The winter ecology of small passerine birds at Northern latitudes provides an example in which counteracting selective forces can be studied in action. Small birds at Northern latitudes face a considerable challenge in winter. The smaller an animal is, the harder it will be to maintain a high body temperature in a cold environment.
The reason is that a small animal has a relatively larger cooling body surface relative to the heating body mass. Also, small birds of this size have a very high metabolism since they have a resting body temperature of 41-42°C (106-108° F). In mid-winter, a 12-g bird such as a chickadee or willow tit has to gain almost 10% of its lean body mass each day, in order to maintain their body temperature overnight.
Furthermore, weather conditions in winter are frequently unpredictable, for example, it may be snowing, windy, or warmer or colder next day. This means that a small bird not only has to carry sufficient fuel for the coming night, it also has to carry a buffer for environmental unpredictability. In fact, a bird should carry sufficient fat reserves to hedge for the worst possible day of winter; otherwise, it will die when this day occurs. Given this, it may seem logical that a small bird should carry huge fat reserves in winter.
However, there is not only a question about eating, not being eaten is also important. The environment these birds live in does not allow small birds to carry much fat. The reason is that they are at constant risk of being attacked by an airborne predator such as a sparrowhawk or pygmy owl. Such predators will typically attack at high speed from above, swooping in towards a foraging or perching bird. When this happens, the attacked bird’s acceleration, maneuverability, and vertical escape ability will be decisive for if it will escape or not. To avoid predation, small birds should thus be as lean and agile as possible. These two selection pressures are counteractive; to avoid death from starvation a bird should be as fat as possible and to avoid death from predation is should be as lean as possible. Natural selection will act on the combined mortality risks from these two sources.
Small, boreal birds that are resident in winter, such as parids (tits, titmice, and chickadees) have a number of adaptations that will help them to cope with their environment. In autumn they will store many thousands of food items that they can use for winter sustenance. Another adaptation is their ability to enter hypothermia. During cold winter nights, these birds can reduce body temperature down to 32°C in order to reduce metabolic expenditure. The disadvantage of this strategy is that they will be helpless in this hypothermic state (as they are not able to move or to react) if a predator spots them. In the morning, it will take 15 minutes for a hypothermic bird to regain normal body temperature.
When one considers how dangerous such a strategy can be, the question is why these birds do not go deeper into hypothermia, to save even more energy. Other small birds, such as hummingbirds, will regularly reduce night-time body temperature much more, down to 7° C, saving much more energy. The model shows that also a more moderate decrease of body temperature (to for example 34°C) may increase the probability of winter survival by almost 50% in these birds.
These findings are described in the article entitled Adaptive temperature regulation in the little bird in winter: predictions from a stochastic dynamic programming model, published in the journal Oecologia. This work was led by Anders Brodin from Lund University.