The influence of visual obstructions on the vigilance and escape behaviour of house sparrows, Passer domesticus

The scanning and flight behaviour of birds that forage in flocks may be influenced by several variables, including the size of the flock and the presence of visual obstructions. Visual obstructions can conceal both potential predators and flock mates from a foraging bird, and individuals may increase their scanning rate accordingly, although concealing flock mates may result in more variable scanning rates as they come in and out of vision. We examined these ideas experimentally by observing house sparrows foraging at a feeder with and without visual obstructions. Birds foraging in the presence of visual obstructions had generally higher and more variable scanning rates. When the birds were approached by a human observer, they took flight earlier in larger flocks, although their reaction was generally delayed when there were obstructions. These data indicate that visual obstructions increase the probability of predation because individuals are less likely to detect a predator and/or the alarm flight of other individuals.