The costs of multiple patch use by birds

Birds living in fragmented habitat may occupy territories comprising more t han one patch. This paper uses a theoretical model to investigate the costs (in terms of time and energy) of crossing gaps between patches for birds f eeding young in the nest, using the great tit (Parus major) as an example. When the proportion of foraging trips involving gap-crossing was small (25% ), gaps of about 300-550 m (depending on body mass and flight speed) could be crossed without exceeding likely maximum sustainable daily energy expend iture (DEEmax). However, a penalty of time lost in crossing gaps of about o ne hour was incurred. For more gap-crossing (due to larger brood size and/o r a greater proportion of gap-crossing trips), distances that could be cros sed decreased rapidly to about 50-100 m and time lost increased to more tha n six hours. Crossing gaps at maximum range speed, rather than at the slowe r minimum power speed, reduced flight times by 42% and slightly reduced ove rall daily energy expenditure because the higher flight costs per minute we re more than off-set by the shorter flight times. Smaller body mass (17 g v ersus 19 g) was advantageous for gap-crossing, the distances which could be crossed without exceeding DEEmax being almost doubled for the smaller mass . The influence of changes in wing morphology, fat load and prey load size on the energetics of gap-crossing were also considered. Although the model was constructed for a woodland bird, problems of time and energy expenditur e associated with gap-crossing will affect many species which exploit patch y resources, especially when the spacing of the patches increases, for exam ple due to habitat loss and modification. In landscapes where semi-natural habitat is highly fragmented and most surviving patches are small (e.g., ma ny farming landscapes) the costs of multiple patch use may represent anothe r mechanism by which habitat fragmentation reduces the reproductive potenti al of the inhabitants of habitat patches which are of acceptable or even go od quality, but are small.