The relative contribution of individual quality and changing climate as drivers of lifetime reproductive success in a short-lived avian species

Abstract

Animal populations are influenced strongly by fluctuations in weather conditions, but long-term fitness costs are rarely explored, especially in short-lived avian species. We evaluated the relative contributions of individual characteristics and environmental conditions to lifetime reproductive success (LRS) of female tree swallows (Tachycineta bicolor) from two populations breeding in contrasting environments and geographies, Saskatchewan and British Columbia, Canada. Female swallows achieved higher LRS by breeding early in the season and producing more fledglings. Other measures of female quality had virtually no influence on LRS. Genetic factors did not predict LRS, as there was no correlation between life-history components for sister pairs nor between mothers and their daughters. Instead, climate variability-indexed by spring pond density (i.e., abundance of wetland basins holding water) during years when females bred-had strong positive effects on female LRS in more arid Saskatchewan but only weak positive effects of moisture conditions were detected in wetter British Columbia. Overall, several life history trait correlates of LRS were similar between populations, but local environmental factors experienced by individuals while breeding produced large differences in LRS. Consequently, variable and extreme environmental conditions associated with changing climate are predicted to influence individual fitness of distinct populations within a species' range.

Figure 1

Lifetime numbers of (a) total eggs laid, (b) fledged nestlings and (c) recruited offspring produced by known-age adult female tree swallows in Saskatchewan (1990–2015; n = 496) and British Columbia (2001–2015; n = 466).

Figure 2

Path diagram summarizing direct and indirect effects of explanatory variables on lifetime reproductive success of known-age adult female tree swallows breeding in (a) Saskatchewan and (b) British Columbia. Line thickness indicates the relative strength of the path. See “Materials and methods” for description of calculation of explanatory variables. Green represents individual characteristics of females, whereas blue represents environmental factors. Age of first breeding for females in Saskatchewan was 1 year old for 48%, 2 years old for 37%, and ≥ 3 years old for 15%, whereas in British Columbia, 90% of females bred at 1 year old and the other 10% bred at ≥ 2 years old.

Figure 3

Percent of known-age female tree swallows in (a) Saskatchewan (1990–2015; n = 496) and (b) British Columbia (2001–2015; n = 466) making 1, 2, or ≥ 3 lifetime total breeding attempts for each age of first breeding (1, 2, or ≥ 3 years old).