Yellow Warbler - Ecology

Ecology

Yellow Warblers breed in most of North America from the tundra southwards, but they do not range far southwestwards and avoid the Gulf of Mexico coasts also. The Mangrove and Golden Warblers occur to the south of it, to the northern reaches of the Andes. Yellow Warblers winter to the south of their breeding range, from southern California to the Amazon region, Bolivia and Peru.

Yellow Warblers arrive in their breeding range in late spring – generally about April/May – and move to winter quarters again starting as early as July, as soon as the young are fledged. Most, however, stay a bit longer; by the end of August, the bulk of the northern populations has moved south, though some may linger almost until fall. At least in northern Ohio, Yellow Warblers do not seem to remain on their breeding grounds longer than they did 100 years ago.

The breeding habitat of Yellow Warblers is typically riparian or otherwise moist land with ample growth of small trees, in particular willows (Salix). The other groups, as well as wintering birds, chiefly inhabit mangrove swamps and similar dense woody growth. Less preferred habitat are shrubland, farmlands and forest edges. In particular Yellow Warblers will come to suburban or less densely settled areas, orchards and parks, and may well breed there. Outside the breeding season, these warblers are usually encountered in small groups, but while breeding they are fiercely territorial and will try to chase away any conspecific intruder that comes along.

These birds feed mainly on arthropods, in particular insects. They acquire prey by gleaning in shrubs and on tree branches, and by hawking prey that tries to fly away. Other invertebrates and some berries and similar small juicy fruits are also eaten, the latter especially by Yellow Warblers in their winter quarters. Caterpillars are the staple food for nestlings, with some – e.g. those of geometer moths (Geometridae) – preferred over others.

The predators of Yellow and Mangrove Warblers are those typical of such smallish tree-nesting passerines. The odds of an adult Yellow Warbler to survive from one year to the next are on average 50%; in the southern populations, by contrast, about two-thirds of the adults survive each year. Conversely, less than one Yellow Warbler nest in three on average suffers from predation in one way or another, while two out of three Mangrove and Golden Warbler nests are affected.

Snakes, corvids and large climbing rodents are significant nest predators. Carnivores, in particular Musteloidea as well as the Red Fox (Vulpes vulpes) and domestic or feral cats, prey on nestlings and fledglings as well as sick or distracted adults. These predators pose little threat to the nimble, non-nesting adults, but certain smallish and agile birds of prey do, especially the American Kestrel (Falco sparverius) and Cooper's Hawk (Accipiter cooperii), and the Sharp-shinned Hawk (A. striatus) and screech owls (Megascops).

These New World warblers seem to mob predators only rarely. An exception are cowbirds, which are significant brood parasites. The Yellow Warbler is a regular host of the Brown-headed Cowbird (Molothrus ater), with about 40% of all nests suffering attempted or successful parasitism. By contrast, the tropical populations are host to the Shiny Cowbird (M. bonariensis), but less than one nest in 10 is affected. Upon recognizing a cowbird egg in its nest, the warbler will often smother it with a new layer of nesting material. It will usually not try to save any own eggs that have already been laid, but produce a replacement clutch. Sometimes, the parents desert a parasitized nest altogether and build a new one. Unlike some cuckoos, cowbird nestlings will not actively kill the nestlings of the host bird; mixed broods of Dendroica and Molothrus may fledge successfully.

Other than due to predation, mortality reasons are not well known. The maximum recorded ages of wild Yellow Warblers are around 10 years. A wintering Yellow Warbler examined near Turbo, Colombia was not infected with blood parasites, unlike other species in the study. It is unclear whether this significant, but wintering birds in that region generally lacked such parasites.