Temperatures in Excess of Critical Thresholds Threaten Nestling Growth and Survival in A Rapidly-Warming Arid Savanna: A Study of Common Fiscals

Abstract

Frequency, duration, and intensity of hot-weather events are all predicted to increase with climate warming. Despite this, mechanisms by which temperature increases affect individual fitness and drive population-level changes are poorly understood. We investigated the link between daily maximum air temperature (tmax) and breeding success of Kalahari common fiscals (Lanius collaris) in terms of the daily effect on nestling body-mass gain, and the cumulative effect on size and age of fledglings. High tmax reduced mass gain of younger, but not older nestlings and average nestling-period tmax did not affect fledgling size. Instead, the frequency with which tmax exceeded critical thresholds (tcrits) significantly reduced fledging body mass (tcrit = 33°C) and tarsus length (tcrit = 37°C), as well as delaying fledging (tcrit = 35°C). Nest failure risk was 4.2% per day therefore delays reduced fledging probability. Smaller size at fledging often correlates with reduced lifetime fitness and might also underlie documented adult body-size reductions in desert birds in relation to climate warming. Temperature thresholds above which organisms incur fitness costs are probably common, as physiological responses to temperature are non-linear. Understanding the shape of the relationship between temperature and fitness has implications for our ability to predict species’ responses to climate change.

Avian thermoregulation in the heat: scaling of heat tolerance and evaporative cooling capacity in three southern African arid-zone passerines

“…For tropical species at lower air temperatures and higher humidity the thermal endpoints may be significantly lower (Weathers, 1997). Studies of wild birds in arid habitats have identified important threshold T a values in the 30-40°C range for variables related to body condition (du Plessis et al, 2012) and provisioning rates during breeding (Cunningham et al, 2013), suggesting that in many cases, detailed models of specific determinants of survival and/or reproduction will be necessary to predict the effects of climate change. However, catastrophic mortality events during extreme heat waves in the arid zones of Australia and elsewhere (reviewed by McKechnie et al, 2012;McKechnie and Wolf, 2010), combined with predicted increases in the frequency and intensity of heat waves (IPCC, 2011), underscore the relevance of models of avian survival over time scales of hours during acute heat exposure, and the need for comparative data on the upper limits of avian heat tolerance.…”

Associations between changing climate and body condition over decades in two southern hemisphere passerine birds

“…They comprise the extreme tails of the temperature distribution at the study site, representing the most extreme conditions experienced by these species. Although various studies have identified air temperatures ≥35°C as thresholds for temperaturerelated mass loss, these are for arid and semi-arid species inhabiting hot environments [10][11][12][13]. Species in cooler, mesic climates may have inflections in evaporative water loss at lower air temperatures, in the region of 30°C [40].…”

“…The need to dissipate heat can lead to trade-offs with foraging, resulting in reduced foraging efficiency or effort, leading to reductions in body condition [10,11]. In addition, nonpanting forms of heat dissipation (e.g.…”

“…In birds, studies of the effects of non-lethal high temperatures have generally focused on air temperatures in the 30-40°C range [8]. Exposure to these temperatures can cause thermal stress and drive reductions in body mass [10][11][12][13]. When exposed to air temperatures above about 30°C, birds actively dissipate heat by evaporative cooling through panting which increases water loss and the potential for mass loss via dehydration [10,14,30].…”

Long-term variability in vegetation productivity in relation to rainfall, herbivory and fire in Tswalu Kalahari Reserve

“…Several studies have reported on how historical warming may already be contributing to the reduced fitness of certain bird species (e.g. Cunningham et al 2013;Du Plessis et al 2012). However, the impact of historical and projected climate change on vegetation change in the southern Kalahari has received less attention, probably because of the difficulty of separating the influence of multiple interacting drivers of change that operate at different temporal and spatial scales.…”