Posts Tagged ‘Poecile atricapilla’

Black-capped Chickadee (Poecile atricapilla)

The 117th edition of I and the Bird has now been posted at The Marvelous in Nature. You won’t want to miss Seabrooke’s artful presentation, which brings together links to the posts of 30 participants in one brilliant drawing! Willow House is represented by Birdie, It’s Cold Outside! Check out the cozily-clad chickadee on Seabrooke’s birdfeeder.

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Tree sparrows, a goldfinch and a junco forage together.

Birds of a feather flock together…don’t they? The answer is “not always”. In winter, mixed flocks made up of several different species of birds are also common. If you watch a backyard feeder over an extended period of time, you might expect that, given a stable supply of food, there would be a steady stream of birds visiting. In fact, it is more usual for there to be an ebb and flow of groups of birds. Periods with many birds of a variety of species at the feeder are followed by lulls with few or no birds feeding. This probably represents the movement of mixed-species flocks through an area.

Junco (Junco hyemalis)

Dark-eyed Junco (Junco hyemalis)

Mixed-species flocks occur on every continent. Flocking seems to offer a number of potential benefits to members and there are a number of theories that seek to explain the purpose of mixed flocks. The convoy theory suggests flocks help to protect individuals because there are a large number of eyes and ears available to detect predators and perhaps confuse the predator when many individuals flee at once. The gang theory proposes that flocking may permit flock members access to resources within a territory whose owner would be able to expel individuals acting alone.

American Tree sparrows (Spizella arborea)

The beater theory notes that some flock members, in the course of their own foraging, flush prey that a different species can capture. However, the most compelling reason for mixed-flocking appears to be the opportunity it gives species to exploit available foraging resources in an efficient manner under difficult conditions. Temperate flocks average 10 to 15 birds of 6 or 7 species. This allows different species to take advantage of different foraging niches with limited competition.

Black-capped Chickadee (Poecile atricapilla)

Black-capped Chickadee (Poecile atricapilla)

Dominant individuals may benefit more because they can usurp the best sites, but foraging in flocks may help ensure that an individual finds at least some food before its energy reserves are exhausted. Thus, small birds with limited fasting abilities tend to flock more than large birds. Typically, a nuclear species provides the main element of a flock structure, with the flock composition changing as it moves along, a result of new individuals joining and others leaving. In the northern woodlands, chickadees are a common nuclear species. However, chickadees are difficult to observe as part of a flock at a feeder because their feeding behaviour involves individuals grabbing a seed and retreating.

White-breasted Nuthatch (Sitta carolinensis)

White-breasted Nuthatch (Sitta carolinensis)

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Black-capped Chickadee (Poecile atricapilla)

The most amazing thing about the little birds that visit the feeder is that they can survive at all. Scarcely more than tiny puffs of feathers and a heartbeat, how do they do it? In fact, they have a number of coping mechanisms, both behavioral and physiological, that help them make it through the winter.

The chickadee, for example, changes its diet. In the summer, chickadees prefer insects and other invertebrates, but in the winter, they change to a predominantly seed diet. Seeds may be a more clumped resource, and seeds tend to have a higher fat content than animal matter in winter, thus reducing the physical costs of foraging. Chickadees also cache seeds against periods when food is hard to find. See A Chickadee Never Forgets for more on this.

When it comes to staying warm, size does matter. Conservation of heat can be maximized by minimizing the surface-area-to-volume ratio. Thus, animals in cold climates tend to be bigger than their southern relatives (Bergmann’s rule). An example is the polar bear, which is larger than black bears that live farther south. In this, little birds are at a disadvantage. Species such as chickadees are distinctive for having the highest ratios of feather weight to body mass, averaging 10 to 11% compared to an average of 6 to 8% for other avian species. Also, chickadees have a prolonged molt after the breeding season, which ensures less feather wear and thus greater insulation at the beginning of the winter.

