Signal honesty in Tree Swallows

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Caraline Gammons (CLAS ’20, Biology) May 2019

For one of the Lab’s weekly meetings, I had the opportunity to lead a discussion on a paper that is relevant to the work I will be conducting this summer and to the lab as a whole.  Taff et al. (2019) focused on a project conducted in Ithaca, New York, that found that achromatic plumage brightness predicts stress resilience and social interactions in tree swallows, Tachycineta bicolor. Their project successfully correlated plumage brightness in songbirds with behavior and fitness, findings that could potentially apply to projects on similar songbirds—specifically Carolina, Black-capped, and hybrid chickadees—underway in the Curry lab. 

Like chickadees, tree swallows also have achromatic patches of feathers whose brightness can be measured by the UV light they absorb and reflect. While these UV signals are unseen to the human eye, they can be detected by other birds, which is especially important for social species. When a signal is reflective of the fitness of an individual, it is called signal honesty (Taff et al. 2019). One measure of fitness this study focused on response to stress. Taff  et al. hypothesized that plumage brightness is an honest signal in female tree swallows, predicting that brighter plumaged birds would have more resilient responses to stress, and also experience higher levels of sociality. 

Taff et al. used a unique method of inducing stress in the wild test subjects without causing permanent damage: rather than clipping the bird’s wings, they instead bound groups of feathers together with zip ties, causing the same stressful effect without permanently hindering the bird’s flight efficiency and maneuverability. Birds either had 0, 1, or 2 groups of feathers bound. To standardize the stress handling measurements, Taff et al. took baseline glucocorticoid measurements upon capture of the birds, and finally after 13 days of the feather binding. The researchers measured plumage brightness using an Ocean Optics USB 2000 spectrometer (the same brand I will be using to measure achromatic plumage brightness in chickadees) on the center of the white breast patch and the center of the blue/green iridescent back patch. 

After the initial capture, the scientists measured social interactions using a radio frequency identification (RFID) system on nest boxes in which the captured females were breeding. Each RFID, located at each active nest, would record the time any bird with a passive integrated transponder (PIT) tag visited there. This RFID and PIT tag system is the same one used in the Curry Lab. Tree Swallows are very social species (Taff et al. 2019); unlike chickadees, the swallows don’t mind nesting extremely close to other members of their social group. This allowed for quick and easy nest-box monitoring and a lot of social interaction data to analyze. 

Taff et al. quantified stress resilience by hatching success (yes or no), corticosterone (baseline and stress response) and adult mass. To measure whether plumage brightness was an honest signal of an individual’s stress resilience, they fit a set of binomial generalized linear models with coloration (breast or back), treatment, and a treatment by coloration interaction as potential predictors. They then used the gathered RFID data to ask whether coloration predicted sociality, measured by patterns of nest visitation and trips to other boxes. Using the Akaike information criterion (AICc) with hatching success as a binomial response variable, there was strong support for models that included either a breast brightness by treatment interaction (model weight = 0.49) or breast bright- ness and treatment as main effects with no interaction (model weight = 0.40). No other model in the set received substantial support.

The significant results of these analyses indicate that female tree swallows with brighter white breasts mounted a stronger physiological response to handling stress, engaged in more extensive social interactions at nest-boxes, and were more resilient to the experimentally imposed flight costs. These results were consistent with the hypothesis of Taff et al. that plumage brightness is a reliable signal of the individual bird’s sociality and ability to cope with stress. 

These findings are important to our work in the lab as they suggest stress resilience, social activity, and plumage signals together are an integrated phenotype in tree swallows. Social activity, plumage brightness, and other behavioral and success indicators are being studied in chickadees in the Curry Lab, and this study can serve as inspiration to find correlations in each of these separate measurements. Additionally, this study indicates the importance of the maintenance of signal honesty in a species of social songbirds. This indicates to me that the achromatic patches on chickadees are likely to be honest signals of some aspect of the bird’s fitness or sociality. However, this study did not include mate choice or extra-pair copulations in their study, as using the RFID alone to determine social interactions could not distinguish the types of social interactions occurring at each nest box. Our work in the Curry lab also includes parental genetic analysis, opening up the potential for deciphering whether signals such as plumage brightness affect mate choice and extra-pair copulations.

Literature Cited 

Taff, C. C., C. Zimmer, and M. N. Vitousek. 2019. Achromatic plumage brightness predicts stress resilience and social interactions in Tree Swallows (Tachycineta bicolor). Behavioral Ecology. https://doi.org/10.1093/beheco/arz010