American yellow warbler

American yellow warbler
Male in breeding plumage, Canada
Conservation status
Least Concern  (IUCN 3.1)[1]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Passeriformes
Family:	Parulidae
Genus:	Setophaga
Species:	S. petechia
Binomial name
Setophaga petechia (Linnaeus, 1766)
Subspecies
About 35 (but see text)
Distribution of the yellow warbler      Breeding range     Year-round range     Wintering range
Synonyms
Dendroica petechia Dendroica aestiva

The American yellow warbler (Setophaga petechia, formerly Dendroica petechia) is a New World warbler species. Sensu lato, they make up the most widespread species in the diverse Setophaga genus, breeding in almost the whole of North America and down to northern South America.

Etymology

The genus name Setophaga is from Ancient Greek ses, "moth", and phagos, "eating", and the specific petechia is from Italian petecchia, a small red spot on the skin.^[2]

Description and taxonomy

Other than in male breeding plumage and body size, 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.^[3][4]

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.^[4] Each of these groups is sometimes considered a separate species, or the aestiva group (yellow warbler) is considered a species different from D. petechia (mangrove warbler, including golden warbler); the latter option is the one currently accepted by the International Ornithological Congress World Bird List.^[5]

Depending on subspecies, the American yellow warbler may be between 10–18 cm (3.9–7.1 in) long, with a wingspan from 16 to 22 cm (6.3 to 8.7 in). They weigh 7–25 g (0.25–0.88 oz), varying between subspecies and whether on migration or not, globally averaging about 16 g (0.56 oz) but only 9–10 g (0.32–0.35 oz) in most breeding adults of the United States populations. Among standard measurements throughout the subspecies, the wing chord is 5.5 to 7 cm (2.2 to 2.8 in), the tail is 3.9 to 5.6 cm (1.5 to 2.2 in), the bill is 0.8 to 1.3 cm (0.31 to 0.51 in) and the tarsus is 1.7 to 2.2 cm (0.67 to 0.87 in).^[4] The summer males of this species are generally the yellowest "warblers" wherever they occur. They are brilliant yellow below and greenish-golden above. There are usually a few wide, somewhat washed-out rusty-red streaks on the breast and flanks. These markings are scientific namesake of the species since petechia roughly translates to "liver spotted", of which the reddish markings of the male were apparently reminiscent of for ornithologists.^[6] The various subspecies in this group mostly in brightness and size as per Bergmann's and Gloger's Rule.^[7]

The golden warbler (petechia group; 17 subspecies^[4]) 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 (D. p. gundlachi) barely reaches the Florida Keys where it was first noted in 1941, and by the mid-20th century a breeding population was resident.^[8] 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 (0.35 oz) or less and sometimes^[9] as little as 6.5 g (0.23 oz). 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 (erithachorides group; 12 subspecies^[4]) tends to be larger than other yellow warbler subspecies groups, averaging 12.5 cm (4.9 in) in length and 11 g (0.39 oz) in weight. 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.^[4] 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.^[4]

The American yellow warbler (aestiva group; 6 subspecies^[4] breeds in the whole of temperate North America as far south as central Mexico in open, often wet, woods or shrub. It is migratory, wintering in Central and South America. They are very rare vagrants to western Europe.^[3]

• 

  Resident adult male mangrove warbler, S. p. bryanti Quepos, Costa Rica

• 

  Breeding male golden warbler, Washington-Slagbaai National Park, Bonaire, (Netherlands Antilles)

• 

  Breeding male S. p. aureola mangrove warbler at Puerto Ayora, Santa Cruz (Galápagos Islands)

• 

  Breeding female, Horicon Marsh, Wisconsin (United States)

• 

  Yellow warbler S. p. gundlachi male resident, Cuba

Vocalizations

	Yellow warbler (aestiva group) song Recorded in Prince Edward Point National Wildlife Area, Ontario, Canada
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The song is a musical strophe that can be rendered sweet sweet sweet, I'm so sweet, although it varies considerably between populations. The call is a soft or harder chip or ship. This is particularly frequently given by females after a male has finished his song. In territorial defence, they give hissing calls, while seet seems to be a kind of specialized cowbird alert (see below). Other calls are given in communication between pair-members, neighbors, or by young begging for food. These birds also communicate with postures and perhaps with touch.^[3]

Ecology

Female yellow warbler attending nestlings, Yukon Flats National Wildlife Refuge, Alaska (USA)

