Understanding avian responses to forest boundaries: a case study with chickadee winter flocks

Avian species diversity and territory location are often associated with sharp forest edges, or boundaries. However, our understanding of behaviours underlying avian distribution near forest boundaries, especially species with large home ranges, remains poor. In a two-year study, we measured chickadee flock movements in forests at 0–300 m from boundaries in an agricultural landscape near Edmonton, Alberta, Canada. Even though flocks foraged in all forested parts of the study area, only six of the 53 flocks studied foraged further from forest boundaries than expected if they had used their home range irrespectively of the distance to boundaries. We found no evidence that the association of chickadee flocks to forest boundaries resulted from vegetation changes near boundaries, or to changes in their foraging sites and foraging success near boundaries. However, chickadee flocks moved parallel to boundaries (B75 m away) twice as frequently as expected from random movement, thus suggesting that forest boundaries act as movement conduits. Even when birds do not apparently seek special features near forest boundaries, they may be closely associated with them, simply because their passive movements are constrained by habitat barriers defined by boundaries.