Mountain chickadees nest in cavities

By Jeff Mitton

The chickadee flew right past me to a tree 15 feet away and disappeared into a hole in a gnarly ponderosa pine.

It emerged less than a minute later, watched me for a moment and left to collect more insects. It returned two minutes later, was only in the nest for a minute and then left again.

I had stumbled on a nest of mountain chickadees, two industrious parents and an unknown number of chronically hungry chicks.

Mountain chickadees, Poecile gambeli, nest in cavities, which seems to me to be a bit risky, for they do not excavate the cavities themselves. They rely on natural cavities or the cavities excavated by nuthatches in previous years. Branches that die and drop off commonly leave holes in the bark that penetrate the xylem or inner wood. Wind damage to trees can also leave cavities. I wonder if cavities ever become a limiting resource.

Cavity nests have distinct advantages. For example, cavity nests with small entrances protect against larger predators, such as ravens. In addition, cavities are insulated by solid wood, which not only protects against winds but also serves as insulation against nighttime winter temperatures. Studies comparing winter temperature inside and outside cavity nests revealed that occupied cavities were 12 to 15 degrees Celsius warmer.

Physiologists estimated that the relative warmth of cavity nests saved chickadees 25 to 38 percent of their daily energy budget, allowing mountain chickadees to stay inside longer without food when deadly winter storms prohibit foraging.

Mountain winters are challenging for small, resident birds that rely primarily on insects but also on seeds. Chickadees cache food in numerous places and are able to recall not only where the caches are, but there is also some indication that they remember what is cached where.

A study of mountain chickadee nest cavities reported that they are most commonly in ponderosa pine and Douglas fir, though nests are also in Engelmann spruce, white fir, subalpine fir and aspen.

Mountain chickadees move about very little, and most of the movement is by sub-adults, which venture above tree line in summer and to lower elevations in winter. But once they become mature, they become monogamous, year-round residents. Mark-recapture studies showed that adults do not move among populations separated by only a mile.

Given the low gene flow among populations, it is not surprising that the geographic range of mountain chickadees is divided into regions marked diagnostically by mitochondrial DNA sequences defining clades or mutually exclusive maternal lineages.

DNA sequences found in California, western Oregon and western Washington are not found in the Great Basin or the Rocky Mountains, and sequences found in Nevada, Utah or Colorado are not found west of the Sierra Crest in California. Just one population, near Mono Lake in eastern California, had both western and eastern sequences. The sequences in eastern and western clades differ by 4.4 percent, leading me to suspect that someone will soon propose that these clades should be recognized as different subspecies or perhaps even species.

Six species of chickadees are found in western North America, but the mountain chickadee's only relative residing in Colorado is the black-capped chickadee, Poecile atricapillus. The two species are most easily distinguished by color patterns on the head. The black-capped chickadee's cap covers the top of the head and extends below the eye to the cheek. The mountain chickadee's cap is interrupted by a white stripe over the eye, and it has a black bar running from the back of the head through the eye.

Locally, I see black-capped more commonly in Boulder, and mountain chickadees seem to be more abundant above 7,000 feet. Mountain and black-capped chickadees hybridize, but that does not appear to be common.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado.  This column originally appeared in the Boulder Daily Camera.

 May 14, 2014