Effects of structural and functional habitat gaps on breeding woodland birds: working harder for less

The effects of habitat gaps on breeding success and parental daily energy expenditure (DEE) were investigated in great tits (Parus major) and blue tits (Cyanistes caeruleus) in urban parkland (Cardiff, UK) compared with birds in deciduous woodland (eastern England, UK). Tree canopy height, the percentage of gap in the canopy and the percentage of oak (in the wood only) within a 30 m radius of nest boxes were obtained from airborne remote-sensed data. Breeding success was monitored and parental DEE (great tits: both habitats; blue tits: park only) was measured using doubly labelled water in birds feeding young. In the park, mean (± SD) tree height (7.5 ± 4.7 m) was less than in the wood (10.6 ± 4.5 m), but the incidence of gaps (32.7 ± 22.6%) was greater (9.2 ± 14.7%). Great tits and blue tits both reared fewer young in the park and chick body mass was also reduced in park-reared great tits. Park great tits had a higher DEE (86.3 ± 12.3 kJ day-1) than those in the wood (78.0 ± 11.7 kJ day-1) and, because of smaller brood sizes, worked about 64% harder for each chick reared. Tits in the park with more than about 35% gap around their boxes had higher DEEs than the average for the habitat. In the wood, great tits with less oak around their boxes worked harder than average. Thus structural gaps, and functional gaps generated by variation in the quality of foraging habitat, increased the costs of rearing young

Bird species distributions across woodland canopy structure gradients

The tree canopy characteristics of two broadleaved woods in southern England were quantified in terms of two independent measures of structure, canopy height (calculated using heights ≥ 1 m) and percentage canopy cover (derived using heights 0.90, p < 0.001), there was a positive correlation across bird species between the mean values of canopy height and canopy cover associated with the mapped locations of each species. We suggest that canopy height acts as an effective surrogate of woodland structure and can be applied as a predictor of bird community composition and distribution, at least in lowland British conditions. Species associated with young growth had narrower niche breadths, as measured by differences in canopy height and canopy cover between the two woods, than did species associated with taller canopies. Remote sensing of canopy height potentially offers a simple, effective way of assessing habitat availability for many species, at both woodland and landscape scales. This may be especially relevant for species dependent on highly transient vegetation structures associated with the early pre-canopy closure stages of forest growth