 64% of the multi-variable models and 27% of the selected variables were floristic rather than structural. Overall, there was a high level of temporal consistency in the bird-habitat models with most species apparently keying into very similar vegetation structures irrespective of browsing pressure. Canopy height is considered to be a surrogate for a complex of micro-habitat elements associated with resource provision for many birds. However, the understory micro-structures that are important determinants of habitat quality for many forest species are difficult to measure in ecologically meaningful ways. New technologies potentially offer better ways of measuring these habitat attributes and of assessing how they are affected by forest management.

► Deer browsing changed responses of some bird species to age of coppice regrowth.
► Canopy height strongly predicted bird distributions over a 16 year period.
► Understory density was consistently a secondary predictor of bird distributions.
► Canopy height acts as a surrogate indicator of key resources for woodland birds.
► Better techniques are needed for measuring understory micro-structures.