The amount of energy that an animal can burn could be limited by the food available in its environment, or how well its body can digest and utilize nutrients. Additionally, because energy metabolism characteristically produces warmth, it has been suggested that some animals have difficulty in offloading excess body heat, which could instead cap their ability to work. High body temperatures can be disastrous, particularly in birds and mammals, in which temperature is normally tightly regulated. This problem is particularly acute when animals are provisioning their growing offspring, when the amount of work a parent can perform has direct consequences for their reproductive success.
In earlier work on rodents in the lab, researchers demonstrated that lowering environmental temperature or shaving body fur – two interventions that increase the ability of the parents to dissipate heat – increases the amount of work that a parent can perform, allowing them to raise larger pups. This supports the concept that, ordinarily, offloading heat is a real constraint on breeding success. In order to perform the first experiment to see whether this principle holds in the wild, Andreas Nord and Jan-Åke Nilsson, both based at Lund University, Sweden, at the time, set their focus on blue tits, a charismatic and agile Eurasian bird species. Working in the southern Swedish countryside during the blue tit nesting season, the duo trimmed the breast feathers of both parents of some nests to reveal their bare skin. This meant that the birds offloaded more heat, which the researchers verified with a thermal camera. They compared the surface body temperature of these birds with that of birds that had not lost any feathers and found it was on average 6.6oC higher (40.4 versus 33.8oC), potentially resulting in 50% greater heat loss from the breast surface. The pair also took internal body temperature measurements and showed that trimmed parents maintained a 0.4oC lower core body temperature (42.7 versus 43.1oC). But what effect would this difference in body temperature have on the birds’ foraging habits?
Nord and Nilsson tagged the birds with uniquely coded transponders and rigged their nest boxes with sensors to record how often they went on feeding excursions. If overheating normally handicaps activity – which the authors predicted – they believed trimmed birds would be capable of making more foraging trips. All of the parents made over 400 trips per day to find food for their chicks yet, surprisingly, the feather-trimmed group made no more trips than the birds with intact plumage.
However, when the duo looked at the body condition of both the parents and their chicks, they saw that the feather-trimmed adults only lost half the weight of untrimmed birds (0.19 versus 0.46 g) during the nesting period. More strikingly, their chicks grew longer wings. Interestingly, in the nests that were tended by females in their first breeding season, the trimmed birds raised chicks that were also over 25% heavier than normal. However, the females that were in their second breeding season reared chicks that were the usual weight, suggesting that experience may partially mitigate the costs of overheating.
Nord and Nilsson saw that when heat dissipation was facilitated, the blue tits became more successful parents, which suggests they normally have difficulty offloading heat. Given that the feather-trimmed blue tits weren't making additional foraging trips, the pair suggest that the trimmed parents can prioritize food quality over quantity. Soberingly, this observation indicates how heat waves – an increasingly frequent occurrence in the current period of climatic instability – may render breeding birds particularly vulnerable.
