Species that Skirt the Season’s Edge

Early Starters

It’s a brisk morning in early March, and you’ve just arrived at your favorite lake. Heater set to high, seats roasting, and a warm beverage in hand, you take a moment to appreciate the landscape.

The lake is all frozen in but for the outflow. You notice a stick nest the size of a couch in a large pine, covered in a layer of fresh overnight snow. The snow fidgets and shakes until a head emerges, sporting a yellow bill gleaming in the morning sun.

The bird is a bald eagle. It shakes the snow off its body, examines its eggs, and resumes incubating, undeterred by the conditions.

In our region, bald eagles return to begin nesting as early as mid-February. They are a species that skirts the season’s edge, risking adverse conditions for a greater reward.

Adult Bald Eagle in a white pine. (photo © Nate Marchessault)

Eagle eggs take just over a month to hatch, and after hatching, the young remain in the nest for approximately three months. It is this long process of rearing young in the nest that makes the early nesting worthwhile.

By early April, many lakes and ponds have started to open up again, and adult eagles take advantage of the increased foraging opportunities to provide fish and occasionally waterfowl for their young.

Another bird that begins nesting early is the great horned owl. In addition to timing the hatching of their young to increased food availability, they also skirt the season’s edge for a slightly more devious reason. They do not build their own nests; instead, they take advantage of the hard work of hawks, crows, and other birds by stealing nests in January and February, before their owners return from their wintering grounds.

They will even steal nests from birds as large as herons, and it’s a favorite pastime of birders to look for the ear tufts of a great horned owl poking out of a heron nest at heron rookeries in the early spring.

Arriving in mid-March, American woodcocks are a cryptic species of shorebird that feed by probing the soil for earthworms and other insects in fields and thickets. These birds can only feed once the ground has thawed, but instead of waiting until there is no threat of refreezing, their migrations follow the snowmelt.

Woodcocks have been known to follow the frost line not only northward as the ground thaws but also on short jaunts back south during sporadic cold snaps that refreeze it. The added moisture from the melting snow may aid their capture of prey, as earthworms are most frequently near the soil surface when the ground is wet. They take advantage of this abundant food source to refuel from the energy spent during migration and can eat their weight in worms in a single day.

Amphibians Don’t Wait Around Either

Also timing their migrations shortly after snowmelt, amphibians like spotted salamanders and wood frogs skirt the edge for a different strategy; to minimize the risk of predation. Their entire reproductive cycle is about speed. They’ve adapted to breed nearly exclusively in vernal pools, which are often filled by precipitation from snow melt and dry out within just a few months’ time.

A spotted salamander on the road. (photo © Dallas Huggins)

They do so because these pools lack fish, which are voracious predators of amphibian eggs, larvae, and adults. In early spring, when the ground has thawed, these amphibians migrate in droves to their breeding pools on rainy nights when temperatures are as low as 40 degrees Fahrenheit.

Their goal is to breed and lay eggs as early as possible so their young have the greatest chance of success before the pools dry out.

Amphibians skirt the season’s edge so tightly that some, like wood frogs, have specially adapted to be able to freeze their bodies entirely.

Though most arrive in May when conditions have become milder, our Neotropic bird migrants, that is, those that are migrating from Central and South America, also skirt the edge. Their long and often perilous migrations are timed to coincide with the emergence of large numbers of caterpillars and other insects following the “green wave” of plant growth in the spring.

As spring arrives earlier and earlier in our region, these birds are unable to see our local environmental cues and are falling out of sync with the dramatic increase in insects that their migrations have historically been timed to coincide with. While it is clear this will impact the survival and breeding success of these birds, what’s a little less clear is how this will affect the ecosystem as a whole.

The lack of a predator means that caterpillars and other insect prey will be more abundant than typical. If other species feed on them, it will increase their fitness and breeding success. If other species do not, there could be detrimental impacts to tree and other plant health.

Each natural system has a delicate balance between interactions of species that live within it, and each species has specific adaptations that make it successful in that niche. Skirting the edge is just one adaptation to gain a competitive edge. It comes with clear benefits like increased resources and a greater chance of reproductive success, but comes with the risk of harsh conditions and delicate timing.

Still, year after year, these species use it to their advantage, proving that the risk is worth the reward.