Great Britain is a notoriously damp isle and spring 2024 was super wet, with relentlessly overcast skies. So how much of a toll does humidity take on the ability of animals to keep warm? ‘Many people have the experience that humid air often feels colder than dry air’, says Cody Porter, from Iowa State University, USA. But it can also be difficult to keep warm in dry air, when your body loses water by evaporation – increasing heat loss – which could force animals to turn up their metabolism to maintain body temperatures. Puzzled by the possibility that small songbirds might have to work harder to keep warm when the air is damp, Cody Porter and Kevin Cortes (Iowa State University, USA), Eric Riddell (University of North Carolina at Chapel Hill, USA) and Ofir Levy (Tel Aviv University, Israel) decided to find out how air humidity affects the ability of tree swallows (Tachycineta bicolor) to maintain their body temperature.
In a colossal series of experiments, the team measured the carbon dioxide production of tree swallows (which rises as birds increase their metabolic rates and use more energy) overnight in the lab as they rested at various temperatures (15, 19, 23, 30 and 32°C) in dry (0% humidity) and damp (60% humidity) air. When the team compared how much energy the birds used to maintain their body temperature, the humidity of the air did not seem to affect the birds particularly, although Porter says, ‘The minor effects we did find in the lab tend to support the “humid, cold air is colder” scenario’. But when the team calculated how much extra energy the birds would expend in the wild when it is damp – based on 9 years of local weather conditions – they realised that on damp days, the swallows use 8% more energy than when it's dry.
So it seems that tree swallows have to work harder to keep warm in cold, humid conditions, and Porter is concerned that the higher likelihood of sudden cold snaps (caused by climate change) at the end of spring will make survival more difficult for small songbirds raising chicks, simultaneously depriving them of food – insects are harder to find on cold days – just when they need the fuel for extra warmth.
In addition, the team compared their lab measurements with measurements taken in the wild and they were surprised by how different the birds’ body temperatures and metabolic rates were. They warn that scientists need to be wary of measurements made in the lab, because they may not reflect what is genuinely happening when animals are out in their natural habitat.
