Birds have an internal rhythm where their physiology and behaviour change at different times of the year. They may change when to migrate, breed, sing more or less, search for mates, fatten up, molt or fly alone or with others. To make sure these changes happen during the best season, they need a way to keep track of time from predictable cues in their environment. So, how do birds prepare for the winter? What are they getting up to at this time of year? And what does the sun have to do with it?
Birds and other animals that live in places that experience cold winters and hot summers face two main choices when it comes to surviving winter. Their options are to stay and hunker down in the harsh cold weather or travel large distances to warmer places.
Birds can also reduce their body temperature and their metabolism at night in response to cold weather and limited food, by entering a type of deep sleep for short periods of time.
To know where they are in their yearly calendars, birds and other animals rely on environmental factors that are reliable and consistent, such as sunlight. The length of the day (known as the photoperiod) is dictated by the tilt of the Earth as it orbits the Sun. If you are at the equator, the daylight lasts 12 hours and does not change much across the year. However, the further north or south you move, the shorter or longer the days are depending on the time of year. The days are shorter in winter with longer nights, which translates into plummeting temperatures as daylight becomes scarce.
A True Bird’s Eye View
So, how do birds sense the short day length? Their eyes! Well, more specifically, the light sensitive cells in their brain. The bird's skull is thin and light can permeate through. This results in a light monitoring system between the eyes and different regions of the brain. Researchers know this through experimentally changing the day to night length with artificial lights and preventing light reaching the eyes or light sensitive cells in the brain to see if they can “trick” birds into behaving like it's shorter day, winter or longer day, summer.
Synchronizing their internal rhythm with the daily amount of sunlight helps birds to make sure their physiology and behaviour match their yearly calendar. Reproduction comes last on the to-do list of most birds. So much so that they shrink their reproductive organs to not waste any valuable energy on them. Surviving the winter comes down to a balance between conserving energy and using it to keep warm.
What do birds do in winter?
1. Slow down
During the winter, the longer nights mean more time to sleep and conserve energy. Birds rest around 3.5 - 5 hours more during this harsh season compared to in the summer. Jackdaws and European starlings make up for their lack of sleep in the summer by taking short daytime naps.
Birds can also reduce their body temperature and their metabolism at night in response to cold weather and limited food, by entering a type of deep sleep for short periods of time. There is one bird, the American poorwill, that takes this to the extreme. It goes to the trouble of migrating to Southern USA and Mexico from Western USA and Canada only to find a good cactus to hibernate under for several days. Researchers found they could hibernate for 45 days when the temperature was cold and the sun was not shining on them.
One way to survive the cold is through building fat reserves, but not too much, otherwise they would find it difficult to take off and fly away from predators. Fat contains up to 10 times more energy than carbohydrates and proteins of the same mass. Migrating birds rely on their fat storages to fuel their travels and whether they stop off to refuel, travel non-stop or travel short distances impacts how much fat they build up. The long haul journeys require more storage.
Researchers found the American poorwill could hibernate for 45 days when the temperature was cold and the sun was not shining on them.
Fat storage isn’t the only way to survive winter. Instead of relying on internal storage of energy, some birds (such as black-capped chickadees and tufted titmice) have a strategy to store food externally to ensure they have access to food throughout the winter. They are busy collecting and storing nuts in the ground or holes in trees that act like larders.
When it’s cold and food is scarce it makes sense to join other like minded-individuals - truth in the old saying ‘birds of a feather flock together’. This is exactly what a lot of birds do in the winter months as defending territories is the least of their priorities. It is a common sight to see chickadees flying around together with kinglets, warblers and even woodpeckers!
Being in a group does help when learning from others where the best food patches are and protecting against predators but it also has other benefits. Blue tits are a monogamous bird meaning they pair up with a partner during the breeding season and may also breed with them the following year. However, even though they are not reproducing during the winter, the dating game starts already. During those winter social forays with other blue tits, they are forming bonds and getting to know other potential partners. When the breeding season arrives, some ‘cheating’ occurs between the individuals that spent time together over the winter. It is common that 10-15% of the chicks have a different father.
What can you do in winter to help?
As the temperature decreases and the days get shorter, there is still a lot of drama packed into a bird's day. One of the most important things for birds during the winter months is getting enough fuel to help them survive. You can help by providing diverse food to your backyard visitors who may come around in groups with different food preferences. You cannot go wrong with suet to provide much needed fat for energy whereas seeds, nuts like peanuts and dried meal-worms will provide them a protein packed nutritious meal. Provide food consistently so birds have a predictable food source they can rely on.