When the tufted titmouse moves in on the chickadee
I like birds. That is why, last Dec. 20, I was standing, at 7 a.m., at Little Buttermilk Bay near Bourne, Mass., about to begin the Buzzards Bay Christmas Bird Count. It was a pretty sunrise. It was also zero degrees F. At one point, as eight of us were trying with frozen hands and noses to survey maps of our territories to divide up the day's duties, we just plain broke out laughing. This was absurd! We were freezing! Suffice it to say that we did the count and all parties together recorded totals of 108 bird species and 25, 259 individual birds, among which there were 60 tufted titmice.
That may seem an insignificant number. But to an ornithologist it suggested an invasion.
The tufted titmouse, or Parus bicolorm to Linnaean fanatics, is a six-inch gray bird with a cardinal-like crest, a stubby beak, and big black beady eyes. The titmouse, like its close relative the black-capped chickadee, is an ecological generalist, munching fondly on a wide variety of insect and plant foods. Both species come readily to bird feeders which offer suet or sunflower seeds.
Like me, the tufted titmouse is a relative newcomer to the commonwealth of Massachusetts. I arrived there in 1970 because Wheaton College offered me a job. The titmouse began appearing in the 1940s, but didn't really come en masse until the early '60s. Since that time the titmouse population has seemingly soared in many parts of the state, the reason, an utter mystery.
In my 10 years at Wheaton, I've watched the tufted titmouse become increasingly abundant. The populations of other species also thought to be traditionally more Southern such as the cardinal, mockingbird, and Carolina wren have also been increasing markedly.
I began to wonder just how dramatic the titmouse invasion had become. What was its magnitude? I further wondered whether or not the coming of the titmouse had perhaps reduced some black-capped chickadee populations in the state. Ecological theory argues that similar species will compete for limited resources , resulting in the population reduction of at least one of the competing species. The titmouse is slightly larger than the chickadee -- a fact that would suggest it would be the victor in most competitive interactions with its evolutionary cousin.
How could I learn about the demography of the titmouse and chickadee? The answer, at least in part, seemed obvious: by looking at those Christmas Bird Count issues of the journal, "American Birds." In those volumes are stored the data from all these counts ever done, on a year-by-year and state-by-state basis. All I'd have to do was look.Well, almost all. . . .
Until very recently, not many papers published in scientific journals used data from Christmas Bird Counts. The data are plentiful and undoubtedly relevant to numerous ornithological questions related to range expansions and other significant matters. The only problem is that these counts are error-prone. How could a scientist seriously compare counts taken on different days in different years, by different people, under different weather conditions? How could he compare data from a count done over an eight-hour period by 12 people with one done for only three hours by 5 people?
The answer to these questions is: easily. Of course the data are fraught with errors, but there arem lots of data. Analyzing the data is sort of like election polling. You know there are lots of opinions, some of them highly local, some highly vocal. How do you predict which candidate will win? You apply statistical sampling techniques, using high-speed data processing technology. In other words, you let a computer do the work.
Now I can tell you lots about the titmouse increase. In fact, the beady-eyed bird is increasing at a pace that does signal a population explosion. The titmouse, however, is confined to areas with some deciduous trees, especially oaks. It has not invaded, nor does it show any signs of invading, the spruce-fir belt in the Berkshires. It is also absent from the islands off Cape Cod, presumably because it migrates by day and seems highly reluctant to venture across a body of water if it cannot see to the other side.
Where does all this leave the black-capped chickadee? The surprising answer is: fat and growing! Despite the dramatic titmouse increase, the chickadee population has not declined. It has actually increased slightly in most areas and more than slightly in some. I think the titmouse, rather than reducing the chickadee population as ecological theory has suggested, has, in fact, indirectly helped to increase it.
The tufted titmouse has, I think, prospered well from winter feeding. Bird watchers anxious to "keep" a titmouse at their feeders have put out literally thousands of pounds of sunflower seed and suet over the past 20 years. But the titmouse is not the only species with a palate for sunflower seed and suet. Chickadees, too, love that cuisine.
Imagine, if you will, the bird watcher of the early '60s, when the titmouse was still considered rare. Such a birder spots a titmouse coming to the feeder and gladly puts out 300 pounds of sunflower seed from October to April.I doubt the titmouse consumed that amount. There was probably rather a lot remaining for the chickadees. Our computer analyses show some correlations between the amount of winter feeding and the size of chickadee and titmouse populations. In some areas in the state, I can accurately predict the numbers of chickadees and titmice on a Christmas count merely by knowing the number of hours of feeder observation.
Although the titmouse increase has not yet affected chickadees negatively, I think it might in the future. Computer comparisons with Pennsylvania, a state where the titmouse and chickadee have coexisted longer than in Massachusetts, indicates that titmouse populations may have a long way to go in Massachusetts. Nowhere in the state is the titmouse equal in abundance to the chickadee. For instance, although our Buzzards Bay count spotted 60 tufted titmice, we recorded 789 black-capped chickadees. At its best, the titmouse is never more than half as abundant as the chickadee. Therefore, I think the titmouse invasion may continue until, at least in some places, the titmouse outnumbers the chickadee as it does in many areas in Pennsylvania.
In other words, I think that although the titmouse invasion may have initially helped the chickadee populations because of increased winter feeding, the titmouse will eventually compete with and reduce the chickadee in some areas. Both species nest in hollow cavities and, with the switch to wood stoves throughout the state, dead tree snags with nest sites may become increasingly difficult to find. Nest sites may prove to be the resource most in contention between the two species. And the titmouse, being larger, will probably win.
Why did the titmouse invade in the first place? I don't know. I do know that our computer analyses showed the titmouse population explosion to be highly correlated with the population increase of the cardinal, an ecologically different bird. Such a correlation suggests perhaps a general trend not tied to any one species. Perhaps the climate has eased. Perhaps winter feeding (cardinals are also fond of sunflower seed) has again had an effect.