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CHICKS ARE COOL ABOUT STAYING WARM
kathryn{at}biologists.com
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Most chicks spend their young lives being waited on by their parents; all they have to do is eat and grow. But precocial chicks are much more independent. Within hours of breaking out of their shells, the youngsters are foraging for themselves. Which is fine in temperate climes, where the chicks stay warm, but for shorebird chicks in the Canadian tundra, the cold conditions place a massive metabolic burden on their tiny bodies. Robert Ricklefs is fascinated by the precise metabolic balance that the birds strike as they stray from the nest's warmth, gathering food to fuel their fast growth rate. He needed to know how newborn chicks cope with the cold, so he began slowly cooling and warming the youngsters in the laboratory while he measured their metabolic rate, and found that the chicks seem to be saving energy by dropping their metabolic rate and letting the environment do the work as they warm up (p. 2883).
Ricklefs and Joe Williams headed north to the Canadian tundra to put the young birds to the metabolic test. But capturing the chicks was tricky. Once the youngsters have hatched, they scatter into the undergrowth, `and become invisible' says Ricklefs. So instead of returning with chicks, the team gathered eggs from six species, incubating them in the lab until they hatched. Having made sure that the chicks were happy foraging in their protected enclosure, the team began measuring a dunlin chick's metabolic rate and body temperature, as the air temperature changed.
At first, the dunlin's metabolic rate rose slowly as the air slowly cooled around it, but even with the extra metabolic effort, the chick's body temperature dropped. After the air temperature had fallen close to freezing, Williams began gently warming the chamber, expecting the chick's metabolic rate to stay high as it's body temperature recovered. But instead, the dunlin's metabolic rate fell while the chamber began warming the chick. And when Williams tested other species' chicks, they all showed the same response. Ricklefs explains that `the hysteresis was completely unexpected'.
But chicks rarely experience a gentle temperature drop in their natural environment; as soon as they leave the nest they are at the mercy of the elements, so the team tested how the chicks faired when the temperature plunged to 5°C. This time the birds' metabolic rate rocketed as the chicks reacted to the large temperature gradient at their warm skins. Ricklefs explains that the rapid metabolic increase must be regulated by peripheral thermosensors, detecting the sudden drop in temperature, rather than a single thermosensor situated in the brain.
Ricklefs' original motivation for following the chick's metabolism was to take their metabolic rate as an index for the youngster's muscular development. But now that he's found that the birds' metabolic rate is dependent on their body temperature, measuring a chick's metabolic rate in the field could give a deceptive impression of their maturity. After all, a cold chick that's recently returned from a foraging trip could have a misleadingly low metabolic rate; especially if it's just been snuggling up to mum.
References
Ricklefs, R. E. and Williams, J. B. (2003).
Metabolic responses of shorebird chicks to cold stress: hysteresis of cooling
and warming phases. J. Exp. Biol.
206,2883
-2893.
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