Experimental evidence that hyperthermia limits offspring provisioning in a temperate-breeding bird

Abstract

In many vertebrates, parental care can require long bouts of daily exercise that can span several weeks. Exercise, especially in the heat, raises body temperature, and can lead to hyperthermia. Typical strategies for regulating body temperature during endurance exercise include modifying performance to avoid hyperthermia (anticipatory regulation) and allowing body temperature to rise above normothermic levels for brief periods of time (facultative hyperthermia). Facultative hyperthermia is commonly employed by desert birds to economize on water, but this strategy may also be important for chick-rearing birds to avoid reducing offspring provisioning when thermoregulatory demands are high. In this study, we tested how chick-rearing birds balance their own body temperature against the need to provision dependent offspring. We experimentally increased the heat dissipation capacity of breeding female tree swallows ( Tachycineta bicolor ) by trimming their ventral feathers and remotely monitored provisioning rates, body temperature and the probability of hyperthermia. Birds with an experimentally increased capacity to dissipate heat (i.e. trimmed treatment) maintained higher feeding rates than controls at high ambient temperatures (greater than or equal to 25°C), while maintaining lower body temperatures. However, at the highest temperatures (greater than or equal to 25°C), trimmed individuals became hyperthermic. These results provide evidence that chick-rearing tree swallows use both anticipatory regulation and facultative hyperthermia during endurance performance. With rising global temperatures, individuals may need to increase their frequency of facultative hyperthermia to maintain nestling provisioning, and thereby maximize reproductive success.

Negative effects of agricultural intensification on the food provisioning rate of a declining aerial insectivore

“…Such weather events may drastically alter the availability of insects as a food resource due to their thermally dependent nature (Grüebler et al 2008, Shipley et al 2020, Garrett et al 2021). Furthermore, recent evidence suggests Tree swallows may already be foraging at some upper thermal threshold (Tapper et al 2020). Our results, combined with other pieces of evidence, suggests food provisioning adults may also be foraging at some upper energetic limit inasmuch, as proxies of body condition suggest they are lower where gain rates are lower (Pigeon et al 2013, Stanton et al 2016, 2017).…”

“…It is thus imperative that future work focuses not only on how agricultural practices or global climate change independently impact declining avian species, but also on how these factors may interact with one another. This importance is due to the ability of growing broods or breeding parents to manage extreme weather is potentially lower within more agro-intensive landscapes (Grue et al 1997, Evans et al 2003, Pérez et al 2008, Eng et al 2019, Tapper et al 2020). Finally, our results provide evidence that aerial insectivore declines may be related to agricultural intensification through a trophic pathway.…”

“…This model contained two weather variables. First, the mean maximum temperature, as insects require a threshold temperature before being active (Taylor 1963, Grüebler et al 2008), and the upper limit of foraging swallow behavior is potentially thermally constrained (Tapper et al 2020).…”

Interacting effects of cold snaps, rain, and agriculture on the fledging success of a declining aerial insectivore

“…Finally, a focus here was on the effects of cold snaps, yet another growing concern of climate change are periods of high ambient temperature [i.e., heat waves; (Perkins-Kirkpatrick and Lewis 2020)]. The energetic expenditure of food provisioning Tree Swallows is potentially constrained by an upper thermal limit (Tapper et al 2020). Moreover, prey availability decreased sharply above 30°C in our study system (Figure 3A).…”

Warming in the land of the midnight sun: breeding birds may suffer greater heat stress at high- vs low-Arctic sites

“…Indeed, birds often decrease activity on days with warmer ambient temperatures, which is likely a thermoregulatory response to avoid heat stress [19][20][21][22][23]. Recent studies have also shown that when a bird's capacity to dissipate body heat is increased (e.g., by experimentally removing insulative feathers), provisioning adults can sustain higher levels of activity and invest more in both their current and future reproductive efforts [24][25][26][27]. Thus, reproductive performance can be constrained by a bird's capacity to dissipate body heat produced during essential breeding activities, suggesting that increasing environmental temperatures could significantly impact reproductive investment.…”