Lee Newman, of Gisborne, asks :-

How do small birds, such as fantails and wrens, keep warm and survive in below zero temperatures

Yolanda van Heezik, a zoologist at the University of Otago, responded.

When keeping warm in freezing temperatures, most of us pull on a down jacket, and huddle together for warmth. Birds do this and more. Keeping warm is challenging for birds, because they have a higher metabolic rate and body temperature than we do, about 40°C compared to 37°C.

Small birds also cool down fast since they have a proportionately larger surface area to lose heat from, but a smaller volume to generate warmth. Some animals use a layer of fat to insulate themselves, but flying birds are limited in the amount of fat they can carry, if they still want to be light enough to fly.

Feathers are amazingly good at keeping the cold out, and when oiled, keep water out as well. Water is a good conductor of heat, so it is essential to stay dry. When it gets really cold birds puff their feathers to trap as much warm air as possible - a little puffer jacket. Those scaly little legs and feet lose hardly any heat to the environment: scales minimise heat loss, and inside the legs there is a counter-current system of blood vessels, where the veins and arteries lie close to each other: as warm blood leaves the body it heats up the cold blood returning to the body. There are mainly bones and sinews in the legs, so not much tissue to be damaged by the cold.

Behaviours that combat heat loss include crouching to cover their legs with their “puffer jackets”, or standing on one leg, tucking the other one up into the warmth. Birds’ faces also get cold, so they tuck their bills into their feathers for protection and breathe air warmed by their bodies.

Huddling reduces the birds’ surface-area-to-volume ratio, since it turns many small birds into a single big group, and larger objects have lower surface-area-to-volume ratios than smaller objects. The birds on the ends of the huddle lose more weight overnight than those in the middle, so they jostle for position, trying not to be the cold one on the end. Birds also find cosy places to roost, protected from wind and warmed by the sun. The final resort is shivering: this is very effective at generating heat, but uses up a lot of energy, so it’s a short-term solution.