in Wood Nests
Mud or earth, of course, is not the only place to make a hole and many birds nest in holes found or excavated in trees, cacti and even in termite nests.
Making a nest in a hole that already exists in a tree is not really an architectural feat as it involves little effort on the bird's behalf. Still, holes in trees, alive or dead, make excellent nest sites and numerous bird use them. Some, like the Blue tit (Parus caerulea), and several of its relatives, Redstarts (Phoenicurus phoenicurus), the common Starling (Sturnus vulgaris), and many of its relatives, as well as various Flycatchers use existing holes to nest in, making very little modification to the hole except to supply some lining as nest material. Others, like the many hole nesting Parrots and Nuthatches, will modify existing holes to varying degrees. Fewer birds excavate their own holes in trees, but of those that do the woodpeckers, with their impressive hammer-drill impersonations are by far the best known.
However, the prize for champion tree hole nester has to go to the various species of Hornbills, Bucerotiformes. These large birds, with the exception of the two species of African Ground Hornbill, all nest in hollows in trees. This is no simple matter. The Great Indian Hornbill is a large bird, nearly a metre from bill-tip to tail-end, which likes to nest between 20-45 metres up the tree trunk so can only nest in trees which have a diameter greater than 1-2 metres at this height. Trees this large are now rare in many forests putting serious pressure on the breeding capability of these birds. Both males and females help excavate the hole which needs to be quite extensive to house the female and several chicks for some weeks. Once the hole is large enough to accommodate the female, she gets inside and helps the male wall up the entrance with a mixture of guano, woodchips and mud. The female will remain in the hole until the young are ready to fledge. Only a small slit will be left in the mud wall to allow the male to feed the female and her young. During this time she will not only raise the chicks in great security but also moult all her feathers in one go. Hornbills are long-lived birds and mate for life, so the male has a considerable vested interest in keeping the female well-fed.
Nesting in holes may be secure form many predators and much of the weather, but it has one drawback. The warm, humid conditions make ideal breeding conditions for various avian pests and nest parasites such as bird and feather lice, ticks and fleas. With a captive food supply in the young birds, parasite loads in nest holes build up rapidly. Hornbills never use the same hole twice and the need to escape these pests may have something to do with this.
Other favourite sites for nest holes are cacti, many of which grow as large as small trees, and termite nets. Each of these provides an interesting example of commensualism. Firstly, the Orange-fronted Parakeet (Cyanoramphus malherbi) of Central America nests almost exclusively in mounds built by Nasutitermes termites. Unlike other birds, mammals, etc, which nest in termite mounds, Orange-fronted Parakeets do not have the nest hole sealed off from the rest of the termite mound. Soldier termites can wander around the nest and workers remove the young birds' faeces and any parasites they can find. Obviously, this is good for the Parakeet, but what the termites get out of it, no-on knows for sure.
The second relationship is quite amazing. The Gila Woodpecker (Melanerpes uropygialis) nests in holes which it excavates in the famous Giant Saguaro cactus of N. America. In the same hole with it, nests the Elf Owl (Micrathene whitneyi). The Elf Owl is tolerated because it has the amazing habit of catching Western Blind snakes and bringing them alive back to the nest. The snakes are insectivores so benefit by having a cosy home and free food in the form of avian parasites. The woodpecker benefits because it gets a reduced parasite loading, thus improving the health of its young. The Owl gets not only a reduced parasite loading and a free nest site but also protection for its young while it is hunting at night when the woodpecker is roosting in the hole.
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