Male (above) and female yellow warblers foraging in a reedbed, Mill Creek Streamway Park, Kansas (United States)

American 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.^[3] The mangrove and golden warblers occur to the south of it, to the northern reaches of the Andes. American yellow warblers winter to the south of their breeding range, from southern California to the Amazon region, Bolivia and Peru.^[3]

American 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.^[10]

The breeding habitat of American 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 American 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.^[3]

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^[11] are also eaten, the latter especially by American yellow warblers in their winter quarters. The yellow warbler is one of several insectivorous bird species that reduce the number of coffee berry borer beetles in Costa Rica coffee plantations by 50%. Caterpillars are the staple food for nestlings, with some – e.g. those of geometer moths (Geometridae) – preferred over others.^[12]

The predators of yellow and mangrove warblers are those typical of such smallish tree-nesting passerines. The odds of an adult American 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 American 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.^[13]

Snakes,^[14] corvids^[15] and large climbing rodents^[16] are significant nest predators. Carnivores, in particular Musteloidea^[17] 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) as well as Cooper's hawk (Accipiter cooperii), and the sharp-shinned hawk (A. striatus). Other avian predators of adults have included peregrine falcons (Falco peregrinus) and merlins (Falco columbarius). Owls such as great horned owls (Bubo virginianus) and eastern screech owls (Megascops asio) have been known to assault yellow warblers of all ages during night.^[3][18]

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. This may be due to the slightly larger size of shiny cowbirds, which are less likely to survive being feed by the much smaller warbler, compared to brown-headed cowbirds.^[18] The yellow warbler is one of the few passerine proven to be able to recognize the presence of cowbird eggs in its nest.^[18] 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 of its 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 Setophaga and Molothrus may fledge successfully.^[13] However, success of fledging in yellow warbler nests is usually decreased by the parasitism of cowbirds due to the pressures of raising a much larger bird.^[18]

Other than due to predation, mortality reasons are not well known. The maximum recorded ages^[19] of wild yellow warblers are around 10 years. A wintering American 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.^[20]

Breeding

Yellow warbler nest with small clutch

As usual for New World warblers (Parulidae), 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.^[3]

The American yellow and mangrove (including golden) 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. American 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; an American 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 American yellow marbler males are bigamous. Very few if any American 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.^[13]

The clutch of the American yellow warbler is 3–6 (typically 4–5, rarely 1–2) eggs. Incubation to hatching usually takes 11 days, but may take up to two weeks. The nestlings weigh 1.3 g (0.046 oz) 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 (somewhat more in the mangrove warbler, somewhat less in the American yellow warbler) 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.^[21]

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 American yellow warbler nestings are successful in raising at least one young.^[21]

Status and conservation

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^[3] as "lovely" and "musical", and they can help to generate revenue from ecotourism. No significant negative effects of American yellow and mangrove warblers on humans have been recorded.^[3]

These birds are generally common and occur over a wide range; consequently, the IUCN^[22] 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 American yellow warblers are prolific, and the stocks will usually rebound quickly if riparian habitat is allowed to recover.^[1][3]

The North American populations are legally protected by the Migratory Bird Treaty Act. The Barbados golden warbler^[23] (D. p. petechia) is listed as "endangered foreign wildlife" by the United States' Endangered Species Act (ESA) since 1970; other than for scientific, educational or conservation purposes (for which a permit may be available) importing it into the USA is illegal. The Californian yellow warbler (D. p./a. brewsteri) and Sonoran yellow warbler (D.p./a. sonorana) are listed as "species of concern" by the ESA.^[24]

The American yellow warbler is sometimes colloquially called the "summer yellowbird".^[25]

Footnotes

References

Further reading

• D. W. Snow (1966). "Annual cycle of the Yellow Warbler in the Galapagos". Bird-Banding. 37 (1): 44–49. doi:10.2307/4511232. JSTOR 4511232. 

External links

Wikimedia Commons has media related to American yellow warbler.

Wikimedia Commons has media related to Setophaga aestiva.

Wikispecies has information related to: Setophaga petechia

• Mangrove warbler breeding ecology
• Yellow warbler species account – Cornell Lab of Ornithology
• Yellow warbler – Dendroia petechia – USGS Patuxent Bird Identification InfoCenter
• Grizzlyrun.com Yellow warbler general information and photos
• Stamps at bird-stamps.org
• "Yellow warbler media". Internet Bird Collection. 
• Yellow warbler photo gallery at VIREO (Drexel University)