Biparental care presents an interesting case of cooperation and conflict between unrelated individuals. Several models have been proposed to explain how parents should respond to changes in each other’s parental care to maximize their own fitness, predicting no change, partial compensation, or matching effort as a response. Here, we present an experiment in tree swallows (Tachycineta bicolor) in which we increased the offspring provisioning of females by presenting them, but not their mates, with additional nestling begging calls using automated playbacks. We performed this experiment in two populations differing in future breeding opportunities. We found that in response to a temporary increase in female parental effort, males in the northern population (with lower future breeding opportunities and thus higher brood value) matched the increased effort, whereas males in the southern population did not. We also found that increases in parental care during playbacks were driven by the females (i.e., females initiated the increased effort and their mates followed them) in the northern population but not the southern population. These results support the idea that with incomplete information about the brood value and need, cues or signals from the partner might become important in coordinating parental care.
Study site and species
In 2014, we studied tree swallows at two field sites where they breed in artificial nest boxes: Queen’s University Biological Station, Ontario, Canada (N 44° 34 ′2″, W 76° 19′ 26″, 121 m elevation) and Davidson College, Davidson, North Carolina, USA (N 34° 31′ 32″, W 80° 52′ 40″, 240 m elevation). These two sites differ in the length of the breeding season‚ with longer seasons in North Carolina. For example, in 2014, the time between the laying date of the first and the last clutch in the season was 60 days in North Carolina, whereas it was only 43 days in Ontario. Annual survival rates are also higher in North Carolina. Tree swallows have high breeding site fidelity, and so return to the breeding population is often used as a proxy of annual survival (Winkler et al. 2004). In our North Carolina study site, return rates are around 50% (51% for females and 49% for males in 2015) which is higher than in the Ontario study site (average 22% between 1978 and 2012, range 13–43%). Return rates were calculated as the proportion of adults banded that were recaptured in later years. Note that return rates are the product of true survival, site fidelity, site propensity, and detection probability. Because of this, return rates are expected to be lower than survival probability and can be considered as a minimum estimate of true survival (Sandercock 2003). Similar findings have been reported in other studies comparing southern and northern populations of tree swallows (Ardia 2005), although we note that some other northern populations have higher survival rates than our Ontario population (Clark et al. 2018; Cox et al. 2018).
We captured birds using traps at their nest box or placing our hands over the nest entrance. We caught females on day 10 of the incubation period and males on day 2 or 3 post-hatching. We recorded body measurements (tarsus, wing chord, weight, skull size) and marked birds with a numbered metal leg band (US Fish and Wildlife Service or Canadian Wildlife Service) and a unique passive integrated transponder (PIT) tag that was integrated into a colored (red for females, blue for males) plastic leg band (EM4102 tags from IB Technology, UK). More details on the field methods can be found in Akçay et al. (2016).