Bluebirds have a resilient quality, which puts them in March spotlight.
March in Kansas is too early, one might think, for birds such as eastern bluebirds to begin nesting. Owls and hawks, yes; bluebirds, no. Nevertheless, as of late February, observers in southern Kansas noted that bluebirds were showing signs of possible nesting. Some local eastern bluebirds have already begun their annual nesting activities: singing, selecting the right places, forming pairs and even gathering nesting material and laying eggs.
Generally in Kansas, bluebirds begin nesting in March, and a major part of the population will be sitting on eggs in April, a time when many other species are just starting to think about pairing, and when some are still far to the south on their wintering grounds. What can the birds have in mind?
Looking on the bright side?
You could say that bluebirds are incorrigible optimists, but I think they do fairly well in March and the penalties for early breeding that we can imagine are to some extent unrealized. If nesting in March really were a poor strategy -- a waste of energy and an exposure to environmental hazard -- then, to the extent that the behavior depends on inheritance, it should gradually disappear.
But bluebirds gamble on good weather in March. Sometimes it pays off: In 1996, a pair of bluebirds began nesting at our old farm in March, and successfully fledged four young. They then had another clutch, and another, and ultimately reared five broods that year. Some 18 new bluebirds entered the world from that one nestbox, and all of them had a reasonably good chance of being early nesters when their turns came around. As we can see from a glance out the window, probability statements in February are pale in comparison to the reality of March weather.
Around 30 to 40 years ago, bluebirds were thought to be real losers in nest site competition with house sparrows. These tough, aggressive survivalists were taking over the nesting cavities that were so necessary for bluebird reproduction. Possible scenarios included bluebirds becoming rare in eastern North America. But this prospect was not realized, at least in part because bluebirds are not as urban as house sparrows, and live in some regions of the countryside that sparrows do not, or cannot, use.
Partly also, competition with sparrows was largely over nest sites about 9 feet or higher above ground-level. "Bluebird trails," that is, long lines of nestboxes placed by humans for bluebirds, proved to be highly satisfactory for bluebirds if boxes were only 4 to 5 feet high. Sparrows much preferred higher sites, and the only real disadvantage for bluebirds was that a few species of snakes had an increased chance to feed on bluebird eggs and young.
Cavity-nesting, such as in a nestbox, is generally a successful way to rear young birds. Most species using cavities have lower egg and nestling mortality than open-nesting species. Some disadvantages are nevertheless associated with cavity-nesting, the most important of which is that the microhabitat of the nest is ideal for certain parasites. The best-known of these are blood-sucking mites, which are tiny but frequently occur in large numbers. They may become very important in nestboxes that have been used at least once before.
Mites live in the litter of the nest and leave it for feeding, either on the adults or nestlings. Because the mites feed on blood, they may cause anemia in a host; in purple martins, mites have been known to kill young nestlings. Controlled studies showed that nests without mites fledged significantly more young martins than nests with mites. So, it is clear that nest parasites are capable of seriously affecting populations of martins. Mites may not now be significant for bluebirds, but the potential for their transfer to bluebirds is real.
In addition to mites, nests in cavities are known to harbor bacteria, some of which can cause serious mortality. The best-known instance of bacterial infestation is documented for old house wren nests in Illinois. Four genera of bacteria, including the dangerous salmonella, were found in five of six wren nests several months after the nesting season of 1995. Presence of potentially dangerous bacteria in cavity nests may help explain an otherwise problematic behavior of some of the cavity-nesting birds: supplying green plant material to the nest. Some cavity-nesting species (martins, starlings) are known to bring leaves to the nest cavity regularly in the egg and nestling period. These green leaves include those of the genus Prunus -- such as cherry, peach or plum -- which on drying out release hydrocyanic acid as a gas; and, the green plant material brought by starlings to nests is known to reduce numbers of pathogenic bacteria.
To get back to eastern bluebirds, we do not know if bacteria live in their nests, but it is a safe bet that they do. And, we do not know if bluebirds supply fresh green plant material to their nests; adult females sit very tightly on nests, and it may be difficult to determine if green leaves have been brought to the nestbox without using binoculars and watching adult behavior for long periods of time during nestbuilding and incubation. It requires bacteriological techniques to determine whether bluebird nests harbor bacteria, but the matter of putting green plant material in nests can be examined by anyone who knows where bluebirds nest and who has binoculars and a few spare hours. Or days. You can't tell.
Bluebirds obey the biological imperative under the most onerous of conditions -- late winter storms, house sparrow invasions, bacteriological warfare -- and promise to remain a significant part of midwestern natural history. This year's unkind cut by late winter weather will cause only a minor blip in bluebird population numbers.
-- Richard F. Johnston, is professor emeritus at the Kansas University Natural History Museum and Biodiversity Research Center and a member of the Kansas Ornithological Society.