I am hanging from the top of a 25-foot balsam fir tree, 3,500 feet up a mountain on a breezy day, counting cones. For more than a decade, I have been studying the fir forests high in the Green Mountains with other biologists from the Vermont Institute of Natural Science.
Since 1920, when a biologist in New Brunswick began monitoring balsam fir cone production, it has been known that balsam firs produce heavy cone crops in odd years and few or no cones in even years. Between 1920 and 1950, this two-year cycle only broke three times. More recently, forest ecologists recorded the same cycle in the Adirondacks and the Greens. On top of that, it appears that balsam fir productivity is synchronized across the Northeast – for 3 years, I drove from Vermont to Newfoundland and found all the trees to be in perfect synchrony each time.
But this is only the beginning of the story. The cone cycle also ties together red squirrels and birds.
Cones form on the trees in the spring, and by August, if there’s going to be big crop, you can see the cones standing on the ends of tree branches like green candles. After just a few years of data collection, we could predict next summer’s squirrel and bird populations by looking at the fall cone crop – lots of cones one year meant lots of crossbills, pine siskins, and red squirrels the next. But it also meant that most other songbirds’ nesting success was going to be terrible.
Crossbills, siskins, and red squirrels dine on the seeds hidden away inside the cones. Crossbills and siskins are so specialized and dependent on these crops that we rarely see a single individual in the summer following a bad cone crop. I have often wondered how crossbills know to immigrate into the region when there are many cones and to stay away when there are none; it turns out that the cone crops are regular enough that crossbills can be certain of food if they arrive only in even-numbered years.
What about red squirrels? Most research suggests that squirrels live longer than two years, so how do they make it? Perhaps they migrate up and down the sides of the mountains, between hardwood and coniferous forests. When cone crops are big, the squirrels move up the mountains to the conifers, and when crops fail, they move back down.
In mid-May, when we arrive at one of our research sites atop Mt. Mansfield, the crossbills and siskins have already finished breeding, and there are juveniles flying about in large flocks. Breeding may begin as early as January. If cones are especially plentiful, the crossbills and siskins may raise several families before the seeds become scarce. But by the end of May, the cones are stripped, and the remaining seeds are lying on the melting snow.
Red squirrel populations (and probably those of other small mammals) rise dramatically following good cone years, and that spells disaster for summer-nesting songbirds like Bicknell’s thrush and blackpoll warblers. The squirrels have a peak mating period from January to February. Females are then pregnant for about 40 days, and the young spend another 30 days in the nest before venturing out onto the family’s territory, just about the time that the songbirds are laying eggs.
Red squirrels and other small mammals have a taste for eggs and nestlings. Each year we spend hundreds of hours finding and monitoring songbird nests in these forests and have found a clear two-year cycle closely following previous fall cone production and the accompanying high squirrel populations. Every other summer is a bad year for most nesting songbirds in the mountaintop forest because the red squirrels have dined on eggs and nestlings.
But crossbills and siskins also breed in high cone years, when red squirrel populations are elevated, so how do they deal with the squirrels? Why aren’t their nests predated, too? These birds have several advantages. They nest in late winter when there are lots of cones for the mammals to eat and the young squirrels have not ventured out of the nest. Crossbills and siskins tend to pick taller and more isolated trees for nesting, with fewer cones on them, so that squirrels are less likely to stumble upon them. And they often nest semi-colonially in trees, which gives them more eyes to watch out for red squirrels and attack them if they come near.
It doesn’t end there. We have a sneaking suspicion that the saw-whet owls that feed on the voles and mice that feed on the seeds may also be tied into the cone cycle. The deeper we dig, the farther the cone cycle seems to reach.