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Identification & Distribution

The downy woodpecker is the smallest of North American woodpeckers. The adult bird is mainly black on the upperparts and wings with a white back, throat and belly, and white spotting on the wings. The male (see first picture below) has a red patch on the back of the head, the female (see second picture below) has no red patch, and juveniles have a red cap. There are also two white bars on the head, one above the eye and one below. The bill is shorter than the head length (cf. hairy woodpecker which has its bill equal to its head length). The tail is black with white outer feathers barred with black (cf. hairy woodpecker which has no black bars on the white outer tail feathers).

First image copyright Betty Matsubara under a Creative Commons Attribution-Share Alike 2.0 Unported License
Second image copyright Russ under a Creative Commons Attribution 2.0 Generic License

Downy woodpeckers are found in open woodlands, particularly amongst deciduous treees. It has also adapted well to the man-made environment being found in orchards, city parks, backyards and vacant lots. The distribution of the downy woodpecker covers most of the United States and Canada apart from the deserts of the southwest and the tundra of the north. They are not generally migratory although northern birds may move further south in winter.

 

Biological Control of Aphids

Downy woodpeckers are omnivorous. Their primary foods include insects and other arthropods, as well as fruit and seeds. Beetles, weevils, ants, bugs, aphids and caterpillars are among the insects eaten, as well as spiders. Downy woodpeckers glean insects from the surfaces of trees, shrubs and large weeds, probe into crevices and excavate shallow holes into wood to find food. Confer & Paicos (1985) looked at predation by downy woodpeckers of insects in 'ball galls' - produced by the gall fly (Eurosta solidaginis) on Canadian goldenrod (Solidago canadens) - in New York, USA. The rate of predation and abundance of abandoned fields with goldenrod galls suggest the assorted insects therein may be a major source of winter food for this woodpecker. The downy woodpecker selectively uses the exit tunnel prepared by the large gall fly larva to extract this prey. If the woodpecker fails to find an exit tunnel, the probability of the gall containing one of the smaller parasitic or predaceous insects is increased and the woodpecker is more likely to abandons such galls without completing a chiselled hole. Downy woodpeckers drink water by scooping it up with their bill.

Predation of willow bark aphids

Tim Hopwood took the wonderful image of a male downy woodpecker on a rather thin stem eating aphids; the aphids are on the stem just above where the leaf in the foreground joins the stem. We cannot be certain of the tree species, but it looks like a Salix. Given their size, feeding position and colour, these aphids are most likely black willow bark aphids (Pterocomma salicis) - or possibly giant willow aphids (Tuberolachnus salignus).

Image reproduced by permission, copyright Tim Hopwood all rights reserved.

We have suggested in the past that bird predation may be a selective pressure acting on the giant willow aphid given the black and red aposematic coloration shown by this aphid, at least close-up. Then we thought treecreepers (Crithea species) were a likely predator, but woodpeckers are equally viable candidates.

Predation of alder woolly aphids

Marne Titchenell obtained some nice images of downy woodpecker eating aphids in Columbus, Ohio, USA. The white wax-covered aphids being eaten in the picture below are the woolly alder aphid (Prociphilus tessellatus).

Image reproduced by permission, copyright Marne Titchenell all rights reserved.

Prociphilus tessellatus is a host-alternating aphid species which lays its wax-covered eggs on silver maple (Acer saccharinum) in autumn. In spring the eggs hatch and these aphids feed along the midrib of the maple leaves. Then in early summer they produce a generation that mature to winged adults, which migrate to alder trees (Alnus species) where they produce large colonies through the summer (see first picture below) often attended by ants. Perhaps surprisingly, despite the additional metabolic cost of producing ant-attractive honeydew, woolly alder aphids retain their heavy waxing when thus attended. The ant in the second picture below is a carpenter ant (Campanotus).

First image copyright Kirill Ignatyev under a Creative Commons Attribution-NonCommercial 2.0 Generic License
Second image copyright Judy Gallagher under a Creative Commons Attribution 2.0 Generic License

In younger colonies, most of the aphids may be along the underside of the branch, necessitating some acrobatics by the bird to get lunch (see picture below).

Image reproduced by permission, copyright Marne Titchenell all rights reserved.

Aphids provide a rich source of both proteins and sugars, which are especially beneficial to birds and their young in the breeding season.

More cases of aphid predation

Jerry Vis captured a wonderful image of a downy woodpecker eating aphids.

Similarly in 'JungleDragon' we found a downy woodpecker taking advantage of leaf aphids - food in plenty.

Although the downy woodpecker may utilise aphids more extensively than other (larger) woodpeckers, one can find evidence of even the great spotted woodpecker having a meal of aphids on occasion.

Further diet and foraging studies

Jackson (1970) studied the foraging ecology of downy woodpeckers. Their foraging niches differ between the sexes according to limb height and diameter. Males tend to forage on small branches, generally 5 cm in diameter or less; females tend to forage on the trunk and larger limbs. The mean foraging height of males in live trees differs from that of males in dead trees, whereas there is no difference for females. Both male and female Downy Woodpeckers use sub-surface foraging techniques to a greater extent during the winter and surface gleaning during the warmer months. There is also greater use of dead trees during winter. Some tree species seem to be favoured and some avoided by one or both sexes.

Peters & Grubb (1983) then assessed whether the different foraging niches of male and female Downy Woodpeckers were determined primarily by genetic preference, or were due to displacement of foraging behavior of one sex in the presence of the other. In one experimental woodlot where males had been removed, the remaining females became male-like in their foraging behavior. In a second experimental woodlot where females were removed, males did not change their foraging behavior. These results supported the hypothesis that sex-specific foraging niches in Downy Woodpeckers are caused by female avoidance of the foraging microhabitat of socially dominant males.

Acknowledgements

We especially thank Marne Titchenell of Buckeye Yard & Garden onLine, and Tim Hopwood, for permission to use their photos of downy woodpeckers 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

  • Confer, J.L. & Paicos, P. (1970). Downy Woodpecker predation at goldrenrod galls. Journal of Field Ornithology 56(1), 56-64. Full text

  • Jackson, J.A. (1970). A quantitative study of the foraging ecology of Downy Woodpeckers. Ecology 51(2), 75-83. Abstract

  • Peters, D. & Grubb, T.C. (1983). An experimental analysis of sex-specific foraging in the Downy Woodpecker, Picoides pubescens Wm. Ecology 64(6), 1437-1443. Abstract