Patch use and departure rules by gull-billed tern Gelochelidon nilotica

Abstract

Abstract The gull-billed tern Gelochelidon nilotica feeds on the fiddler crab Leptuca uruguayensis, which settles forming patches in south-eastern temperate mudflats of Argentina. Through field observations, we evaluated whether gull-billed terns used patches following the marginal value theorem (MVT). Gull-billed tern residence time in a patch was not related to crab density or travel times. The number of captures was also unrelated to crab density. Most of the times (44.6%) terns captured only one crab from each patch, and 35% of the times they left without a capture. However, crab density was lower when terns left the patches than when they arrived. This suggests that following several capture attempts by terns, crabs hide, producing a temporary decrease in their availability, forcing tern departure from the patches, which are no longer profitable. Thus, when prey availability is affected more by predator activity than by consumption, the MVT may not necessarily apply.