American Goldfinch (Carduelis tristis)

In winter, night may be the most difficult time for most small birds because of its colder temperatures and length, combined with a relatively short daylight foraging period. Physiological adaptations allow winter-acclimatized goldfinches to maintain a constant 40° body temperature for up to eight hours at -70° C. To maintain their body temperature in such extreme cold, they shiver intensely and produce heat at a rate four to five times their basal rate. This high level of thermogenic endurance is seasonal. Summer-acclimatized goldfinches cannot maintain normal body temperature for more than an hour in such frigid temperatures.

Chickadees have a different strategy. They utilize a period of regulated hypothermia at night, letting their body temperature drop by as much as 10° C. This reduction in body temperature lowers the metablolic expenditure for the night by as much as 23%. The benefit of this strategy decreases as the size of the bird increases. Large birds would require too much enegy to reheat their bigger bodies to use this system.

The elevated metabolic rates that small birds must maintain through cold nights place a premium on energy reserves. These reserves consist primarily of the undigested food in the crop and other parts of the digestive tract, and fat. Some birds begin roosting with relatively large amounts of seed in their crops. However, the primary energy reserve for most species is fat. In winter-acclimatized goldfinches, fat stores at the onset of roosting are significantly higher than in summer, and may reach 15% of lean body size.

It’s all pretty amazing, but I’m glad I’m inside, sipping hot chocolate and thinking about those poor goldfinches, shivering in the night.

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Perhaps you’ve heard the expression An elephant never forgets, but did you know that chickadees have amazing memories? If you watch the birds that come to your feeder you will notice that some, like the American Goldfinches (Carduelis tristis), will perch on the feeder as long as they are able before being chased off by others seeking the perch. Black-capped Chickadees (Poecile atricapilla), however, don’t spend much time at the feeder. They grab a seed and fly. They may take the seed back to a safe branch to eat right away, or they may hide the seed for later.

Chickadees cache seeds under bits of lifted tree bark and in other safe hiding places for times when food is scarce. They may retrieve a seed a few hours later or a few weeks later. Unlike some animals that build up a large cache in a safe hiding place, chickadees are scatter-hoarders. That is, every seed is hidden in a different spot. A chickadee can cache as many as 100,000 food items in a year. The surprising thing is that it can find where it hid all those seeds with amazing accuracy!

Scientists have long been interested in this memory feat, not just zoologists, but psychologists and neurologists who study memory as well. Many experiments have looked at how chickadees relocate their caches. Generally, it has been found that they depend upon a hierarchy of visual cues to locate hidden seeds. The position of the hidden item relative to larger landmarks such as trees is the most important cue, with more localized cues such as a particular arrangement of pine needles next to the cache being less important.

Their amazing spacial capabilities are related to the hippocampus region of the brain. Surprisingly, the geographic location of birds can affect the size of this structure. For example, Alaskan chickadees have a larger hippocampus than do Colorado chickadees. It is hypothesized that birds living in more extreme conditions are more dependent on being able to locate their caches reliably and thus have a larger hippocampus. However, a finding that some European species such as starlings have a larger hippocampus than their North American relatives remains puzzling.

While it may not be true that a chickadee never forgets, these little birds are pretty cool!

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It’s baby bird season. Throughout the nature blogosphere, posts on baby birds are popping up. Here’s a great post over at The Marvelous in Nature about chickadee fledglings. And how about these cuties? Baby field sparrows. Over at A Passion for Nature, Winterwoman is showing off little house wrens. Not being as intrepid a birder as Seabrooke or Jennifer, I let the baby bird find me. When I walked into the kitchen, there was a young robin sitting on the windowsill. It was probably newly out of the nest, and still being cared for by its parents. My windowsill wasn’t the best place for it to be sitting. I was concerned about a parent running into the window. However, I did enjoy getting a good look at the youngster. I love the speckled breasts of baby robins. It’s when the species looks its most thrush-like, I think. I wasn’t the only one who found the baby of interest. Moey, left, and Tonka were quick to notice the new arrival. I shooed them away from the window, and when I came back a little later, the little robin was gone. I hope he found a better spot to perch.

Speaking of baby birds, here’s the latest on the boreal forest bird nursery. The Save Our Boreal Birds petition, with 60,000 signatures, was presented to the federal government on June 15th by MP Linda Duncan. May it help to preserve a future for many more baby birds.


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