Bird Nesting Boxes

I just got a note from a friend asking about bird box cleaning. I told him to clean out the boxes every spring, especially ones with tree swallow nests. Tree swallow nests have a layer of poop if they have been successful. Eastern bluebirds leave their boxes cleaner, but still leave parasites. What’s the official word on nest box cleaning?

Cheers, and stay safe,

Fred Gralenski, Pembroke

My official word on nest boxes is to clean them out in the offseason. Cleaning out bird boxes and letting the birds “start fresh” each nesting season doesn’t hurt; if anything, it helps! Old nesting material can rot or decompose, and may hold parasites like mites. You may also see other critters that have used the box, or even the nest, throughout the winter. Rodents, especially field mice, will often take up residence in bird boxes during the winter for the shelter it provides. They’ll even cap over bird nests, creating a dome where they’ll store food, mostly seed. In my experience, it is always wise to tap on the box a few times to make sure it is vacant before opening it up. It is also good to make sure there are no birds already using it! Most box — or cavity — nesters haven’t started nesting yet, but some resident species like chickadees or titmice are just beginning, so make sure you’re not evicting a new tenant … unless it is a house sparrow! These non-native, invasive species are more aggressive and will out-compete our native species, especially those in decline like tree swallows. It is prudent to make sure that if you are putting nest boxes up, you are taking steps to limit access to house sparrows.

Another pro tip: beware of bees, or more specifically, wasps. These are less likely in early April but birds and mice aren’t the only wildlife that finds boxes useful. The droning buzz of wasp wings from inside a bird box are probably better left until a day with below-zero temperatures to clean out.

The offseason is also a great time to do a little maintenance on bird boxes. While cleaning old material out of boxes, check for moisture. Bird boxes can unfortunately be deadly if they are retaining water, which can be cold, promote disease, and in some extreme cases, even flood nests. You want to make sure that the box has good drainage in the bottom. Most boxes are made with the corners of the bottom piece cut out. If not, you can drill holes in the bottom of a box using a quarter-inch bit. One hole in each corner is recommended to ensure drainage even if the box shifts positions through the summer.

Putting up a bird box is one of the easiest and most helpful things you can do to support backyard birds. In nature, most cavities are created by primary cavity nesters, like woodpeckers who will make a hole one year to nest in, then not return. We have a whole suite of what we call secondary cavity nesters: species that nest in cavities but don’t make their own. These secondary cavity nesters (tree swallows, eastern bluebirds, house wrens, great crested flycatchers, American kestrels) all benefit from humans providing nest boxes, as natural cavities are in short supply.

The last thing to keep in mind: if you forget to clean your boxes out, don’t worry. In nature, there is no being that goes around and cleans out all the natural cavities for birds each spring. They’ll probably appreciate the help, though.

They’re not bald, but why are they white?

Uncle Doug,

Why do bald eagles have white heads?

Johnny Enright (age 3), Bedford, Masachusetts

I may have been biased in selecting this question, but it was a really good question because I don’t know the answer! It is a fun one to ponder, though. An important starting place is that only adult bald eagles have white heads (and white tails). In their first year, eagles are all dark brown, from their heads to their tails. They do show some white on the inner lining of the wings, specifically the under-wing coverts, but that is impossible to see unless they are flying. These young bald eagles are often misidentified as golden eagles, a very rare transient in Maine. A lot of people reporting golden eagles to us talk about their huge size, “larger than any bald eagle I’ve seen!” Fun fact: Bald eagles are larger than golden eagles. Goldens are often called “larger” because they have more mass.

Into a bald eagle’s second year of life it starts to get white splotches on its feathers, its head, body, tail, etc. As third-year birds, they are about as white as you’ll ever see. By Year 4, their bodies and wings are getting dark brown but the heads, which are now mostly white, still show some dark feathers near the cheeks (behind the eyes, on the auriculars), and dark speckles on the tips of the tails. Around five years of age the bald eagle is ready for the cover of National Geographic.

The reason for this change in plumage over those years is a visual signal of status and dominance. Young eagles are not a threat to adults, in the sense of competition for breeding locations or mate selection, and having an obvious sign like a dark or smudgy head is a “white flag” of subordination. So we know that the white-headed birds are of breeding age. Eagles will live to be more than 30 years old, and so they have apparently evolved white heads through some mate choice. Perhaps a white head is an honest trait of a bird’s health? Fortunately for the eagle, they are apex predators, having no natural predators, and don’t need to worry about the lack of camouflage that a white head or tail presents. So, why do bald eagles have white heads? It probably has to do with the choice a female eagle made about a million years ago, an ancestor species of what would become the bald eagle, when she chose a male eagle with some white on its head.

Do you have a nature question of your own? Email questions to ask@maineaudubon.org and visit www.maineaudubon.org to learn more about volunteering, classes, bird walks, and other programs about wildlife and habitat.