Behavioural responses to changing landscapes: flock structure and anti-predator strategies of tits wintering in fragmented forests

We analysed the effects of forest fragmentation on the flock structure of i nsectivorous forest passerines (Parus, Aegithalos, Certhia, Regulus, etc.), and on the anti-predator behaviour and energy management of blue tits in t hese flocks. We surveyed flocks in Central Spain during two winters. Flocks in fragments comprised fewer individuals and species than flocks in unfrag mented forests. The most abundant species in forest flocks (blue tit, Parus caeruleus, and firecrest, Regulus ignicapillus) were also the most abundan t in fragments, while the rarest species in the area never occurred in smal l woodlots. We investigated how fragmentation and related changes in flock structure affect anti-predator behaviour of blue tits, a widely distributed species in the area. In fragments but not in forests, blue tits increased scanning rates with decreasing flock size. Vigilance was relaxed when great tits, Parus major, were abundant as flock mates, suggesting that the absen ce of this dominant species in fragments could intensify anti-predator beha viour of blue tits. Blue tits enhanced anti-predator behaviour in the secon d winter parallel to an increase in the abundance of raptors. This behaviou ral change was stronger in fragments, where blue tits foraged deeper in the canopy and increased scanning and hopping rates. Under increased predation risk, birds are expected to reduce body mass to improve predator avoidance . On average, blue tits weighed similar in fragments and forests the second winter. However, they accumulated fat along the day in-fragments only, and adjusted body mass to body size more closely in that habitat type. This su ggests that blue tits perceived fragments as unpredictable habitats where f attening would help avoid starvation, but also as dangerous sites where ove rweight would further increase the risk of predation. In summary, our resul ts. support that fragmentation affects individual behaviour of blue tits, a nd show the potential of behavioural approaches to unravel how different sp ecies face the advancing fragmentation of their habitats.