Abstract
The frequency of extreme weather events, including heat waves, is increasing with climate change. The thermoregulatory demands resulting from hotter weather can have catastrophic impacts on animals, leading to mass mortalities. Although less dramatic, animals also experience physiological costs below, but approaching, critical temperature thresholds. These costs may be particularly constraining during reproduction, when parents must balance thermoregulation against breeding activities. Such challenges should be acute among seabirds, which often nest in locations exposed to high solar radiation and predation risk. The globally endangered bank cormorant Phalacrocorax neglectus breeds in southern Africa in the winter, giving little scope for poleward or phenological shifts in the face of increasing temperatures. Physiological studies of endangered species sensitive to human disturbance, like the bank cormorant, are challenging, because individuals cannot be captured for experimental research. Using a novel, non-invasive, videographic approach, we investigated the thermoregulatory responses of this seabird across a range of environmental temperatures at three nesting colonies. The time birds spent gular fluttering, a behaviour enhancing evaporative heat loss, increased with temperature. Crouching or standing birds spent considerably less time gular fluttering than birds sitting on nests (ca 30% less at 22°C), showing that postural adjustments mediate exposure to heat stress and enhance water conservation. Crouching or standing, however, increases the vulnerability of eggs and chicks to suboptimal temperatures and/or expose nest contents to predation, suggesting that parents may trade-off thermoregulatory demands against offspring survival. We modelled thermoregulatory responses under future climate scenarios and found that nest-bound bank cormorants will gular flutter almost continuously for several hours a day by 2100. The associated increase in water loss may lead to dehydration, forcing birds to prioritize survival over breeding, a trade-off that would ultimately deteriorate the conservation status of this species.
Highlights
The effects of climate change on animals have been a focus of research for over four decades (Parmesan, 2006; Urban, 2015)
We examined the thermoregulatory responses to high temperatures in breeding bank cormorants Phalacrocorax neglectus
To predict the effect of climate change on gular fluttering behaviour in breeding bank cormorants, we focused on sitting birds, because they should be more sensitive to heat than crouching or standing birds
Summary
The effects of climate change on animals have been a focus of research for over four decades (Parmesan, 2006; Urban, 2015). Hot weather events have been occurring with increasing frequency across the species’ breeding range in coastal southern Africa (Kruger and Sekele, 2013) and bank cormorants have been observed to abandon nests during heat waves (Sherley et al, 2012), suggesting that rising temperatures may be exacerbating their population decline. By filming birds from a distance over long periods of time, we were able to investigate non-invasively patterns of gular fluttering in wild bank cormorants, a behaviour that enhances heat dissipation through evaporative heat loss but may lead to dehydration when sustained We examined how this behaviour relates to parental behaviour during nest attendance in relation to changes in temperature, wind speed and humidity. This approach allows us to predict how patterns of behaviour will change under various climate change scenarios and to explore the potential impacts of rising temperatures for southern African seabirds and for seabirds in general
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