Supplementary tables and analyses.Data used in the analyses. from Strategic adjustment of parental care in tree swallows: life-history trade-offs and the role of glucocorticoids

Life-history theory predicts that optimal strategies of parental investment will depend on ecological and social factors, such as current brood value and offspring need. Parental care strategies are also likely to be mediated in part by the hypothalamic–pituitary–adrenal axis and glucocorticoid hormones. Here, we present an experiment in tree swallows (<i>Tachycineta bicolor</i>), a biparental songbird with wide geographical distribution, asking whether parental care is strategically adjusted in response to signals of offspring need and brood value and if so, whether glucocorticoids are involved in these adjustments. Using an automated playback system, we carried out playbacks of nestling begging calls specifically to females in two populations differing in their brood value: a northern population in Ontario, Canada (relatively higher brood value) and a southern population in North Carolina, USA (relatively lower brood value). We quantified female offspring provisioning rates before and during playbacks and plasma corticosterone levels (cort) once during late incubation and once immediately after playbacks. Females in both populations increased feeding rates temporarily during the first 2 h of playback but the increase was not sustained for the entire duration of playback (6 h). Cort levels from samples at the end of the playback did not differ between control females and females that received playbacks. However, females that had higher increases in cort between the incubation and nestling period had greater fledging success. These results suggest that females are able to strategically respond to offspring need, although the role of glucocorticoids in this strategic adjustment remains unclear

Proportion of variance explained (R²) and its 95% confidence interval generated by bootstrapping, statistical significance (p-values), and the sample size (N) of the relationship between 1h-samples and the total daily visit rate based on the time of onset of the 1h-sample for female and male tree swallows.

<p>Proportion of variance explained (R²) and its 95% confidence interval generated by bootstrapping, statistical significance (p-values), and the sample size (N) of the relationship between 1h-samples and the total daily visit rate based on the time of onset of the 1h-sample for female and male tree swallows.</p

Visit rate (the number of feeding visits/h) of female and male tree swallows inferred from 1h-behavioral observations (y-axis) and RFID readings (x-axis).

<p>Open circles denote influential data points that have disproportionate effect on the relationship as measured by the ‘influence.measures’ function in R. Note that the statistical analyses provided in the main text were carried out including these data points, and therefore provide a conservative estimate of these relationships.</p

Optimal durations of observation periods for female and male tree swallows.

<p>The solid lines show the best fit curve to the data (a three parameter Michaelis-Menten model) for the relation between R² and observation period duration (15 minutes—4 hours). The dashed lines show three alternative model fits (Gompertz, Asymptotic regression and General Additive Model). Red and blue dots indicate the optimal sampling effort for females and males respectively, that maximizes R² and minimizes the duration of observation (indicated by the dashed arrows).</p

Summary of published results testing different sampling regimes.

<p>Summary of published results testing different sampling regimes.</p

The cumulative number of parental visits in tree swallow nests in (A) Canada and (B) North-Carolina.

<p>In both (A) and (B), each panel corresponds to one nest (the nest identifier is printed above each panel), with the blue line representing the male and the red line the female parent.</p