Field Notes - Knotholes

~by Jim Eagleman

I set aside some interesting pieces of wood gathered from my walks in the forest and from working at the woodpile with a splitting maul. They all show a knothole where a branch had attached at one time and had either grown over or the wood had rotted to soften it. Ones with what looked like the right size hole for a bird to enter could become a part of a nest birdhouse. I’ll design some birdhouses from the collection. They needn’t be fancy—and that’s good, given my limited carpentry skills.

I once read cavity nesters comprise a near majority of the songbirds that nest here (over 80 species). And cavities can exist more readily in old and dying trees, dead snags of living trees, or in the main trunk. Dead and declining trees may seem an eyesore, but they are an important habitat component of cavity nesting birds. Cavity nesting is a reproductive strategy used by some species to protect their nests from weather and predators.

There are two types of cavity nesting birds: primary and secondary. Primary cavity nesters excavate nest holes, secondary cavity nesters rely on the abandoned cavities. As with many things in nature, recycling occurs.

The best example of a primary cavity nester is the woodpecker. They are excellent at excavating holes in trees using their chisel-like beak to chip away at a hole, sometimes for several days, until the cavity is complete. They are also considered to be a keystone specie by providing shelter and nesting sites for other birds.

There are seven species of woodpeckers in our area and that breed here. The most common are the northern flicker, downy, hairy, redbellied, and pileated woodpecker, all frequently seen. Less likely seen, but still occurring, are the red-headed and the yellow-bellied sapsucker.

The sapsucker breeds in young, northern deciduous forests. Redheadeds prefer mature hardwood forests with dead trees and nut crops. Older stands of oak are its habitat choice.

Secondary cavity nesters rely on previously excavated holes. Chickadees, titmice, and nuthatches are examples.

Bluebirds used to rely heavily on old woodpecker holes, but numbers began to decline when starlings and house sparrows competed with bluebirds. Luckily for many bluebird lovers, nesting boxes mimic the nest cavity, and the birds readily take to it. Bluebird trails now exist in many communities and the population is doing well.

Similarly, purple martins used to rely on cavity trees along forest edges, rivers and open fields but have since moved almost exclusively to human-made structures for nesting. You’ve probably seen the white, round, plastic houses looking like gourds hanging together. Installed near mowed fields or lakes, martins soar back and forth as they feed on insects that rise from both surfaces.

Other species like the greatcrested flycatcher, prothonotary warbler, and tree swallow take advantage of both tree cavities and nest boxes that mimic cavitylike conditions.

It isn’t only small songbirds that utilize tree cavities. Waterfowl like wood ducks, hooded merganser, and the common merganser use tree holes and waterfowl nesting boxes. The American kestrel, barred owls, barn owls, and eastern screech owls use available tree cavities and nest boxes for roosting and nesting.

And many readers know about our native wildlife like squirrels, raccoons, bats, and opossums that also rely on abandoned cavities for shelter and rearing young.

A professor of mine once used a stick to beat on a tree with a prominent hole overhead. “We’ll see who’s home,” he said. After several loud taps, poking a head halfway out, a sleepy raccoon looked around, then retreated inside. I used this same technique on many hikes over the years. Sometimes it was flying squirrels, screech owls, and bats that halfway emerged. Disturbed for a few seconds, they disappeared back inside. My hikers enjoyed the spectacle.

A reason for cavity nesting birds declining is a lack of dying and dead trees left standing on the landscape. No question, dead trees pose a threat to homes, buildings, and places where people gather, but also offer a great way to promote wildlife. To combat this problem, city planners have erected artificial nesting boxes, often taken on as a community project, or by a local conservation club. Dead trees in our woods attract pileated woodpeckers, and there’s no threat from falling timber in those remote areas. It’s a tradeoff. I reason if a big dead tree near the house needs to come down, while I assume other dead trees in our woods can remain. When I fashion my home-made birdhouses, the unique wood shape will limit my design. Birdhouse instructions usually state the precise hole size, cavity depth and roof shape. I visualize my houses taking on a more natural look. Whatever bird finds it suitable, that’s fine. I’d hope for any animal. I know from previous experience a birdhouse can hang unused for sometimes years at a time. That’s fine too. The weathering of the wood may add to its appeal. My indoor winter birdhouse project using nature’s knotholes awaits.

To inquire about this or other articles, contact the author at: jpeagleman@gmail.com