Freezing water, lack of food, greater risk. Why is winter the hardest time for birds?

Food hidden under the snow

In summer, the range of food available to birds is enormous. Many species feed on insects, larvae, spiders, small invertebrates, as well as seeds, fruits and green parts of plants. In winter, many food options suddenly disappear. Insects hide in bark, in the soil or in dead wood, plants do not bear fruit, and snow covers everything birds could find on the ground. Even if food can be found somewhere, access to it is often difficult, because it requires digging through a layer of snow or searching places sheltered from frost.

Winter food is also less calorie-dense and less diverse. Birds therefore have to make decisions that are not as important in summer: is it better to stay nearby and look for leftovers in familiar places, or to move farther in hope of better conditions. Every flight costs energy, and in winter saving it is extremely important.

In practice, many species change their diet and feeding behaviour. Birds that are “insect-eaters” in summer switch to plant food in winter, if it is available. Tits, greenfinches, siskins and sparrows more readily look for seeds, and blackbirds or fieldfares can move to places where rowan, wild rose or hawthorn fruits still hang. Woodpeckers more often use larvae hidden in wood, and nuthatches and treecreepers scour tree trunks for small organisms hidden in cracks in the bark.

In cities the situation can paradoxically be easier, but also riskier. On the one hand, birds find more warm microclimates there, and some plants bear fruit longer. On the other hand, the share of human food grows, and it is not always safe for birds. Bread leftovers, salty snacks, food with lots of spices or low-quality vegetable fats can lead to health problems, and when feeding waterbirds, bread is often one of the main causes of poor condition and disease.

In winter, access to food in space also matters. Birds more often approach houses, gardens and roads, because it is easier to find leftovers and vegetation there that is not completely covered by snow. This increases the risk of collisions with cars, strikes against windows, as well as contact with predators. From a bird’s perspective, winter is not only less food, but also a harder and more dangerous “hunt” for every bite.

A shorter day means less time to eat

In winter the day is short, and this matters enormously. Birds have fewer hours to obtain food, and at night the temperature drops, so the body uses more energy to keep warm. This means birds must “fit” their entire energy balance into a very limited time. If during the day they fail to obtain enough food, the night can be critical for them.

This also explains why in winter birds feed so intensively in the morning and before dusk. Every hour is valuable, and weather conditions can suddenly make searching impossible.

Greater risk from predators and stress

In winter predators also fight to survive, which means pressure on birds often increases. When easily available food is scarce, predators hunt more intensively, take risks more often and move closer to areas inhabited by people. Birds that are weakened in winter have to forage longer and spend more time in open spaces, making them easier targets.

The most obvious predators are birds of prey. The sparrowhawk is a typical “feeder hunter” and can take advantage of moments when birds gather in one place. The goshawk hunts larger birds, and in winter conditions may be more active near housing areas. Owls, though less visible, also hunt intensively, especially when rodents are harder to reach.

In addition, there are ground predators. The fox, marten or weasel can hunt birds feeding low to the ground, especially in places where snow limits escape. In cities and on the outskirts of villages, the role of domestic and free-roaming cats also increases; in winter they can be exceptionally effective, especially against weakened individuals. In practice, a bird often takes a risk in winter: it has to come closer to people to find food, but then it encounters predators more often.

Another important factor is stress and being startled. In winter a bird does not have the same energy reserve as in summer. A sudden take-off after being startled is not just a momentary effort. It is lost calories that on a frosty day can determine whether a bird survives the night. That is why seemingly small human behaviours matter so much: walking into thickets, chasing birds for fun, noise, letting dogs run off-leash in forests or meadows.

In practice, the most harmful is a pattern repeated many times. If a bird is startled several times a day, it must constantly interrupt foraging, loses energy on escapes and fails to replenish its reserves. In winter even small disturbances can have consequences greater than we intuitively assume.

Ways to survive

Birds have many mechanisms that increase their chances of survival in winter conditions. Some are behaviours we can observe during a walk, and some are physiological tricks of the body. The most important strategies are:

• Building up energy reserves in the form of fat
  Many birds increase their body mass before frost arrives and try to “top up” daily to a safe level. Fat is crucial because it provides a lot of energy and helps them survive the night, when there is no possibility of feeding and the temperature drops.
• Fluffing up feathers and better thermal insulation
  In winter birds often look “puffier” because fluffed feathers trap a layer of air next to the body and act as insulation. This is a simple but very effective way to reduce heat loss.
• Choosing sheltered places to roost
  At night birds look for places that protect them from wind and moisture. These can be dense shrubs, conifers, the inside of hedges, crevices in buildings, and for some species also tree cavities. Roosting in a good place can reduce energy loss and lower the risk of attack.
• Roosting in groups
  Some species readily roost close to one another. In a group it is easier to keep warm and detect danger faster. In winter you can observe birds gathering at dusk in specific places, forming distinctive “roosting sites”.
• Limiting activity and avoiding unnecessary flights
  In winter birds try to conserve energy, so they often choose shorter routes and avoid unnecessary activity. This is one reason why it is so important not to disturb them. Every additional flight means lost calories.
• Dietary change and flexibility in choosing food
  Many birds switch in winter to a more seed-based and fatty diet, if they have the opportunity. Tits are an example, as they readily use high-calorie food. Birds that hunt insects in summer can in winter search for seeds, buds and fruits left on shrubs.
• Using microclimates
  In cities birds are more often found in places where it is slightly warmer and where snow does not linger as long. These can be areas near buildings, parks with dense vegetation, waterside areas or sheltered courtyards. Even a difference of one or two degrees matters when the energy balance is on the edge.
• Short-term lowering of body temperature at night
  In some species there is a phenomenon that can be compared to an “energy-saving mode”. Birds briefly lower their metabolism and body temperature to reduce calorie use at the hardest moment, which is at night.