Disentangling climatic and nest predator impact on reproductive output reveals adverse high‐temperature effects regardless of helper number in an arid‐region cooperative bird

Abstract

Climate exerts a major influence on reproductive processes, and an understanding of the mechanisms involved and which factors might mitigate adverse weather is fundamental under the ongoing climate change. Here, we study how weather and nest predation influence reproductive output in a social species, and examine whether larger group sizes can mitigate the adverse effects of these factors. We used a 7‐year nest predator‐exclusion experiment on an arid‐region cooperatively breeding bird, the sociable weaver. We found that dry and, especially, hot weather were major drivers of nestling mortality through their influence on nest predation. However, when we experimentally excluded nest predators, these conditions were still strongly associated with nestling mortality. Group size was unimportant against nest predation and, although positively associated with reproductive success, it did not mitigate the effects of adverse weather. Hence, cooperative breeding might have a limited capacity to mitigate extreme weather effects.

Egg components and offspring survival vary with group size and laying order in a cooperative breeder

“…We monitored the breeding activity of 16 sociable weaver colonies during 8 breeding seasons (from 2010/2011 to 2017/2018) to obtain data on egg mass, egg laying order, and fledging success (see online Appendix A1 for details on data collection; D’Amelio et al 2021 ; Fortuna et al 2021 ). We obtained a sample of 779 eggs (in 326 nests from 14 colonies) with known mass and laying order, and for which breeding female’s identity, tarsus size, and group size were identified (see below).…”

“…This represented 7.5% of the clutches laid by this population in the two breeding seasons ( N = 784 clutches) and is considerably lower than the estimated annual brood failure only due to predation (ca. 22.5%; see D’Amelio et al 2021 ), to which sociable weavers usually respond by laying one or several replacement clutches (Covas et al 2008 ; Fortuna et al 2021 ). Sociable weavers can re-lay over ten times in the same season, and we therefore consider that egg removal had similar, or smaller, effects in their reproduction to egg loss under natural predation conditions (Covas et al 2008 ; Fortuna et al 2021 ).…”

“…In cooperative breeders, females experience variable social conditions due to variation in the number of ‘helpers’ that assist with offspring care. Helpers provide food and other types of care to the offspring (e.g., protection from predators), often resulting in a positive correlation between number of helpers and offspring success and/or parental survival (Brouwer et al 2005 ; Downing et al 2020 , 2021 ; D’Amelio et al 2021 ), although sometimes varying in strength and direction depending on environmental conditions (Rubenstein 2011 ; Capilla-Lasheras et al 2021a ; Groenewoud and Clutton‐Brock 2021 ). Mothers could benefit from helpers’ presence by adopting one of two opposite prenatal reproductive strategies: (1) load-lightening, whereby females save energy by investing less in eggs when breeding with helpers, who compensate for this by providing food to the offspring, ultimately benefiting mothers’ survival and/or future reproduction (Russell et al 2007 ; Taborsky et al 2007 ), or (2) increased pre-birth investment when breeding with helpers, often called a differential allocation strategy (Russell and Lummaa 2009 ; Dixit et al 2017 ), thereby increasing current offspring survival by investing more energy in reproduction when breeding under favorable conditions (i.e., life-history theory; Stearns 1992 ; Cunningham and Russell 2000 ; Sheldon 2000 ; Russell and Lummaa 2009 ; Savage et al 2015 ; Valencia et al 2017 ; Capilla-Lasheras et al 2021b ).…”

“…This reduction may be a consequence of nutrient depletion in female reserves and/or a strategy to allocate less resources to offspring that are less likely to survive (Slagsvold et al 1984 ; Williams et al 1993 ; Crean and Marshall 2009 ; Vedder et al 2017 ). Since helpers may generally increase offspring survival (although such effect may be modulated by climatic conditions; Rubenstein 2011 ; Downing et al 2020 ; Capilla-Lasheras et al 2021a , b ; D’Amelio et al 2021 ; Groenewoud and Clutton‐Brock 2021 ), the adaptive value of laying later-laid eggs richer in hormones or poorer in nutrients could be modulated by helpers’ presence (Fig. 1 ), and it is therefore important to study how laying order and helpers’ presence interact to shape egg composition and offspring survival.…”

Avian Behavioral and Physiological Responses to Challenging Thermal Environments and Extreme Weather Events

“…Given that birds face the dual threats of dehydration and hyperthermia daily during this period, elucidating the limits of their capacity to cope with the increased resource (food, water, and time) demands associated with breeding is critical for understanding variation in reproductive success under current and future climates . There is a growing body of literature on the impacts of high temperatures and drought on nest success of aridzone birds at the scale of nesting attempts and seasons (e.g., Catry et al, 2011Catry et al, , 2015Salaberria et al, 2014;Cruz-McDonnell and Wolf, 2016;Bourne et al, 2020a,b;D'Amelio et al, 2021), but fewer studies investigate these impacts on population-level breeding outputs over longer timescales in relation to ongoing climate change trends (however, see e.g., Frederiksen et al, 2013;Hatch, 2013;Amélineau et al, 2019;Bourne et al, 2020a;Ridley et al, 2021).…”