Patterns of Fluctuating Selection on Morphological and Reproductive Traits in Female Tree Swallow (Tachycineta bicolor)

Abstract

Temporally replicated studies are essential to describe and understand selection in natural populations. Selection patterns can differ among life stages representing different fitness components. Despite the increasing number of long-term studies, yearly estimates of fluctuation in strength and direction are mostly available from studies conducted on a limited number of years. Based on a population of Tree swallows (Tachycineta bicolor) monitored over 10,200 km 2 in Southern Québec, Canada, since 2004, we investigated how patterns of selection may change across breeding stages by dividing the overall selection at the nesting stage (number of fledglings produced) into hatchling (number of hatchlings produced) and fledgling (number of hatchlings having successfully fledged) selection stages. We assessed fluctuation in selection gradients on two morphological (body mass and wing length) and two reproductive (laying date and clutch size) traits in females. We found significant positive selection gradients for body mass and clutch size and negative selection gradients for laying date, though the latter only during the fledgling selection stage. We also found that selection gradients on reproductive traits significantly fluctuated in direction and/or strength among years but only during the hatchling breeding stage. Our results thus emphasize the need to consider how selection events may be fluctuating in time and among breeding stages and the importance of these patterns for the maintenance of phenotypic variation in wild populations.

Multidimensional environmental influences on timing of breeding in a tree swallow population facing climate change

“…A previous analysis in our study system showed that selection favoured earlier laying date in this population but that patterns of selection fluctuated in strength and direction through time (Millet et al. ). Also, the time lag observed in the studied area between spring arrival (eBird, http://ebird.org/) and reproduction suggests that further adjustments of laying date are possible.…”

“…While we believe that the pattern described here is likely to be nonadaptive, given that tree swallows breeding later show a reduced fitness in most years (Millet et al. ), further investigations are needed to clearly conclude on the effects of reduced plasticity in lower density habitats (e.g. compare selection gradients between low and high breeder density farms).…”

“…reduced individual plasticity) that we suspect in lower density habitats is worrying for tree swallow populations in the context of concurrent climate change, population decline and reduced fitness for individuals breeding later (Millet et al. ). Multiple environmental drivers of phenotypic changes can act in synergy and accelerate the rate of extinction (Brook et al.…”

Offspring mass variation in tree swallows: A case of bet‐hedging?

“…In our system, environmental conditions vary greatly within and between reproductive seasons and can have a large impact on within-year reproductive success of tree swallows (Lessard et al 2014, Rioux Paquette et al 2014, Bourret et al 2015, Millet et al 2015. Indeed, a post hoc analysis showed that there is no temporal autocorrelation (Appendix S1: Figs.…”

“…For instance, the positive relationship between female mass and reproductive success could suggest that females with greater energy supplies consistently produce more fledglings, as female mass positively affects clutch size and fledging success (see Fig. 2A; Winkler and Allen 1995, Rioux Paquette et al 2014, Millet et al 2015, Pellerin et al 2016. Our repeatability analyses controlled for female mass, but several other factors linked to female identity could influence Notes: CI, confidence interval.…”