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Generalized Aphid Predator (Passeriformes : Fringillidae)

Spinus tristis

American goldfinch

On this page: Identification & Distribution  Biological Control of Aphids  Predation of currant aphids,  milkweed aphids,  other aphids 

Identification & Distribution

The American goldfinch (Spinus tristis) is a small, sexually and seasonally dimorphic finch with a short notched tail and (in breeding season) a conical peachy-orange bill. Males in the breeding season are lemon yellow with a jet black cap. The wings are also jet black with white tips. The lesser wing coverts are yellow, and greater coverts are white-tipped. Uppertail coverts, parts of the ventral region, and the undertail coverts are white. Females in the breeding season are brownish olive-yellow on the back, and greenish-yellow on the forehead, throat, abdomen and rump. Parts of the ventral region, and undertail coverts are whitish. Winter birds have a grey or black bill and are drab, unstreaked brown with blackish wings and two pale wingbars.

First image Copyright Mdf under a Creative Commons Attribution-Share Alike 3.0 Unported License 
Second image Copyright Eric Dunham under a Creative Commons Attribution-NonCommercial ShareAlike 2.0 Generic License 

American goldfinches are found in floodplains, weedy fields and cultivated areas including back yards. They fly with a characteristic undulating pattern, often calling in flight. Spinus tristis are reputed to be almost exclusively seed eaters, although there is considerable evidence that they may be insectivorous on occasions. In summer the breeding range stretches across North America from coast to coast, extending northwards to Saskatchewan and southwards to northern California. Their winter range extends from southern Canada to parts of Mexico.

 

Biological Control of Aphids

There have been a surprising number of photos taken of American goldfinches predating insects - especially aphids.

Predation of currant aphids

The first picture below shows an American goldfinch predating aphids, on a golden currant (Ribes aureum) bush.

Image reproduced by permission Copyright Jeanne Dammarell of the Spokane Audubon Society,  all rights reserved

The birds were observed to fold back the leaves with a foot to expose the aphids underneath. A number of aphid species feed on golden currant in the USA. Judging by the lack of leaf blisters characteristic of Cryptomyzus ribis,  the most likely species are Hyperomyzus lactucae  (see picture below) or Aphis mimuli (not pictured), both of which can cause leaf curl.

The aphids on the golden currant could instead be Hyperomyzus ribiellus, but there was no evidence of the ant-attendance usual for that species.

Predation of milkweed aphids

A more surprising case of aphid predation by an American goldfinch is shown in the first picture below. The bird is feeding on milkweed aphids (Aphis nerii ) on swamp milkweed (Asclepias incarnata).

The milkweed aphid (see second picture below) is a distasteful aposematically-coloured aphid which obtains cardiac glycosides from its foodplant (e.g. Crawley (ed), 1992 ). Rothschild (1970)  found cardiac glycosides in Aphis nerii fed on both Nerium oleander and Asclepias curassavica.

First image reproduced by permission Copyright Hummer Haven,  all rights reserved
Second image reproduced by permission Copyright Alan Outen,  all rights reserved.

Examining the pair of images above, and noting the similarity in the yellow-black colour scheme of both bird and aphid, it is tempting to at least speculate whether Spinus tristis is itself a distasteful aposematically-coloured bird, and if it regularly samples Aphis nerii to obtain cardiac glycosides - to which it has evidently evolved some tolerance. However, there are very few aposematic bird species that are both toxic and brightly coloured - the only known examples being two species of the genus Pitohui from New Guinea, and two other genera from the same area. The most poisonous is the hooded pitohui (Pitohui dichrous, see picture below) which contains high concentrations of bratrachotoxins produced by the melyrid beetles which these birds eat (Dumbacher et al., 2004 ).

Image Copyright Mark A. Harper under a Creative Commons Attribution-Share Alike 2.0 Generic License 

It seems extremely improbable that such a common and widely distributed bird as the American goldfinch would be regularly feeding (largely unobserved) on such a noticeable insect as the milkweed aphid. So until someone reports that Spinus tristis is highly unpalatable, we may assume that the bright yellow colour of the (male) American goldfinch (derived from carotenoid pigments obtained from the diet) is related to species-recognition in order to reduce the risk of hybridization - rather than aposematism. There is some evidence that females prefer to mate with males that exhibit the brightest colours, and thus acquire the most skilled foragers (Rosen & Tarvin, 2006 ).

Predation of other aphids by American goldfinches

There is photographic evidence of an American goldfinch eating Cavariella aphids on dill.  Various Cavariella species  can occur in large numbers, on several umbellifers, where they are targeted by different predators. There are also photos of an American goldfinch eating aphids on a yellow composite,  and of an American goldfinch eating aphids on vine leaves. 

American goldfinches have also been filmed predating Eriosoma lanigerum,  the woolly apple aphid: Gold finch eats apple pest at Washington State University research orchard. 

Contrary to the evidence above, many authorities state that very little insect food is taken by the American goldfinch. Cornell Lab of Ornithology  suggests that goldfinches eat seeds "almost exclusively". Coutlee (1963)  notes that field observations have shown that a few insects are eaten, but in most areas insects act as "merely as a dietary supplement, since they are eaten in such small quantities". The Audubon field guide  states that the diet is primarily seeds, especially those of the daisy (composite) family, also those of weeds and grasses, and small seeds of trees such as elm, birch, and alder. It also eats buds, bark of young twigs, and maple sap. Young are fed regurgitated matter mostly made up of seeds. It feeds on insects to a limited extent in summer. A gardening website Yardener  puts a contrary view, stating that "in the spring, as much as half the goldfinches' diet is composed of insects that they eat and feed to their young". The issue remains open to debate at present, but we would caution that, even in the most rigorous studies, aphid predation by birds is under-recorded - partly owing to the difficulty of visually identifying their prey to be an aphid, but also because soft-bodied insects may disintegrate in crop contents (obtained by using purgative or neckband methods often used in bird feeding behaviour studies).

Acknowledgements

We especially thank Jeanne Dammarell of the Spokane Audubon Society  and Margy Terpstra of Hummer Haven Unltd  for permission to use their photographs. We also thank Plumpton College  for their kind assistance, and permission to sample.

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 high resolution photos of living specimens, and/or host plant identity. We have used 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

  • Coutlee et al. (1963). Maintenance behavior of the American Goldfinch. The Wilson Bulletin 75(4), 342-357. Full text 

  • Crawley, M.J. (ed). (1992). Natural enemies. The population biology of predators, parasites and diseases. Blackwell Scientific Publications, London. Google 

  • Dumbacher, J.P. et al. (2004). Melyrid beetles (Choresine): A putative source for the batrachotoxin alkaloids found in poison-dart frogs and toxic passerine birds. PNAS 101(45), 15857-15860. Full text 

  • Rosen, R.F. & Tarvin, K.A. (2006). Sexual signals of the male American Goldfinch. Ethology 112(6), 1008-1019. Full text 

  • Rothschild, M. et al. (1970). Cardiac glycosides in the oleander aphid, Aphis nerii. Journal of Insect Physiology 16(6), 1141-1145. Abstract