FRIENDS OF EDGEWOOD NATURAL PRESERVE
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FRIENDS OF EDGEWOOD NATURAL PRESERVE
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BIRDS THAT NEST IN CAVITIES By Lee Franks Birds build nests to protect themselves, their eggs, and their young from predators and from adverse weather. Other animal species also build nests, but birds do so in a greater variety of forms, from a greater variety of materials, and on a greater variety of sites. One form of nesting that several species residing in the Park employ is called “ cavity nesting.” Cavity nesters are birds that build their nests in tree cavities or holes. These cavities can be either natural ( i.e. created by decaying wood ), or excavated ( i.e. created by woodpeckers ).
There are two types of cavity nesters, primary and secondary. The primary cavity nesters are woodpeckers, who can chisel cavities into living hardwood trees which they then use for nesting sites. When they are finished nesting, the cavities become available for the secondary cavity nesters, who are unable to excavate. There are five primary cavity nesters in the Park (Northern Flicker and the Acorn, Downy, Nuttall’s, and Hairy woodpeckers) , and seven secondary cavity nesters (Western Bluebird, Violet-Green Swallow, Ash-Throated Flycatcher, Bewick’s Wren, Chestnut-Backed Chickadee, Oak Titmouse, and White-Breasted Nuthatch). While there are five species of woodpeckers present in the Park, current census data shows that they are not very abundant. Thus, few cavities are excavated, resulting in intense competition among the more abundant secondary cavity nesters for available cavities. One way to remedy this situation is to provide artificial housing (man-made nest boxes), which the birds accept as a suitable alternative to tree cavities. The boxes have an added advantage for the users in that they can be placed closer to food sources. Several of these well-built, well-ventilated and predator-resistant boxes have been installed in the Park. The most frequent users of the boxes are Bluebirds, Swallows, Flycatchers, Wrens, and Titmice. Beginning in mid-March, when the hormones of these birds are triggered, and courtship begins, the male will show the female several boxes from which she will eventually choose one to her liking and begin to bring in nest material. The Western Bluebirds use dried grass to form a cup shape by the female’s rotation around and around inside the box. The nest cup is then lined with small, finer dried grass. The Violet-Green Swallow also uses dried grass, but it lines the cup with feathers which usually come from the brooding female. The Chickadees and Titmice use lots of moss materials, lined with soft animal fur. The Flycatcher will use moss mixed with coyote scat which it lines with small animal fur. The Wren relies on small twigs.
With favorable weather, the first nest of the season, which is usually built within 5 or 6 days, shows up in early April. When the nest is complete, the female lays one egg per day until her clutch is complete. The average clutch size for Bluebirds, Swallows, and Flycatchers is 5, and for Wrens, Chickadees, and Titmice, the average is 7. The female does not usually begin the 12- to 15-day incubation period until the last egg is laid, thus insuring that all hatch at the same time. The Violet-Green Swallow is the exception to this rule. With this species, the female begins incubation before the clutch is complete, resulting in sequential hatching. The eggs are turned and rearranged frequently and incubation continues during the night. Though occasionally fed by the male, the female usually leaves the nest during the day for varying lengths of time, depending on prevailing temperatures. The incubating temperature for these birds is about 95° F. The newly hatched young, weighing just over 2 grams, are altricial (almost entirely naked), with a few bits of down sprinkled on the head and along the spine. They are extremely susceptible to cold and weather changes at this time and are brooded by the female almost constantly until they are about 6 days old. During this time the male will bring food to the nestlings and the female. After six days, both parents share equally in bringing small, soft insects about once every 5 minutes. Each nestling, regardless of the total number in the brood, is fed 2 to 3 times each hour until it fledges. The food changes to larger insects, such as beetles and grasshoppers as the nestlings mature. By the eighth day, the nestlings’ weight has increased 10 times, their eyes begin to focus, and pin feathers break through the skin, supplanting the down. Development rapidly increases and by day 13 or 14, the body is completely covered with feathers. They are ready to take their first flight (about 150 ft.) when they are 21 to 25 days old. Detailed records of nesting activities in each nest box are kept through the breeding season. Over the past two seasons, the records show that 84% of the nestlings raised in nest boxes successfully fledge. It is highly unlikely that such positive results could be achieved using tree cavity housing. There are far too many predators of these birds that are ready to interfere when they have unrestricted access to nest sites. It is hoped that the nest boxes at Edgewood will continue to aid secondary cavity nesting birds in their survival, so that they continue to eat insects, pollinate plants and disperse their seeds, and carry out all the other roles birds play in keeping the ecosystems of Edgewood functioning, while giving future generations of people the kinds of pleasures they have given us. |
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