Setophaga nigrescens

Identification & Distribution

Adult male black-throated gray warblers (see first picture below) have a black and white face. They have a black stripe on the crown, throat and below the eye, and a white stripe below the chin and above the eye. There is also a characteristic small yellow spot between its eye and bill. The female black-throated gray warbler (see second picture below) has more dingy plumage on the head, with a white throat and dark grey cheeks. Both sexes have a grey back, white underparts and black side streaks. Both sexes give a call described as a flat 'chupp' or 'tup'. The male sings a buzzy song variously described as having the 'earlier notes doubled and the next to last note high' or 'zeedle zeedle zeedle zeet-chee'.

Both images reproduced by permission, copyright Mark Rauzon, all rights reserved.

In the breeding season black-throated gray warblers live in in open pine forests, pine-oak woodlands, and pinyon-juniper forests with a brushy understory. During migration and on the wintering grounds, they use a variety of woodlands, scrub, and thickets. The breeding grounds are in western North America from British Columbia to northern California and east to New Mexico and Montana. It overwinters from southern California to Mexico.

Biological Control of Aphids

No detailed studes have been carried out on the diet of the black-throated gray warbler, but they are known to be primarily insectivorous, eating insects and their larvae that they pluck from trees and shrubs. They also hover briefly to pick insects from various surfaces, and may fly out after flying insects. They tend to forage in the lower to middle levels of the forest, moving with slow, deliberate hops while looking around for insects.

Predation of ?Artemisia aphids

The bird feeding on aphids below was observed by Mark Rauzon in California, USA. We do not know the plant species (it is possibly an Artemisia), but the bird can be seen to be feeding on the clearly visible aphids infesting the plant.

Image reproduced by permission, copyright Mark Rauzon, all rights reserved.

Predation of alder aphids

Similarly Mark observed and photographed a black-throated gray warbler gleaning aphids from an alder (Alnus) leaf in Emeryville, California in October.

Gleaning aphids 1.

Gleaning aphids 2.

From the appearance and feeding positions of the aphids, the most likely prey species is the widely distributed common alder aphid (Pterocallis alni, see picture below of the aphid in Britain).

Pterocallis alni is native to Europe where it feeds on common alder (Alnus glutinosa) and other European alders. It was introduced to America early in the last century, where it proved able to feed on local American alders. For example in British Columbia it attacks red alder (Alnus rubra).

Further diet and foraging studies

Keane and Morrison (1999) found some variation in habitat-use patterns and foraging behaviour of the black-throated gray warbler in pinyon-juniper woodlands of the White and Inyo Mountains of eastern California from April to August. Use of shrubs was greater in the early summer of each year; which correlated with greater relative abundances of arthropod numbers sampled on bitterbrush and sagebrush. The greater number of arthropods on bitterbrush was associated with early season flowering phenology. Greater use of pinyon pine in the late summer of each year corresponded with increases in numbers of arthropods sampled on pinyon pine and decreases in arthropod numbers on bitterbrush and sagebrush.

Gleaning was the most frequently used foraging maneuver by each sex, ranging from 61-81% for males and 69-79% for females. Hover-gleaning was the second most frequently used foraging manoeuver by both sexes: It ranged from 12-20% for males and 8-24% for females. Use of aerial manoeuvers increased in late summer versus early summer of each year for both sexes, and ranged from 7-19% for males and 6-19% for females. No information was gathered on the types of insects eaten by the warblers.

Acknowledgements We are extremely grateful to Mark Rauzon for permission to reproduce his excellent pictures of black-throated gray warblers, both at at rest and feeding on aphids. For bird identification we have used Cornell Lab of Ornithology for the key characteristics, together with the latest Wikipedia account for each species. For aphids we have made provisional identifications from photos of living specimens, along with host plant identity using the keys and species accounts of Blackman & Eastop (1994) and Blackman & Eastop (2006) supplemented with Blackman (1974), Stroyan (1977), Stroyan (1984), Blackman & Eastop (1984), Heie (1980-1995), Dixon & Thieme (2007) and Blackman (2010). We fully acknowledge these authors as the source for the (summarized) taxonomic information we have presented. Any errors in identification or information are ours alone, and we would be very grateful for any corrections. For assistance on the terms used for aphid morphology we suggest the figure provided by Blackman & Eastop (2006).

Useful weblinks

References Keane, J.J. and Morrison, M.L. (1999). Temporal variation in resource use by black-throated gray warblers. The Condor 101, 67-75. Full text