Oreothlypis peregrina

Identification & Distribution

The Tennessee Warbler (Oreothlypis peregrina) has long wings, a short tail and a thin pointy bill. The breeding male has an olive back, shoulders, rump and vent (see first picture below). It has a slate grey neck, crown and eyeline, and brownish-black flight feathers. The underside is plain whitish, although some some have yellow-washed undersides. There are no strong wingbars. The female is similar to the male, but is duller and has a green tinge to the underside (second picture below is either a female or immature).

First image Copyright Jerry Oldenettel under a Creative Commons Attribution-Share Alike 2.0 Generic license
Second image Copyright Brian Plunkett under a Creative Commons Attribution 2.0 Generic License

The Tennessee Warbler breeds in the north of the USA and throughout much of Canada. It migrates to Central America and northern Colombia and Venezuela where it overwinters. Oreothlypis peregrina is a diet generalist feeding mainly on insects, especially spruce budworm, but also taking nectar, fruit and seeds.

Biological Control of Aphids

The Tennessee warbler is best known as a predator of spruce budworm (Choristoneura species, of the tortricid moth family). Bird numbers show a large and rapid response to budworm outbreaks, and there is evidence that birds may play a role in determining the mean level of oscillations (Venier & Holmes, 2010). Oreothlypis peregrina will also feed on aphids if available, and from the observations below birds appear to aggregate in areas where there are large aphid populations.

Predation of beech blight aphids

Tracy, from Ohio USA, made a fascinating set of observations one autumn which she recounts in Autumn Warbler Incidents, part 3 - An insect feast. She watched a Tennessee warbler feeding on a vast colony of the beech blight aphid (Grylloprociphilus imbricator).

Image reproduced by permission, Copyright Seasons Flow, all rights reserved.

The beech blight aphid is covered in a light bluish-white wax. This is commonly thought to act as a defense against predators, either by hiding prey-identification cues from vision-guided predators ( Moss et al., 2005), or by making them distasteful.

Image reproduced by permission, Copyright Seasons Flow, all rights reserved.

Grylloprociphilus imbricator amasses by the thousands on the twigs, small branches, and the undersides of foliage primarily of the American beech (Fagus grandifolia).

Image reproduced by permission, Copyright Seasons Flow, all rights reserved.

Beech blight aphid-infested beech trees can appear to have their smaller branches and twigs covered with snow. Sooty mould fungi often colonise the honeydew on the surface of the plants. When they are disturbed, these aphids will raise the posterior end of their body and sway. This action produces a dance-like effect that ripples throughout the colony, leading to Grylloprociphilus imbricator getting the name the "boogie-woogie aphid".

This dance-like effect can be seen in a video: 'Dancing Boogie-Woogie Aphids'. Should you wish to compare it to a real boogie-woogie dance, feel free (but be advised - this is not for the delicate nor infirm).

Large, wax-covered colonies of Grylloprociphilus imbricator are known to last over several months on exposed twigs of American beech (Fagus grandifolia) Aoki et al. (2001) hypothesized that the colonies could not persist for such a long period without defense against predators. They found that nymphs of the second generation attacked moth larvae that had been artificially introduced into the aphid colony. This was interpreted to mean that other intruders to the colony (for example predatory coccinellid larvae) would also be attacked.

Image reproduced by permission, Copyright Seasons Flow, all rights reserved.

Whether these nymphs would have any effect at all on vertebrate predators such as birds is unknown, although Aoki et al. found the nymphs would 'sting' their fingers.

Predation of elm aphids

Joe Sebastiani, of the Delaware Nature Society, reports that aphids swarm around an American elm tree each fall, around late September into early October. Many types of birds gorge on them, including Tennessee warblers. The picture below shows a Tennessee Warbler hunting for aphids, some of which are visible in flight (as blurred grey objects) behind and beneath the bird.

Image reproduced by permission, Copyright Joe Sebastiani, all rights reserved.

The aphids attracting this warbler are shown below massed on the trunk of the elm tree.

Both images reproduced by permission, copyright Joe Sebastiani, all rights reserved.

The aphids in question are in the Eriosomatinae subfamily, and were most likely either Eriosoma americanum or Colopha ulmicola. These overwinter on elm in the egg stage. In spring these eggs hatch and the aphids form pale green elongate cockscomb-type galls on the leaves of elm. Winged forms emerge in June-July and fly to grasses (Eragrostis species) where they form waxy colonies - which reproduce parthenogenetically on the upper grass leaves and stems. Winged forms return to elms in September-November where they produce the sexual generation (and also tend to get eaten by birds). After mating the females then lay their eggs on the elm bark.

Other ornithologists have also found Tennessee Warblers feeding on elm aphids in autumn.

Predation of other aphid species

Dave Leatherman, in Colorado Birds - More chasing sites, noted several warbler species including the Tennessee Warbler feeding on aphids in pine trees at Christmas time. The aphids were probably Eulachnus rileyi (the picture below shows this aphid in Sussex, UK).

At Twin Lakes, in northeast Boulder, Leatherman observed Eulachnus rileyi being fed upon by Tennessee Warbler, Yellow-rumped Warbler, Nashville Warbler and Hammond's Flycatcher. He felt they were probably the same aphid in pines utilized by the Black-throated Green Warbler found by another observer in Boulder.

A photograph by Gerald Markhoff, also shows a Tennessee Warbler feeding on aphids, on the underside of leaves of an unidentified bush.

Anich (2014) reported ground foraging for invertebrates by mixed flocks of typically arboreal passerines, including the Tennessee warbler, during an unusually cold spring. He attributed this to the fact that leaf burst was delayed, thus limiting typical arboreal foraging. Newell et al (2014) described the foraging behaviour of the Tennessee warbler and other migrant warblers in Venezuelan shade coffee. They noted that the warblers provided an important ecological service by reducing the number of arthropods among the coffee. The Tennessee warblers, along with the other leaf-gleaning warblers, selected shimbillo trees (Inga species) which are commonly planted shade trees in shade-coffee farms for foraging. They are known to support a greater abundance of arthropods than other trees. Huange found that that warblers possessing specialist diets are most likely to be those now identified as declining, threatened, endangered or extinct species, whereas warblers that were diet generalists had a conservation status of 'least concern'.

Acknowledgements We especially thank Tracy of Seasons Flow and Joe Sebastiani of the Delaware Nature Society for permission to use their photographs. For bird identification we have used Cornell Lab of Ornithology for the key characteristics, together with the latest Wikipediaaccount for each species. For aphids we have made provisional identifications from high resolution photos of living specimens, along with 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 Anich, N.M. (2014). Ground foraging by typically arboreal passerines during the cold spring of 2013. The Passenger Pigeon 76(1), 89-95. Full text Huang, A.H. . Generalists and specialists warblers and their conservation statuses in North America. Undergraduate research paper University of British Columbia Full text Moss, R. et al. (2006). Mask of wax: Secretions of wax conceal aphids from detection by spider's eyes. New Zealand Journal of Zoology 33(3), 215-220. Full text Newell, F.L. et al. (2005). Foraging behavior of migrant warblers in mixed-species flocks in Venezuelan shade coffee: interspecific differences, tree species selection, and effects of drought. Journal of Field Ornithology 85(2), 134-151. Google Scholar Venier, L.A. & Holmes, S.B. (2010). A review of the interaction between forest birds and eastern budworm. Environmental Review 18, 191-207. Full text