Heat tolerance limits of Mediterranean songbirds and their current and future vulnerabilities to temperature extremes

Songbirds are one of the groups most vulnerable to extreme heat events.
Although several recent studies have assessed their physiological
responses to heat, most of them have focused on arid-zone species solely.
We investigated thermoregulatory responses to heat in eight small-sized
songbirds occurring in the Mediterranean Basin, where heatwaves are
becoming more frequent and intense. Specifically, we determined their heat
tolerance limits (HTL) and evaporative cooling efficiency and evaluated
their current and future vulnerabilities to heat in southwestern Iberia, a
Mediterranean climate warming hotspot. To do this, we exposed birds to an
increasing profile of air temperatures (Ta) and measured resting metabolic
rate (RMR), evaporative water loss (EWL), evaporative cooling efficiency
(the ratio between evaporative heat loss and metabolic heat production)
and body temperature (Tb). HTL ranged between 40 and 46°C across species,
and all species showed rapid increases in RMR, EWL and Tb in response to
increasing Ta. However, only the crested lark Galerida cristata achieved
an evaporative cooling efficiency greater than 1. The studied songbirds
currently experience summer Ta maxima that surpass their upper critical
temperatures of their thermoneutral zone and even their HTL. Our estimates
indicated that five of the eight species will experience moderate risk of
lethal dehydration by the end of the century. We argue that the limited
heat tolerance and evaporative cooling efficiency of small-sized
Mediterranean songbirds make them particularly vulnerable to heatwaves,
which will be exacerbated under future climate change scenarios.