Are birds feeling the heat in a warming Arctic?

Blog written by Ryan O’Connor. Read the full paper here. All photos by François Vézina

It is common knowledge that the Earth’s climate is rapidly changing. However, what is likely less known is that the Arctic region is warming at a rate faster than the global average. Unfortunately, information pertaining to how Arctic animals can physiologically tolerate increasing temperatures is largely limited. More distressing is the fact that Arctic animals have evolved physiological mechanisms to withstand extremely cold environments at the potential cost of higher sensitivity to moderate heat. Consequently, data on how Arctic animals can cope with increasing temperatures is desperately needed for scientists to reliably predict how species will be directly impacted by a rapidly warming Arctic.

We studied the heat tolerance and evaporative cooling capacity of snow buntings (Plectrophenax nivalis), a circumpolar migrant, Arctic songbird. This study is part of the ArcticSCOPE research project, which seeks to understand the impacts of warming on Arctic biodiversity and species range limits. Buntings are known cold specialists and regularly experience sub-zero temperatures, except for during a small window when breeding. Therefore, buntings are a prime model species to investigate how a known cold specialist can tolerate increasing temperatures.

We exposed snow buntings to increasing air temperatures and recorded four common physiological traits: 1) body temperature, 2) resting metabolic rate, 3) rate of evaporative water loss and, 4) evaporative cooling efficiency. Evaporative cooling efficiency is an index of how efficient an individual is at dissipating body heat and is expressed as a ratio between evaporative heat loss and metabolic heat production (i.e., EHL/MHP). The higher the ratio, the more efficient an individual is at dissipating body heat and any value above 1 signifies that a bird is shedding more heat through evaporation than they are producing metabolically. We also looked at snow bunting’s evaporative scope, which compares the maximum rate of evaporative water loss to baseline rates measured at non-stressful air temperatures. In general, the higher a species evaporative scope the more heat tolerant they are. Finally, we determined the onset of heat stress in buntings by analyzing the data for significant inflection points in each response variable. These inflection points represent the air temperature where a physiological trait changes abruptly due to increasing heat.

There were several key findings to highlight from our results. Firstly, snow buntings began to exhibit heat stress, as indicated by their inflection point in evaporative water loss, at an air temperature of 34.6°C. This inflection point is consistently lower than those reported in songbirds from hotter climates (Figure 1). Secondly, snow buntings tolerated a comparatively low maximum air temperature, which stemmed from their inability to substantially increase evaporative water loss (i.e., a low evaporative scope; Figure 1) and low evaporative cooling efficiencies (Figure 2). For example, we found that only 2 birds were able to evaporatively dissipate an amount of heat greater than what they were producing metabolically (i.e., EHL/MHP above 1). To put it another way, heat stressed snow buntings are not good at shedding body heat.

It is necessary to mention that free-living birds will actually experience heat stress at temperatures lower than those found under laboratory conditions because active birds have higher metabolic rates and thus produce more heat. Additionally, we have to consider the influence of solar radiation and wind, in conjunction with air temperature, when determining the total heat load experienced by a bird in the wild. For example, preliminary data collected by the ArcticSCOPE team on snow bunting breeding grounds indicates that they can experience environmental temperatures up to 30°C. Therefore, with Arctic temperatures rapidly increasing, we argue that highly active, breeding buntings will experience more periods of heat stress and will have to reduce their activity to limit their heat production, potentially interfering with other essential activities.

Figure 1. Maximum air temperatures tolerated in relation to (a) evaporative water loss inflection and (b) evaporate scope among 24 arid-zone songbirds and snow buntings.
Figure 2. Evaporative cooling efficiency of snow buntings. The higher the values the more efficient the bird is at evaporative cooling.
Dashed line indicates when heat lost = heat produced. Below the line, buntings are producing more heat than losing through evaporation

So where do we go from here? Well, as our data show, buntings experience heat stress at temperatures lower than those seen in other songbird species and, once heat stressed, are very bad at dissipating heat. Additionally, we know that the environmental temperatures that buntings experience in the wild can reach 30 °C, once we have accounted for solar radiation. In other words, wild buntings can experience environmental temperatures approaching those that elicit heat stress when at rest. The next step is to apply our findings to free-living snow buntings in the Arctic and determine whether their inability to efficiently lose heat interferes with their breeding success. For example, a string of recent studies conducted on wild birds inhabiting hot, arid climates have shown that under increasing air temperatures adults feeding their young have to reduce feeding trips because of the need to cool off, but at the expense of their nestling’s development. If breeding populations experience multiple seasons of nestlings leaving their nest with poor body condition, this has the potential to reduce a species fitness over the long term. Over the next few years, the ArcticSCOPE team will study whether buntings, along with other Arctic birds, such as thick-billed murres (Uria lomvia), experience similar thermal constraints when breeding in the Arctic. Elucidating the relationships among thermoregulation and breeding behavior will be fundamental for scientists to determine the impacts of climate change on Arctic biodiversity.

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