Studies of avian escape behavior have mostly focused on nonbreeding birds, however the costs and benefits of escape behavior are predicted to vary depending on whether or not birds are breeding. Passive nest defenders may escape an approaching threat earlier than would otherwise be the case as an adaptation that helps conceal their cryptic eggs. Alternatively, the investment in the clutch may mean that birds delay escape. We tested these predictions on a population of passively defending Little Tern (Sterna albifrons) and Kentish Plover (Charadrius alexandrinus) within the Saemanguem reclamation area, South Korea. Incubating terns exhibited longer flight-initiation distances to approaching pedestrians than did non-incubating conspecifics, suggesting early escapes may function to help conceal the locality of the nest. No such effect was evident for Kentish Plover.

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