Friday, July 12, 2024

A January chickadee finds some seeds to cache in Arlington. (Photo: Richard George)

Black-capped chickadees (Poecile atricapillus) are our state bird. Unlike many birds, they do not migrate. Like squirrels, they survive the winter by caching food – both seeds and insects. One chickadee can store hundreds of food items per day in nearby crevices, under bark, in hollow stems, under fallen leaves. But what is really amazing is that chickadees remember where they cache these items.

How can they remember so many items in so many locations? The chickadee brain does something that other bird brains do not do. Each fall, it sheds some neurons in its brain and grows new ones. It is thought that the neurons store memories. The old memories are shed and new memories are stored in their place. The part of the chickadee’s brain that is involved with spatial memory – the hippocampus – grows by 30 percent. Chickadees can remember where they have stored food for about a month. In the spring, when they no longer need to remember so many items, the brain shrinks back to normal size. 

A chickadee drinks water from an icicle in March 2018. (Photo: Tom Murray)

Chickadees are tiny, and they cannot have brains that are too heavy because they need to fly. Evolution has solved this problem by having these birds lose their old memory cells and replace them with new memory cells.

A chickadee does something else that few other birds do: enters a state of torpor, slowing its metabolism and lowering its body temperature at night to conserve body fat. On a cold night, a chickadee can lower its body temperature by about 50 degrees Fahrenheit, reducing its metabolic rate by about 32 percent to 45 percent. It still takes a lot of energy to survive in winter, and chickadees need 20 times as much food in cold weather as in warm summer weather. Insects become less available in winter, so seeds and berries make up half of a chickadee’s winter diet.

A chickadee picks caterpillars off apple blossoms in May. (Photo: Tom Murray)

In winter, other birds, such as nuthatches and woodpeckers, may flock with chickadees because chickadees provide so much valuable information. Chickadees call out when they find a good food source. Other birds have learned what this call means, and therefore, benefit from the chickadees’ finds. In addition, chickadees sound an alarm call – chick a dee dee dee – that alerts other birds when there is danger. The more dees in the call, the greater the danger. In one study, a chickadee added 23 dees when it spotted a small owl nearby. 

A winter chickadee in Strawberry Hill, Cambridge, on Jan. 24, 2022. (Photo: Richard George)

Flocking chickadees have a rigid social hierarchy. Males outrank females, and older birds outrank younger ones. This dominance hierarchy allows chickadees to coexist without too much squabbling and wasted energy. Higher-ranking chickadees have larger territories, larger bodies, higher singing rates, better mating success and lower mortality rates. Lower-ranking chickadees, for example, tend to forage on the outer parts of trees where predators can see them more easily. Dominant birds forage low and close to a tree trunk, where they are well hidden. Of course, this hierarchy is not permanent: chickadees can move up and down the ranks.

During the breeding season, up to 90 percent of a chickadee’s diet is insects and spiders, including caterpillars, cocoons and eggs. Caterpillars form the largest part of the diet probably because they have much more protein than other stages of insects. Caterpillars even contain twice the protein of beef (beef is 25 percent protein). Ninety-six percent of North American land birds feed insects – largely soft caterpillars – to their young. Each chickadee chick consumes thousands of caterpillars before it is old enough to leave the nest!

A chickadee along the Charles River sings its springtime song on a warm February day. (Photo: Tom Murray)

There are more than 550 species of moths and butterflies in Massachusetts, which live mostly on native plants, especially trees. Chickadees feast on the caterpillars of these moths and butterflies. According to University of Delaware studies, when native plant biomass falls below 70 percent, bird populations decline. One large meta-analysis indicates that insect populations have declined by 45 percent in the past 40 years. Although chickadee populations are stable right now, bird populations in general have declined by about 29 percent since the 1970s.

What can we do to help insect-loving birds? Many birds prefer areas with trees that support the greatest numbers of caterpillars. In our area, these trees are oak, native cherries and plums, willows, poplars, birches and crabapples and apples. Because birds need so many caterpillars to survive, planting trees is the best way to help. Bird feeders can help adult chickadees survive the winter, for example, but they do not provide the caterpillars that baby chickadees need.

A chickadee digs out a caterpillar from inside a dead cattail stem in January. (Photo: Tom Murray)

People often seem to think that animals do not speak to each other, but they do, though in way a different from humans. Chickadees, for example, produce an array of calls – at least 16 – each with a different meaning. They sing more in the spring than at other times of year because they are looking for mates, defending territories and raising young, which takes more communication than winter foraging.

As long ago as 1896, author and artist Rowland E. Robinson wrote about our chickadees:

[In winter,] none is more welcome than that feathered atom of life, the chickadee … Set forth a feast of suet on the window-sill, and he will need no bidding to come and partake of it … He will repay you with an example of cheerful life in the midst of dreariness and desolation … On some February day, when the first promise of spring is drifted to you in the soft south wind, the tenderness of spring is voiced in his love-note, brief but full of melody.

A chickadee in North Cambridge on Jan. 11, 2022. (Photo: Richard George)


Seen nearby

Scott Wieman spotted this ash-throated flycatcher in Neighborhood 9 on Nov. 28.


Have you taken photos of our urban wild things? Send your images to Cambridge Day, and we may use them as part of a future feature. Include the photographer’s name, date and the general location where the photo was taken as well as any other relevant information.

Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.