A careful observer hiking in the late afternoon of a winter or early spring day might see birds fly into shrubs and disappear into hollowed out openings in branches or behind peeling bark. If the observer can get close enough to look into the hollow, he or she might see a bird almost filling the space. One might ask why a bird should go into such a cramped space when it wasn't nesting season yet.
Benefits of Winter Roosts
There are several factors in the answer to this question. The first and most obvious one is that by being out of sight, the sleeping birds is not visible to predators. But at bird could sleep under leaves or a rock ledge and be invisible, so hiding alone would not be a really good reason to sleep in a cramped space. The shrub has fairly thin branches. A climbing predator would disturb the shrub and awaken the bird in time for it to fly away from danger. Additionally, a flying predator such as an owl would find it difficult to fly through the branches and twigs to a chickadee, even if it was able to hear the smaller bird moving inside its roost.
Another benefit of a roosting site is that the bird can keep warmer in a semi-enclosed space than if in an open area or under a rock ledge. K. Weidinger (Microclimate of tree cavities during winter nights-implications for roost site selection in birds. International Journal of Biometeorology. March 2007) found that the temperature of occupied tree cavities during winter nights were up to nine degrees warmer than outside the cavity. In Alaska, chickadees drop their body temperatures up to 15 degrees overnight. Wintering birds commonly do this to conserve energy. The combination of lowered body temperature (hypothermia) and roosting inside a cavity can save the bird up to 50% of its overnight energy expenditure. This energy savings is valuable on a day-to-day basis, but when the weather turns cold and wet for several days, a 50% energy savings overnight can mean the difference between staying alive until better weather arrives and starving to death.
Birds Return to Favorable Sites
Once a bird selects a roosting site, it will return to the same hollow every night unless the roost is discovered or damaged. The bird that is sleeping there in April is most likely the same bird that was found there on an early October evening. Birds with identifiable bands regularly use the same overnight roost over long periods of time. Many conservationists use this behavior to capture individuals of endangered species as they enter their overnight roosts and then transport them to distant areas for restocking populations.
How to Help Birds Find Roosting Boxes
Setting out roosting boxes will increase the number of native birds in your neighborhood. Just as putting out nest boxes increases opportunity for breeding, putting out winter roosting boxes and feeding in the winter increase survivorship of resident birds. By setting out roosting boxes that are relatively airtight and have entry openings down low, many birds can survive extreme winter conditions. Larger boxes can be used by more birds. Although this produces increased temperatures in the roosts and greater energy savings for each bird, boxes that are too large can be dangerous to the health of the birds. A low box reduces the problem of birds becoming covered by excrement from birds on higher perches. The openings should be 1.5 inches round for wintering birds. Instructions for making these roosting boxes can be found here.
An important way to help native birds is to remove winter roosting boxes in late spring and replace them in early fall. This reduces the number of nesting locations for house sparrows and starlings that often kill native nesting birds found in their territories. By making sure the entry holes to winter roosting and nest boxes are the minimum size suggested for the species to be attracted, these alien and more aggressive birds will be deterred from nesting and will have to range further to find suitable housing.
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