Age-related microhabitat segregation in willow tit Parus montanus winter flocks

It is expected that through flexibility in behaviour, flock living birds re spond to the asymmetries in resource access derived from dominance relation ships. We analysed the microhabitat use of willow tits in winter flocks and assessed possible factors which shape habitat segregation between adults a nd juveniles in different temperature regimes. When foraging in mild condit ions (ambient temperature > 0 degreesC), flocks split up into subgroups wit h adults foraging in inner parts of trees more often than juveniles. Howeve r, no differences were recorded in the vertical position occupied in trees. In harsh conditions (< - 4<degrees>C), flocks re-united and juveniles furt her moved to outer parts of trees, increasing horizontal segregation betwee n age classes. In mild conditions, vigilance behaviour was not related to t he position of birds in trees, but in harsh conditions, scanning frequency was higher in outer parts of trees only for adults. In mild weather, juveni le position in trees was associated with body size and mass. The foraging m icrohabitat segregation detected in harsh conditions fits the age-related h oarding distribution previously described in the same population. This supp orts the hypothesis that hoarded food is important in determining future fo raging habitat use. Adult preference and intraspecific competition for safe r or richer inner parts of trees as foraging sites during harsh conditions seems to determine the habitat segregation between adults and juveniles. Fu rthermore, we suggest that in mild weather, when foraging in the absence of adults, juveniles balance the costs of using a potentially dangerous micro habitat with the benefits of building energetically cheap and large food re serves through hoarding. The expected patterns of microhabitat segregation may differ in parids, depending on whether predation risk or other factors such as food availability are the main factors controlling habitat quality.