Summary
Sexually selected traits can signal an individual's ability to contribute offspring care. Differential allocation theory posits that when these same traits also influence brood value, it may be adaptive for partners to adjust care in response to their mates' traits. Evaluating the strength and direction of parental quality signalling and differential allocation is thus essential to understand selection on ornamental traits. We examined relationships between plumage colour and parental care in tree swallows, Tachycineta bicolor, a mutually ornamented species in which plumage colour is related to male and female reproductive performance. Using a model of avian vision to quantify parental colour trait variation, we found that male and female tree swallows that were paired to partners with greener, more saturated plumage colour fed offspring at higher rates. Among tree swallow pairs where both partners were in their second year of breeding or older, individuals with greener, more saturated plumage colour also fed their offspring at higher rates. We show that offspring of males that provisioned more often tended to achieve greater body mass independent of the colour traits of their parents. Our results suggest a role for partner parental care in selection on female ornamentation in this species.
Methodology
We studied box-nesting tree swallows at the Queen's University Biological Station (QUBS) in Ontario, Canada (44°34′N, 76°19′W). We determined the date of the first egg to hatch (hatch date) for broods laid in May 2013, and quantified both maternal and paternal provisioning rates at 31 nests where both parents were still present >3 days posthatch. Three days posthatch, we observed nests for 1 h between 0730 and 1300 hours Eastern Daylight Time and counted the number of times each parent entered the nestbox as a measure of provisioning rate (McCarty, 2002, Ouyang et al., in press). During observations, we differentiated males and females using plumage coloration at 14 nests where the female was in her first year of breeding. Females in their first year of breeding have predominantly brown dorsal plumage and are easily distinguished from their mates (Hussell, 1983; see Supplementary Fig. S1). We classified any females with ≤90% blue-green upperparts as ‘brown’ females (note that our classification of ‘brown’ includes both ‘brown’ and ‘intermediate’ females in Hussell's (1983) terminology). Females with >90% blue-green upperparts were classified as ‘blue-green’. At the remaining 17 nests where the female had uniformly blue-green plumage, parents were differentiated using either leg bands from previous years (N = 4 nests), or acrylic paint that was applied during the incubation period (N = 13 nests). Paint was applied passively by mounting a small sponge with a dab of paint on the nestbox entrance to mark the incubating bird (males do not incubate; Ardia, Cooper, & Dhondt, 2006).
At 26 nests, we captured males and females the following day (4 days posthatch) to measure body mass (±0.1 g) and skull length (cranium–bill tip, ±0.1 mm), and we removed five to seven rump feathers for colour measurements. At five remaining nests, we captured parents 13 days posthatch instead, because these nests were not part of a separate hormone manipulation experiment (Ouyang et al., 2015). Note that the hormone treatment was applied after the observations of parental care for this study. Birds at the five nests captured 13 days posthatch did not differ significantly from those captured on day 4 in parental visit rate (Gaussian linear regression models: all F1,29 < 0.81, P > 0.37), body condition (see below; all F1,29 < 1.45, P > 0.23) or plumage colour metrics (see below; all F1,29 < 1.60, P > 0.21). Thus, in all following analyses, we present results without controlling for the timing of adult capture. At 25 nests, we measured the body mass of each nestling (±0.01 g) immediately after capturing both parents 4 days posthatch. At the six remaining nests (five controls in a separate experiment plus one that was missed on day 4 due to error), we weighed nestlings 6 days posthatch. We calculated average nestling mass for each brood, and, as a measure of offspring quality comparable across nests, we took the residual from a linear model of average nestling mass controlling for age when weighed (estimate ± SE = 1.52 ± 0.34, N = 31; F1,29 = 20.2, P = 0.0001). Results of analyses involving nestling mass were qualitatively the same when using only the 25 nests measured 4 days posthatch.