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Male tree swallow (Tachycineta bicolor) sitting on a birdhouse. Photo credit: Mykola Swarnyk [CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0/)]

Parenting is not for the faint hearted. Dedicated tree swallow mums and dads commit themselves to 16 h of exertion each day, snapping up insects for their ravenous offspring. And they maintain this herculean effort for up to 22 days until their young are ready to leave the nest. It seems there are no limits to the lengths these parents will go. But Simon Tapper and Gary Burness from Trent University, Canada, with Joseph Nocera from the University of New Brunswick, Canada, weren't so sure. Tapper explains that the amount of energy a warm-blooded animal can invest in rearing its young – which is the most energetically demanding time of any animal's life – seems to be restricted by the amount of heat it can dispose of. However, most of the evidence supporting the idea is based on the experiences of new mouse mothers. Also, it wasn't clear how much of an effect lifestyle might have on an animal's ability to remain cool. Thinking about parent tree swallows, Tapper and colleagues wondered whether the industrious mothers might be able to labour even harder while raising their young if they were able to lose more heat through a bald patch in their feathers.

Staking out nesting boxes in two nearby nature reserves, Tapper waited until clutches of tree swallow eggs hatched before briefly capturing the new mothers and gently snipping off the feathers covering the patch of warm skin that they use to incubate their eggs. He also inserted a minute tag into the necks of both parents to record when they returned to feed their demanding brood, in addition to keeping track of the weather conditions and monitoring the hatchlings as they grew.

Initially, it looked as though the trimmed mothers weren't working any harder than females with an intact plumage; both visited their offspring roughly every 5.2 min. It was only when the team factored in the effect of the weather that the trimmed mums seemed to have an advantage. They were able to push themselves 25% harder than the untrimmed females on hot days, making an average of 43 more foraging trips per day to feed their young, although the trimmed mothers struggled to keep up with the untrimmed mothers’ feeding rate on chillier days. In addition, the trimmed mums were able to make ∼3 more visits per hour to their nests than the untrimmed females when feeding large broods of 7 nestlings. Most impressively, the offspring of the trimmed mums piled on the grams faster than the hatchlings of untrimmed mothers, weighing 1.82 g more when they departed, and they had better chances of survival after leaving the nest.

It seems that the exertions of tree swallow mothers are restricted by the amount of heat that they can lose from their bodies, explaining why mothers that had had feathers removed were able to forage more on hot days than those with a full set of feathers that could overheat. However, the team suspects that the boosted growth of the trimmed mothers’ chicks could have more to do with the extra warmth available through their mother's bald patch. They are also concerned that rising temperatures caused by climate change could place chicks at greater risk, as their frenetic parents may be unable to meet their relentless demands as a consequence of heat exhaustion, making it harder for the chicks to grow swiftly to join their parents on the wing.