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Generalist Aphid Predator (Passeriformes: Sylviidae)Sylvia cantillans sp. grp.Subalpine warblerOn this page: Identification & Distribution Biological Control of AphidsIdentification & DistributionThe adult male of Sylvia cantillans species group (first image, below) has a grey back, brick red underparts and white malar stripes. The female (second image, below) is mainly brown above with a greyer head, and whitish below with a pink flush.
First image copyright Mick Sway under a Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0). The most recent proposals by Svensson (2016) split the complex into three different species:
The subalpine warbler species group are birds of dry open country, often on hill slopes. They are insectivorous, but will also take berries. The breeding ranges of the different species are given above, but vagrants can be found over much of western and northern Europe. Biological Control of AphidsThe subalpine warbler forages by actively moving through dense vegetation and picking prey off vegetation with its bill. It is therefore well adapted to being an aphid predator when the prey is available, and there are several accounts of subalpine warblers feeding on aphids. Predation of sycamore aphidsOne set of observations of subalpine warblers eating aphids comes from Jim Almond (the Shropshire birder) on the Scilly Isles in October 2016. A subalpine warbler was observed foraging through the leaves of a sycamore tree (Pseudoplatanus platanoides) which were covered in aphids (see pictures below). The aphids are light green and can be seen feeding alongside the main veins of the leaf. The white objects are the aphid exuvia.
Both images above by permission copyright Shropshire Birder, all rights reserved. The aphid prey is the 'common sycamore aphid', Drepanosiphum platanoidis, one of the most commonly-observed aphids in Britain. Given the photos were taken in October, most of the aphids feeding on the senescing sycamore leaves are likely to be immature sexual forms, either winged males or wingless oviparae. Several species of birds have been observed feeding on sycamore aphids including blue tits (Cyanistes caeruleus).
Predation of bird cherry-oat aphidsOver 25 years Glutz von Blotzheim (2010) conducted a very substantive study on bird species feeding on the bird cherry-oat aphid (Rhopalosiphum padi) on bird cherry (Prunus padus) bushes in autumn. In all he recorded 36 species of birds preying on the aphids. Chiffchaff (Phylloscopus collybita) and blackcap (Sylvia atricappilla) were the most numerous and most intensively-feeding aphid predators, but in addition there was a four-day-stopover of a Subalpine Warbler, in his region an extremely rare vagrant. The warbler fed alternatively on aphids and fruits.
The bird cherry oat aphid (see picture above) is a host alternating species whose winter host is bird cherry. Its summer hosts are cereals and grasses. Very large numbers of these aphids can build up on bird cherry in spring to early summer prior to migration, and again in the autumn when these aphids return to their primary (winter) host. The aphids above are mature and developing oviparae which will mate with winged males before laying overwintering eggs on the bird cherry twigs. Videos of subalpine warblers eating aphids'Fligue11' provides an excellent video of a male subalpine warbler feeding on aphids in Malta. Unfortunately the type of tree is not given, so we cannot identify the prey aphid. The author notes that subalpine warblers are common in Malta during migration. The Internet Bird Collection (IBC) provide a video by Alex Carcia of a subalpine warbler foraging in a flowering broom bush. We cannot be certain, but the broom appears to be heavily infested with dark-coloured aphids, most likely the broom aphid (Aphis cytisorum) which this bird is avidly consuming. Further diet and foraging studiesThere are few detailed studies of the diet of subalpine warblers, and the few studies that have been done provide no further evidence for aphids forming an important component. Stoat & Moreby (1995) used faecal analysis to look at the premigratory diet of migrant passerines including three European Sylvia warblers in the western Sahel. Since their faecal analysis was microscopy following washing, soft-bodied insects such as wingless-aphids, would not have been recorded. For the subalpine warbler, 21% of the remains were of arthropods with the rest being fruit. Of the arthropods identified, most were ants or beetles, but many of them were unidentifiable. Fruits of orimeet (Salvadora persica, see picture below) were exploited by all species, with berries over 5 mm diameter being selected.
The fruits are not only eaten by birds - the image above shows a young Maasai gentleman enjoying them in Kajiado District, Kenya. Orimeet, the Maasai name for this shrub, has peppery-tasting fruits - and roots (which are occasionally used to spice their milky tea). Morel & Morel (1992) looked at habitat use in West Africa by migrant warblers. An important feature of West Africa as a wintering zone is the presence of the two great river basins, the Senegal and the Niger, which are responsible for the Sahel being far richer than would be expected at this latitude. Morel & Morel noted that previous observers found thousands of Subalpine Warblers in the reedbeds of Lake Horo, Mali, and in the shrub vegetation along the river Niger from January to March. These reedbeds, mostly Typha species, may cover hundreds and sometimes thousands of hectares. Reedbeds provide major roosting sites, in addition to being a likely source of insect food. It is unclear whether subalpine warblers exploit the rich aphid resources in wetlands. Vafidis et al (2014) also looked at habitat use among warblers in the Sahel Region during the non-breeding Season. Although subalpine warblers and chiffchaffs were regularly captured in wetlands, these captures occurred mainly in the scrub-Phragmites interface, rather than in the pure stands of Phragmites reeds where Eurasian and African reed warblers and sedge warblers were all regularly captured. This is consistent with more limited exploitation by subalpine warblers and chiffchaffs of the invertebrate-rich Phragmites stands compared to reed and sedge warblers. And some nectar feeding as wellSalewski et al. (2006) made observations on nectar feeding in spring by Palaearctic migrant birds at the oasis of Ouâdane stopover site in the Sahara. A total of seven warbler species were observed consuming nectar, including the Subalpine Warbler, from five different tree species (Balanites aegyptiaca, Maerua crassifolia, Capparis decidua, Acacia raddiana, Ziziphus mauritiana).
The picture above shows a butterfly taking nectar at a Maerua bush in Kenya. Flowering trees were available throughout the entire presumed migration period and, as well as providing nectar, they attracted many insects. They suggested that the phenology of flowering trees might be crucial for bird migration in spring, and might offer a solution to the phenomenon known as 'Moreau's paradox" - that migrants successfully lay down fuel reserves prior to spring migration during the dry season, when potential resources are thought to be at their lowest. There were hardly any observations of nectar-feeding migrants in autumn, owing to the scarcity of flowering trees.
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