Friday, July 6, 2012

Breeding Birds

We have a persistent presence of Turkey Vultures throughout the breeding season
Evidence of breeding birds is not necessarily confirmed only by finding a bird at its nest. It can also be inferred by observing certain bird behavior. First, the mere presence of a bird on a given territory during the breeding season is an indication that the bird is possibly nesting in the area. Birds that persistently defend territory –sing or give alarm calls in the same location for more than a week, are providing us with evidence that they are probably breeding there.

Brown Thrashers are frequesntly found at the Preserve during the nesting season

Positive breeding evidence short of finding the active nest or parents feeding fledglings includes seeing a bird with a fecal sack in its bill. Fecal material is generally removed from the nest site by the parent songbird; so if a parent is seen in the act of taking out the trash, breeding is confirmed.

The male Indigo Bunting protects his unseen young
On the basis of the above criteria, I can get a good idea of what birds are breeding around our nature preserve or for that matter, any other place that I visit regularly. I know that we had at least one pair of Indigo Buntings raising young at the preserve this summer. I saw no nest or young birds, but I did see both parents vigorously defending about 6 square yards of territory. Seeing both parents with their beaks occasionally packed with food and giving alarm calls was strong evidence that they were feeding young fledglings in the nearby bushes.

The Common Yellowthroat was also defending his young
Close by the buntings, a pair of Common Yellowthroats were behaving in a similar manner –issuing sharp alarm calls and frantically guarding some unseen offspring. The Yellowthroat is probably the easiest member of the warbler clan to find in the Northeast, but seeing their nest is not so easy. They build on or near the ground and most often under a tangle of vegetation. Unfortunately, they tend to raise more than their share of cowbird nestlings, so evidently, the female cowbird doesn’t have any trouble locating the Yellowthroat’s nest.

The Clay-colored Sparrow may be our newest nesting songbird species
This year, on the north side of the nature preserve, we’ve hosted a small colony of Clay-colored Sparrows. This is of particular interest to me since the species has never spent the breeding season with us before, and there are no known breeding colonies in our region. Consistent singing and holding territory for over a month leads me to the conclusion that they are likely nesting, but I have yet to obtain any proof positive on that score.

A parent Red-eyed Vireo defends its territory from marauding Jays
As I’ve mentioned in a previous post, seeing which songbirds pursue or “mob” nest predators (like Jays, Grackles and Crows) entering a given territory is a good indicator of what species are breeding in the area. Watching Red-eyed Vireos getting exercised over a marauding Blue Jay told me that the vireo had an active nest nearby.

The male Rose-breased Grosbeak on the breeding territory
Seeing birds mate or collect nest material also counts as positive evidence of a species breeding, though not always a guarantee of their subsequent breeding success. Recently while taking tour group through our old woods, we had the good fortune to have a pair of Rose-breasted Grosbeak land on a low branch right in front of us on the trail and proceed to mate. Mating is a very quick affair with songbirds, but it was proof that the species had plans, and likely already had already built a nest somewhere nearby.
The Black-billed Cuckoo is nesting somewhere nearby
The other day I saw a pair of Black-billed Cuckoo traveling together through the young trees growing along one of our brushy meadows. Seeing a pair together suggests the possibility that the species is breeding, but when one of them was seen to have a stick in its bill, proof positive was at hand that the birds were starting to nest. European Cuckoos don’t generally build their own nests, but in this part of the US, our 2 species (Yellow-billed Cuckoo and Black-billed Cuckoo) typically do.

An immature and flighted Baltimore Oriole still beggs to its parents
Hearing begging calls of nestlings or fledglings is another way to confirm breeding. Though the similar vocalizations given by many young birds make it difficult to distinguish which species they belong to, some do give quite distinctive calls. This season, Baltimore Orioles seemed to have had a bad year; far fewer nests have been seen than usual at our preserve and in the region. However, I have been able to determine that the species has had some breeding success, and this is largely by being able to recognize the begging calls of Oriole fledglings. Those Oriole begging calls sound a little like “Tu – tu – tu – tu”, which is often repeated.

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