Neighbor visibility is thought to influence territory size and shape, and visual obstructions may allow for greater nesting density within a breeding colony. We studied the effects of neighbor visibility and neighbor proximity on settlement patterns in a colony of Barn Swallows (Hirundo rustica) using artificial nest cups placed in parallel rows on the underside of a pier. Nest cups were placed 61 cm apart so that the distance between adjacent cups matched the distance between adjacent rows. Rows were separated by the vertical panels to which the nest cups were attached. To a swallow perched on a cup, cups in the same row were visible but cups in adjacent rows were visually obstructed. We compared the observed nest-settlement patterns with expected distributions generated both by calculating cup availability and by simulating random settlement events. Barn Swallows avoided settling 61 cm from an active nest only if that neighbor was visible; visually obstructed neighbors at the same distance were not avoided. Nest cups where the nearest neighbor was both visually obstructed and >61 cm away were occupied significantly more often than expected by chance. Barn Swallows thus consider both the proximity and visibility of established pairs when choosing nest sites within a colony.
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