Latitudinal variation in clutch size–lay date regressions in Tachycineta swallows: effects of food supply or demography?

Abstract

In a study of almost 16 000 nest records from seven swallow species across the entire Western Hemisphere, clutch sizes decline with relative laying date in each population, but the slope of this decline grows steeper with increasing distance from the equator. Late-laying birds at all latitudes lay clutches of similar sizes, suggesting that latitudinal differences may be driven primarily by earlier-laying birds. Focused comparisons of site-years in North America with qualitatively different food availability indicate that food supply significantly affects mean clutch size but not the clutch size-lay date regression. Other studies on the seasonality of swallow food also indicate that steeper clutch size-lay date declines in the North are not caused by steeper earlier food peaks there. The distribution of lay dates grows increasingly right-skewed with increasing latitude. This variation in lay-date distributions could be due to the predominance of higher quality, early-laying (and large-clutched) individuals among populations at higher latitudes, resulting from latitudinal variation in mortality rates and the intensity of sexual selection. Our results underscore the importance of studying clutch size and lay date in tandem and suggest new research into the causes of their joint geographic variation.

Differences in size between first and replacement clutches match the seasonal decline in single clutches in Tree Swallows Tachycineta bicolor

“…This is consistent with the results of a study across the entire Tachycineta genus that demonstrated that the slope of the decline in clutch size with laying date varies across the Western Hemisphere, being less steeply negative in populations breeding closer to the equator (Winkler et al . ). A similar pattern was also found in the House Wren Troglodytes aedon , another widely distributed, single‐brooded passerine (Young ).…”

“…It is also similar between first‐year and older females (Winkler & Allen ), and is not affected by variation in local food abundance (Winkler et al . ). Such consistency provides further support for the possibility that the rate of seasonal decline in clutch size may be facilitated by the seasonal dynamics of photoperiod.…”

“…These swallows vary in when they lay and how much they invest in the rearing of offspring (Ardia ,b, Winkler et al . ) but these adjustments in effort are made against the backdrop of a remarkably regular seasonal decline in clutch size.…”

Determinants of making success in female Tree Swallows (Tachycineta Bicolor)

“…The T-implanted females skipped egg laying for 2-4 days after initiating a clutch, while the ATD+F-implanted female continued to lay eggs; nevertheless all four females were included in subsequent analyses because the effects of silastic implants on T and aromatase activity can be detected within a day of being implanted (see Smith et al, 1977;Balthazart et al, 1990). I tested whether clutch size and average egg mass differed among females in each treatment using separate analysis of covariance (ANCOVA) for each variable that included clutch initiation date as a covariate to control for the seasonal decline in clutch size that occurs in tree swallows (Winkler et al, 2014). I used a z-transformation to center and scale clutch initiation date so that the effect of treatment on clutch size or average egg mass could be interpreted in the presence of a treatment x initiation date interaction (Schielzeth, 2010).…”

“…I also tested whether clutch size and average egg mass differed by the plumage brightness treatment of the female. I included year as a categorical variable and standardized clutch initiation date as a covariate to control for the seasonal decline in clutch size that occurs in tree swallows (Winkler et al, 2014). For these analyses, I used the R package lmerTest (Kuznetsova et al 2016) to calculate P values for fixed factors and covariates.…”