Yellow Warbler

Appearance

Other than in male breeding plumage, all subspecies are very similar. Winter, female and immature birds all have similarly greenish-yellow uppersides and are a duller yellow below. Young males soon acquire breast and, where appropriate, head coloration. Females are somewhat duller, most notably on the head. In all, the remiges and rectrices are blackish olive with yellow edges, sometimes appearing as an indistinct wing-band on the former. The eyes and the short thin beak are dark, while the feet are lighter or darker olive-buff.

The 35 subspecies of "D. petechia sensu lato" can be divided into three main groups according to the males' head color in the breeding season. Each of these groups is sometimes considered a separate species, or the "aestiva" group is considered a species different from "D. petechia"; the latter option is the one currently accepted by the International Ornithological Congress "World Bird List".

The Golden Warbler is generally resident in the mangrove swamps of the West Indies. Local seasonal migrations may occur. On the Cayman Islands for example, "D. p. eoa" was found to be "decidedly scarce" on Grand Cayman and apparently absent from Cayman Brac in November 1979, while it had been a "very common" breeder in the group some 10 years before, and not frequently seen in the winters of 1972/1973; apparently, the birds disperse elsewhere outside the breeding season. The Cuban Golden Warbler barely reaches the Florida Keys where it was first noted in 1941, and by the mid-20th century a breeding population was resident. Though individual birds may stray farther north, their distribution is restricted by the absence of mangrove habitat.

They are generally smallish, usually weighing about 10 g or less and sometimes as little as 6.5 g. The summer males differs from those of the Yellow Warbler in that they have a rufous crown, hood or mask. The races in this group vary in the extent and hue of the head patch.

The Mangrove Warbler measures 12.5 cm and weighs 11 g on average. It is resident in the mangrove swamps of coastal Middle America and northern South America; "D. p. aureola" is found on the oceanic Galápagos Islands. The summer males differ from those of the Yellow Warbler in that they have a rufous hood or crown. The races in this group vary in the extent and hue of the hood, overlapping extensively with the Golden Warbler group in this character.

The Yellow Warbler long, with an average wingspan of about 20 cm. They weigh 7–25 g, varying between subspecies and whether on migration or not, globally averaging about 16 g but only 9–10 g in most breeding adults of the United States populations. The summer males of this group are generally the yellowest "warblers" wherever they occur. They are brilliant yellow below and golden-green above. There are usually a few wide washed-out rusty-red streaks on the breast and flanks. The various subspecies in this group mostly in brightness and size as per Bergmann's and Gloger's Rule.

Status

To humans, these birds are quite beneficial. For one thing, in particular the young devour many pest insects during the breeding season. For another, the plumage and song of the breeding males have been described as "lovely" and "musical", and they can help to generate revenue from ecotourism. Adult Yellow Warblers are significant dispersers of plants via the ingested seeds, though this may of course spread any invasive weed whose fruits they find tasty. No significant negative effects of Yellow and Mangrove Warblers on humans have been recorded though.

These birds are generally common and occur over a wide range; consequently, the IUCN does not consider them a threatened species. A local decline in numbers has been noted here and there. This is generally due to habitat destruction and pollution, mainly by land clearance and herbicide and pesticide overuse of agriculture, and sometimes overgrazing. However, especially Yellow Warblers are prolific, and the stocks will usually rebound quickly if riparian habitat is allowed to recover.

The North American populations are legally protected by the Migratory Bird Treaty Act. The Barbados Golden Warbler is listed as "Endangered Foreign Wildlife" by the United States' Endangered Species Act since 1970; other than for scientific, educational or conservation purposes importing it into the USA is illegal. The Californian Yellow Warbler and Sonoran Yellow Warbler are listed as "Species of Concern" by the ESA.

The Yellow Warbler is sometimes colloquially called the "Summer Yellowbird".

Habitat

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. 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 – 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 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 and Cooper's Hawk, and the Sharp-shinned Hawk and screech owls.

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, with about 40% of all nests suffering attempted or successful parasitism. By contrast, the tropical populations are host to the Shiny Cowbird, 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.

Reproduction

As usual for New World warblers, they nest in trees, building a small but very sturdy cup nest. Females and males share the reproductive work about equally, but emphasize different tasks: females are more involved with building and maintaining the nest, and incubating and brooding the offspring. Most of the actual feeding is also done by them. Males are more involved in guarding the nest site and procuring food, bringing it to the nest and passing it to the waiting mother. As the young approach fledging, the male's workload becomes proportionally higher.

The Yellow and Mangrove Warblers differ in some other reproductive parameters. While the former is somewhat more of an r-strategist, the actual differences are complex and adapted to different environmental conditions. The Yellow Warbler starts breeding in May/June, while the Mangrove Warbler can be found breeding all year round. Yellow Warblers have been known to raise a brood of young in as little as 45 days, but usually take about 75 days. The tropical populations, by contrast, need more than 100 days per breeding. Males court the females with songs; a Yellow Warbler has been observed to sing more than 3,200 songs in one day. They are, like most songbirds, generally serially monogamous; some 10% of Mangrove Warbler and about half as many Yellow Warbler males are bigamous. Very few if any Yellow Warblers breed more than once per year; around one in twenty Mangrove Warbler females will do so however. If a breeding attempt fails, either will usually try and raise a second brood successfully though.

The clutch of the Yellow Warbler is 3–6 eggs. Incubation to hatching usually takes 11 days, but may take up to two weeks. The nestlings weigh 1.3 g on average, and are brooded for an average 8–9 days after hatching, and leave the nest the following day or the one thereafter. Mangrove Warblers, on the other hand, have only 3 eggs per clutch on average and incubate some 2 days longer. Its average post-hatching brooding time is 11 days. Almost half of the parents attend the fledglings for some time after these leave the nest. This post-fledging care can extend for two additional weeks or more, and sometimes the pairs separate early, each accompanied by one to three of the young.

Some 3–4 weeks after hatching, the young are fully independent of their parents. They become sexually mature at one year of age, and attempt to breed right away. Only about one in four Mangrove Warbler nests successfully fledge any offspring, due to accidents and predation frequently causing total loss of the clutch. By contrast, 55% of all Yellow Warbler nestings are successful in raising at least one young.