Larry Cornelis – President, Sydenham Field Naturalists
Numerous species of birds, from wood ducks to black-capped chickadees, nest in the cavities of trees.
Today, due to logging and farming practices and the drive to clean up our landscapes, trees with cavities are in short supply.
The result is that cavity nesting birds are struggling to find nesting sites.
This, combined with habitat loss, has caused the decline of many species of these birds.
Natural nesting cavities are produced in a number of different ways. Many are made by woodpeckers, commonly known as the housing developers of the woods and hedgerows.
Every year they make a new nesting cavity, never using the same one twice (except northern flickers who will sometimes reuse old cavities).
The old woodpecker holes become nesting sites for other birds and small mammals such as nuthatches and flying squirrels.
Woodpeckers primarily make nesting holes in dead trees and limbs.
Other natural cavities are created when trees are damaged in wind and ice storms.
Broken branches eventually rot back into the trunks and larger limbs resulting in cavities.
When cavity nesting birds can’t find natural cavities, they will use man made bird houses to nest in and we can all help by putting up bird houses.
Different species of birds use different sizes of bird houses with specific sizes of entry holes.
Generally speaking larger birds need larger houses and entry holes to accommodate their size but sometimes a smaller species will use a larger bird house than they really need.
As an example, you may find a European starling nesting in a large bird house designed for a wood duck.
In regards to entry holes, house wrens can use the smallest holes (7/8″) and wood ducks the largest (3 x 4″).
A 1 1/2″ hole will accommodate most common species. It should be noted that European starlings cannot enter a hole less than approximately 1 5/8″ so keeping the entry holes 1 1/2″ or smaller restricts starlings which are an introduced alien species having a negative impact on our native birds.
Common local bird species that will use bird houses include wood duck, eastern screech owl, tree swallow, black-capped chickadee, American kestrel, white-breasted nuthatch, house wren, eastern bluebird, European starling and house sparrow.
Rarer species such as great-crested flycatcher, purple martin, tufted titmouse, prothonotary warbler (an endangered species) and hooded merganser are also possible.
Northern flickers usually make their own cavities but may use a bird house.
I had a pair of flickers successfully use a wood duck box after the baby wood ducks fledged.
Baby wood ducks leave the nest hours after hatching.
Other woodpeckers will not typically use bird houses. Different bird species live in different habitats (forests, wetlands and savannahs) and nest at different heights.
Wood ducks and great crested flycatchers may nest as high as 15 metres but I’ve also seen wood ducks nest much lower, even as low as a metre over the water.
Most species prefer a height of about 2 metres.
I mentioned purple martins, the largest member of the swallow family. Purple martins are a species at risk ranked “Threatened”.
Humans have been making housing for purple martins for so long, for centuries by first nations people, that they no longer nest in natural cavities, only in man made bird houses.
In the case of purple martins it’s a large multi-unit house like a T-14.
Only we can help save purple martins.
So consider helping cavity nesting birds by building and erecting bird houses.
Winter is a good time to make plans for where to erect bird houses and building them. It’s an easy project we can all do which provides environmental and ecological benefits for more than just the birds.