Imagine you are a female coastal plain swamp sparrow sitting on eggs. You get hungry and decide to go forage. You slip off the nest, fly low to the ground, and then utter a loud series of call notes. What? Why advertise your presence? Isn't that a dead giveaway to predators as to where your nest is?
Listen to a coastal plain swamp sparrow nest departure call:
Not only female coastal plain swamp sparrows but 16 other species in North America and Europe exhibit the same behavior. Several reasons have been proposed for this odd behavior:
All the birds that call when leaving the nest are birds that live in grasslands, marshes, or other open habitats. Typically, the birds live in small territories packed close together. The most likely reason appears to be that the females are trying to avoid harassment by males that are very aggressive in protecting their territories.
This article summarizes the information in this publication:
McDonald, M. V. and Greenberg, Russell S. 1991. Nest departure calls in female songbirds. The Condor, 93: 365-373.
Females of at least 15 species of North American passerines give distinctive nest departure calls. These calls consist of a series of notes uttered while flying away from the nest. less often, the call is given while flying to the nest. It is heard during nest-building, incubation, brooding, and nestling care. The behavior is found in emberizid finches and icterines in North America, and possibly muscicapids (Sylviinae) in Europe. It occurs almost exclusively in birds nesting in marshes or grasslands. Birds giving this call could incur a great cost if predators locate nests using these calls. Proposed benefits of calling include reduction of harassment by males, discouragement of settlement by females, promotion of anti-predatory vigilance by mates, advertisement of receptivity for copulation, and distraction of potential predators away from the nest. The reduction of harassment and increased vigilance hypotheses have the strongest support. The behavior is more common in species in which visibility of birds close to the ground is poor and in which the intra-sexual competition among males is intense which thus increases the risk that an already mated female will be mistaken for an intruding male.
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