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A plumage patch signalling nest occupancy is shaped by social environment
Hannah R Reeb1,2, Matthew B Toomey1,3, Charles R Brown1
1Department of Biological Science, The University of Tulsa, Tulsa, OK, USA.
Biology Letters
|January 21, 2026
Summary
Cliff swallows use white forehead patches to signal nest occupancy, deterring intruders in crowded colonies. This plumage trait evolves with social environment and sex differences in nesting behavior.
Area of Science:
- Animal Behavior
- Evolutionary Biology
- Ornithology
Background:
- Cliff swallows (Petrochelidon pyrrhonota) are colonial birds nesting in groups.
- Adult cliff swallows possess a white forehead patch, hypothesized to signal nest occupancy.
- This signal may mitigate costly nest intrusions in dense colonies.
Purpose of the Study:
- To test the hypothesis that the cliff swallow's white forehead patch signals nest occupancy.
- To investigate how patch size and brightness relate to sex, colony size, and collection date.
- To experimentally assess the functional role of the forehead patch in intraspecific interactions.
Main Methods:
- Measured forehead patch area and brightness in 241 preserved specimens spanning 43 years.
- Analyzed variation in patch characteristics relative to sex, colony size, and date.
- Experimentally darkened forehead patches of nesting birds and observed interactions with other cliff swallows.
Main Results:
- Females exhibited larger forehead patches than males.
- Forehead patch area increased with colony size.
- Nests with darkened patches attracted more visits from transient cliff swallows compared to control nests.
Conclusions:
- The white forehead patch functions as a signal of nest occupancy in cliff swallows.
- Patch evolution is influenced by social environment (colony size, time) and intersexual differences in behavior.
- Plumage signals play a role in mediating social interactions and reproductive success in colonial birds.
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