The costs of multiple patch use by birds

Birds living in fragmented habitat may occupy territories comprising more than 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 feeding 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 expenditure (DEEmax). However, a penalty of time lost in crossing gaps of about one hour was incurred. For more gap-crossing (due to larger brood size and/or a greater proportion of gap-crossing trips), distances that could be crossed decreased rapidly to about 50-100 m and time lost increased to more than six hours. Crossing gaps at maximum range speed, rather than at the slower minimum power speed, reduced flight times by 42% and slightly reduced overall daily energy expenditure because the higher flight costs per minute were more than off-set by the shorter flight times. Smaller body mass (17 g versus 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 expenditure associated with gap-crossing will affect many species which exploit patchy resources, especially when the spacing of the patches increases, for example due to habitat loss and modification. In landscapes where semi-natural habitat is highly fragmented and most surviving patches are small (e.g., many farming landscapes) the costs of multiple patch use may represent another mechanism by which habitat fragmentation reduces the reproductive potential of the inhabitants of habitat patches which are of acceptable or even good quality, but are small.