How kinglets survive cold winter nights

How does a tiny bird stay warm through a freezing night? The opening gives us several sensible answers. Nuthatches shelter in holes, which helps them hold on to heat. Chickadees let their body temperatures drop, so they need less energy to stay alive. Grosbeaks stay near fatty seeds that provide fuel for shivering. Different birds solve the problem in different ways.

Then we reach the turn: kinglet survival “remains something of a mystery.” Now we know where to focus. We are not getting a general tour of birds' winter habits. We are trying to understand why these particular birds survive when the explanations we have do not seem to cover their needs.

Their size creates the first problem. Kinglets are tiny, but they maintain very high body temperatures even when their surroundings are below freezing. That takes energy. In fact, they would need to burn 75 percent more food per unit of body mass than birds twice their mass to maintain the same temperature. Keep the comparison straight: more food relative to their size, not necessarily more food in total than a much larger bird eats.

What about their feathers? Those certainly help, but they do not solve the problem. Feathers make up roughly the same fraction of a kinglet's body mass as they do for other northern birds. The smaller bird still has less insulation in absolute terms. So far, we have a bird that should lose heat quickly and needs a lot of fuel relative to its size. Something else must help it get through the night. Could it shelter somewhere? Could several birds huddle together and share warmth? Those are possibilities to keep in mind, not conclusions we have established.

The diet makes the puzzle harder. Kinglets eat only insects. They have a short winter day in which to find those insects, do not store food for later, and do not forage at night. A big meal before bed would be a tempting explanation, except a full stomach supplies enough energy to stay warm for only one hour. That will not cover an entire night.

The final paragraph keeps offering possible answers and then showing why we cannot stop there. Kinglets can triple their body fat during a day. Great—stored energy could help! But that increase supplies only about half of what they need overnight. Torpor could reduce the remaining demand, as it does for chickadees, but researchers found no evidence that kinglets use it.

Huddling is the remaining idea discussed. A recent study of related goldcrests may support it, and kinglets already spend their days in small flocks. However, two or three birds would not be enough. The hypothesis is that several of those small groups call to one another after dark and gather into a larger group. Notice the difference between an appealing explanation and a demonstrated one: this theory is “still untested.”

It is easy to finish thinking, “They survive by huddling.” We need to pull that back. Huddling may help explain their survival, but we have not been told that it does. The central idea is the unresolved puzzle, with the last paragraph offering partial answers and a promising lead.