The idea of hibernation to humans usually sums up the idea of going to bed early in winter and not being in any rush to get out. But the idea of hibernation to animals is more a necessity to survive than to get comfortable and have a nice long sleep. Several animals have evolved to their environment so that when the weather is cold, and food is scarce, they can enter a state that we refer to as hibernation. Although many other animals are known to take this approach, we do not hear about wild birds hibernating, but why?
The primary reason birds do not hibernate is that they can simply fly to another country where the conditions may be more favourable. With warmer weather comes a more generous supply of insects, riper berries, and generally better wild bird conditions.
Therefore, there is no need for wild birds to have developed a survival strategy that includes hibernation. However, there is one bird in the USA that we know does hibernate. But what bird is it, and why does it do it?
Seasons in the UK
The UK is quite a unique little set of little islands due to its location and comparatively mild climate. We also have year-round breeding sites in the UK as they are relatively protected sites compared to much of Eastern Europe which can suffer from extremely harsh winters and blisteringly hot summers. This is also true of North America who shares some characteristics with them. The UK shores see an almost monthly passage of birds migrating, coming and going across the UK and in all North, South, East and West directions.
The main reason for migration away from the UK for many wild birds aside from the UK temperatures in winter is the reduction in food. Many birds such as Robins and Blackbirds are renound for protect their territory, especially in winter, but if the conditions are particularly tough, they are known to engage in this less and just try and survive.
When the UK encountered an extreme cold snap (the beast from the east) last year, there were reports from around the UK that Robins, a notoriously territorial bird had begun tolerating the presence of other Robins at feeding stations. An example of just how the need to survive became more important than the strategy of maintaining a territory over winter. Live to fight another day, springs to mind!
For those that do not migrate, difficult winters can often mean that birds need to change their usual feeding habits and increase their food consumption. Flying around uses a considerable amount of energy, and it is a delicate balancing act which could be a life or death situation. In winter, birds such as great tits, blue tits, goldcrests and sparrows begin to feed throughout the day to stock up before the night.
They need to carefully manage their energy levels, finding food while keeping themselves topped up by as much as 30% of their usual body weight, so that they survive the night when temperatures drop considerably. They do this by avoiding unnecessary flight wherever possible.
Sparrows are one of the least adapted birds to migration and one of the most sedentary. You may have noticed that sparrows will feed almost in a frenzy when in greater numbers in the summertime but adapt their behaviour during winter. In winter they tend to fly very short distances, popping out of the cover of a bush, feeding and then darting back into cover where they will digest their food and conserve energy.
Their activities, in particular, give us a clue as to how much they need to plan. Smaller wild birds cannot carry much fat. To a blue tit or great tit, adding fat makes them more susceptible to predators, but it is thought that they will lose between 10-15% of their body weight overnight. With no guarantees of food first thing or the conditions they may awake too in the morning, a careful balancing act is required.
Feeding stations are busiest first thing in the morning and again late afternoon during the winter. This reflects wild birds need to seek out the nearest and most accessibly energy source first thing in the morning following a cold night and last thing in the afternoon a short time before the night draws in. That is of course not to say that birds will not feed during the day, but the morning and afternoon are typically the busy times.
When humans consume food, we increase our weight over time, and it can take many days before we lose the weight gained. However, it is known that wild birds can lose the equivalent of a stone of weight in 24 hours due to decreasing energy levels. Just think about that for a moment!
Food sources
Wild birds are experts at knowing which food sources are of high value. Sunflower seeds for example, are oil-rich and are therefore considerably more desirable than bread and if we look at the following infographic, it’s not hard to see why!
As the popularity of bird boxes with cameras built into them has grown in the UK. We have gained a greater insight into the activities of wild birds at night. These cameras have revealed that birds will often arrive at dusk, to maximise the time spent feeding. They will then fluff themselves up, tucking their bill and heads up as theses are the areas that are known to lose the most heat. Some of the smaller birds take on an almost spherical appearance!
A birds body temperature is known to drop by as much as 10 degrees at night to minimise heat loss a tactic known as Nocturnal Hyperthermia. This energy-saving tactic reduces the difference between the bird’s body temperature and that of the outside. It is thought to save as much as 75% of their energetic cost but drastically limits the birds’ ability to respond quickly to predatory attack.
Other common tactics that wild birds use to survive in cold weather include gathering in small flocks. Being together improves their chances of identifying food sources and huddling together in these groups conserves body heat. Some of the species that are known to take this approach include blue, great and coal tits, goldcrests and chaffinches. The RSPB has reported that they once counted as many as 50 wrens sleeping together in a nest box in particularly cold weather.
Huge flocks of starlings are known to come together in murmurations, before roosting together in the evening, another tactic to conserve heat as well as providing protection against predators.
There is, however, one bird that is known to hibernate through the winter period and it is called the Poorwill. This small bird which is a relative of the nightjar, can be found in California and New Mexico. It is a nocturnal hunter of insects which can be notoriously tricky to find at night during cold winter nights.
This little bird is known to tuck itself away when food sources are low, and the temperature has dropped only to emerge in Spring when the temperature has risen along with the number of available insects.