On Friday, we asked which species was different nesting type than the others among Red-headed Woodpeckers, Great Crested Flycatcher, Northern Cardinal, Eastern Bluebird or Eastern Screech-Owl. If you are like me when I volunteered to write this blog, the only birds I knew that were cavity nesters were owls and woodpeckers. The only species above that is NOT a cavity nester is the Northern Cardinal.
What is a cavity nester? Cavity nesting birds are ones that build nests, lay eggs and raise young inside sheltered chambers or cavities. The term cavity-nesting does not typically apply to completely constructed cavities, such as birds that weave elaborate, enclosed nests, but instead is reserved for birds that rely on nesting shelter from other sources and build their nests within that shelter. Furthermore, those cavity nesters come in two types: Primary Cavity-Nesting Birds and Secondary Cavity-Nesting Birds.
- Primary Cavity-Nesting Birds: These birds excavate their own holes, such as woodpeckers that drill out chambers in suitable trees or ground-nesting species that may dig out burrows in riverbanks. The labor to create a new cavity may take several days or weeks, depending on the birds’ nesting needs.
- Secondary Cavity-Nesting Birds: These species take advantage of natural or abandoned cavities, or in some cases will usurp them from other birds through aggressive intrusion. They may make minor adjustments to the cavity, such as adding or removing nesting material or changing the entrance, but do not do substantial modifications.
In addition to these different types of nests, there are various cavities that different species will use. The size, shape and placement of cavities varies depending on the bird species and their individual nesting needs.
Types of chambers birds may use include:
- Holes excavated in dead or decaying trees, stumps, logs, or poles
- Burrows in soft, vertical riverbanks, dirt mounds, dunes or similar banks
- Rock niches or crevices, either in natural cliffs or stone walls and structures
- Exposed pipes, chimneys or similar artificial cavities
- Supplemental nesting boxes and bird houses
Some birds use a bare, empty chamber, while others line the floor or interior of the chamber with grass, twigs, wood chips, feathers, fur or other materials, even creating an entire nest within the chamber.
Now that we have a better understanding of the basics, let’s go in to some of more common, specific species.
The Red-headed Woodpecker is a good example of a Primary Cavity-Nesting bird. Red-headed woodpeckers most commonly excavate holes in the trunks of dead trees. Holes are excavated from 24 to 65 feet above the ground and the 1.8-inch diameter entrance hole often faces south or west. These woodpeckers may excavate new holes each year, or use old nest sites. When the Eagles had their nest on the top of the dead pine on Ocean Winds, I was surprised when a Red-headed Woodpecker popped out of the trunk of that same tree. When the Birders had a Learning Together on Crooked Oaks in early May, we were treated to this beautiful bird looking out from a tree near the green of the 10th hole. Its mate was flying to the hole, apparently bringing food for possible babies in the cavity.
The Tufted Titmouse nest site is in hole in tree, either natural cavity or old woodpecker hole. It is a Secondary Cavity-Nester as it does not excavate its own nest hole. It will also use nest boxes. Nest (probably built by female) has foundation of grass, moss, leaves, bark strips, lined with soft materials, especially animal hair. Birds may pluck hair from live woodchuck, dog, or other animal, even from humans. In fact, we’ve seen a video of a Tufted Titmouse doing this to one of our Seabrook Island neighbors!!!
Great Crested Flycatchers use natural cavities or excavations made by other species. Nests are found in a variety of tree species anywhere from 3 to 70 feet above the ground (mostly below 20 feet). They build a bulky nest, and therefore prefer deep cavities. Before constructing a nest, they will generally fill a deep cavity with trash to a level of 12 to 18 inches from the top. They are well known for their habit of including a snake skin in the nest or dangling it from the cavity opening. As the Seabrook Island Birders were on a Learning Together at the fourth tee of Crooked Oaks, we saw the Great Crested Flycatcher bring nesting material to his cavity and then looking out as if to say “I’m doing well in my fixer-upper.”
In late April, David Gardner reported an Eastern Screech-Owl had taken over the nesting cavity used last year by a Great Crested Flycatcher. This cavity is in a tree next to the benches the children use to look at the Camp St. Christopher bird feeders. Obviously the nearby activity did not deter the Screech-Owl from becoming a Secondary Cavity-Nester in this cavity. Eastern Screech-Owls build no nest. The female lays her eggs on whatever debris is at the bottom of her nesting cavity, be it wood-chips, twigs, or the cast-off feathers and droppings from a previous year’s nest. Settling in, she makes a body-shaped depression where her eggs lie.
A Northern Rough-Winged Swallow was recently seen excavating a hole in the steep cliffs along Bohicket Creek. Audubon reports this species’ nest site is “usually in burrow in vertical dirt bank; may be bank along running stream, or road cut or similar bank miles from water. Birds may dig the tunnel themselves, 1-6′ long, or may use an old burrow of Bank Swallow, Belted Kingfisher, or ground squirrel. Sometimes you will find them in other kinds of cavities, such as drainpipe, culvert, crevice in bridge support, hole in side of building. It is a bulky nest at the end of a burrow made of twigs, weeds, bark fibers, lined with finer grasses, occasionally with fresh horse manure added.” Northern Rough-Winged Swallows are frequently seen flying near the “dryer vents” on the south side of the Island House. No nesting activity, however, has been observed.
Yes, the Eastern Bluebird is also a Cavity-Nester. This is a good example of a bird that often uses supplemental nesting boxes and bird houses. Eastern bluebird nesting sites (snags) are often eliminated because of their unsightliness or interference with cultivation. When available, eastern bluebirds nest in old woodpecker holes, hollows of decayed trees, and crevices of rocks. They will readily take to hollows in wooden fence posts or correctly sized and placed nest boxes. On Seabrook Island, we have 4 Bluebird trails that have over 75 boxes that are monitored and cleaned as needed. Bluebirds are very tolerant of the monitoring activity and their nests can be identified prior to the eggs being laid by their construction being primarily of grass and pine straw within the box. A Carolina Chickadee using the same box would build a nest of moss, pine needles and pieces of bark then line with fur, dryer lint or some other soft material.
Finally the Carolina Wren may or may not be a Cavity-Nester. Carolina Wrens are quite universal in their choice of nesting sites. These wrens prefer nesting sites that are fairly well enclosed, but they are not totally dependent upon cavities. They are well adapted to habitat conditions provided by man, but also nest in the woods where they prefer tangles and brushy undergrowth. Nests have been found in natural cavities, mailboxes, newspaper cylinders, old hornet nests, and bird houses.
Now that I have a better understanding of the diverse nature of cavity nests, I expect to find nests I’ve never noticed in the past including for species not mentioned above.
Article submitted by: Judy Morr
Photographs submitted by: Various – see credit on individual photos