Nest sanitation does not elicit egg ejection in a brown-headed cowbird host

Area of Science:

• Behavioral Ecology
• Ornithology
• Evolutionary Biology

Background:

• Most passerine birds engage in nest sanitation, removing foreign objects.
• Nest sanitation is hypothesized as a precursor to egg ejection in hosts of avian brood parasites.
• Few North American hosts, like the brown-headed cowbird (Molothrus ater), exhibit egg ejection behavior.

Purpose of the Study:

• To investigate if nest sanitation behavior in red-winged blackbirds (Agelaius phoeniceus) elicits egg ejection.
• To test the hypothesis that mechanical similarity between debris removal and egg ejection triggers the latter.

Main Methods:

• Artificial cowbird eggs were introduced into red-winged blackbird nests alongside debris (flagging tape or pine cone bract scale).
• Nest sanitation responses and egg ejection rates were monitored.
• Control nests received only an artificial cowbird egg.

Main Results:

• All red-winged blackbirds removed the added debris within 24 hours.
• However, all individuals accepted the artificial cowbird eggs.
• The rate of egg ejection did not differ significantly between experimental and control nests.

Conclusions:

• Nest sanitation and egg ejection, while mechanically similar, involve different cognitive processes.
• The act of nest cleaning does not elicit egg ejection behavior in red-winged blackbirds.
• This suggests nest sanitation is not a direct pre-adaptation for cowbird egg ejection in this species.