Cyanistes caeruleus

Identification & Distribution

The Eurasian blue tit, Cyanistes caeruleus, has an azure blue and dark blue line passing through the eye, and encircling the white cheeks to the chin, giving the bird a very distinctive appearance (see first picture below) (cf. the great tit which has a bluish-black crown, a black neck, throat, bib and head and white cheeks and ear coverts). The forehead and a bar on the wing are white. The nape, wings and tail are blue and the back is yellowish green. The underparts are mostly sulphur-yellow with a narrow blackish line in the breast centre. The bill is black, the legs bluish grey, and the irises dark brown. The sexes are similar, but under ultraviolet light, males have a brighter blue crown. The body length is usually 12 cm (4.7 in) long with a wingspan of 18 cm (7.1 in) for both sexes. Juvenile blue tits (see second picture below) have pale yellow cheek patches (cf. adults which have white cheek patches) and greyish-green upperparts and crown.

First image copyright Francis C. Franklin under a Creative Commons Attribution-Share Alike 3.0 License.
Second image copyright Andrew Morffew under a Creative Commons Attribution 2.0 Generic License.

The Eurasian blue tit is found in a range of habitats from gardens to forest, wherever there are trees. They mainly feed on invertebrates but switch to seeds and other plant material in the winter when insects are scarce. Cyanistes caeruleus are found throughout most of Europe, as well as North Africa, Turkey and Iran.

Biological Control of Aphids

The blue tit is well known as an important predator of several species of aphids, and doubtless makes an important contribution to the biological control of aphids in gardens where supplementary feeding ensures artificially high blue tit populations. They appear to especially target aphids when feeding older nestlings.

Predation of rose aphids

For gardeners, rose aphids are one of the biggest banes they have to cope with. Heavy aphid infestations on the young shoots can cause considerable damage to the plant, and black sooty moulds may grow on the honeydew that aphids excrete. Blue tits are very efficient predators of rose aphids - the bird below is picking them off the leaves.

First image above, reproduced by permission, copyright 2015 Marsha Levine all rights reserved.

There are twelve different species of aphids on roses in Britain, with more in other parts of Europe, and at least 30 worldwide. The common rose aphid Macrosiphum rosae (second picture above) is the most common and the most damaging.

Predation of sycamore aphids

Blue tits also consume large numbers of tree-dwelling aphids. In a blog on bird behaviour - The Rattling Crow - Africa Gomez reports a fascinating set of observations about blue tit predation of Drepanosiphum platanoides, the common sycamore aphid.

Adult blue tits were moving about amongst the leaves (see picture below) searching for (and eating) the winged aphids and green caterpillars.

Image copyright Africa Gómez under a Creative Commons Attribution-Share Alike 3.0 License.

The author's attention was drawn by the rasping begging calls of blue tit fledglings (see picture below) coming from a sycamore tree.

Image copyright Africa Gómez under a Creative Commons Attribution-Share Alike 3.0 License.

The underside of some sycamore leaves were covered with winged aphids. Drepanosiphum platanoides aggregate on particular leaves, but maintain a minimum space between each other (Kennedy & Crawley, 1967) giving the evenly-spaced pattern shown below. Very few nymphs are present at this time of year, because most adult Drepanosiphum platanoides undergo a reproductive aestivation or diapause.

First image copyright Africa Gómez under a Creative Commons Attribution-Share Alike 3.0 License.

Diapausing Drepanosiphum platanoides (see above) look quite different from their predecessors earlier in the year. In spring this aphid has dark cross-bars on the abdomen but in summer there are no cross-bands, and the aphid is generally paler.

The adult blue tits, and sometimes the youngsters, were foraging for the aphids, resulting in clouds of sycamore aphids cascading down from the tree - visible as white spots against the dark leaves in the image below.

Image copyright Africa Gómez under a Creative Commons Attribution-Share Alike 3.0 License.

Dr. Gómez also posted a video of adult blue tits foraging and the aphids flying off.

Predation of thistle aphids

In their natural history blog - Moorhen - Roy & Marie Battell recorded adult and juvenile blue tits feeding from an aphid infestation on thistles growing in their wildlife-friendly garden. In the picture below a blue tit is eating an ant which was attending the aphid colony, but it is doubtless consuming both the aphids and their attending ants. The ants provide a degree of protection to the aphids, since they will undoubtedly attack the intruding blue tit.

Image reproduced by permission, copyright Moorhen all rights reserved.

Those aphids were Aphis fabae cirsiiacanthoidis (the 'spindle-thistle aphid', shown below). Like many species of aphids, Aphis fabae cirsiiacanthoidis is usually attended by ants enjoying honeydew produced by these aphids. These ants provide a measure of protection to the aphids against predators.

Note: Although this subspecies is very similar to the black aphid found on garden beans (Aphis fabae fabae, = Aphis fabae), the spindle-thistle aphid does not like to feed on beans, so a thistle aphid infestation is unlikely to spread to a crop - even if those aphids survive the blue tits.

Predation of umbellifer aphids

Graham Hall found this fledgling blue tit feeding on black aphids (probably a mixture of Aphis fabae and Cavariella species) on an umbellifer.

Image reproduced by permission, copyright Graham Hall all rights reserved.

Umbellifers provide a rich source of aphids for several species of birds.

Predation of pea aphids on broom

Smith (1966) studied the predators of the pea aphid (Acyrthosiphon pisum, see picture below) on broom. He found that birds were always present as predators, but were only numerous when aphid populations were high. The most common bird species predating pea aphid were blue tits (Cyanistes caeruleus) and chaffinches (Fringilla coelebs). They fed on aphids mainly in the morning and late afternoon, but when aphids were at their peak, birds were present at all times of day. Winged aphids (alatae) were preferentially eaten by blue tits, and then wingless adults & (near-adult) fourth instar nymphs were taken, before the birds moved to other shoots.

An attempt was made to assess the importance of predation by birds by enclosing a broom bush in a cage at the aphid peak time and comparing the population with those on uncaged bushes. Over a three week period, numbers declined in both caged and uncaged bushes, but the reduction on the uncaged bushes was much greater. It was thought that birds were the most important predators because they consumed large numbers of aphids and they selected the largest, reproductively active, aphids. This study is one of very few that attempt to quantify aphid predation by birds.

More cases of aphid predation by blue tits

Back in the days before blogs, Way & Banks (1964) assessed egg mortality of Aphis fabae in Hertfordshire during the winters of 1958-59 and 1959-60. Mortality ranged from 18 to 73% on individual bushes of Euonymus europaeus (spindle) at different sites. Field and laboratory experiments indicated that the average mortality was unlikely to exceed about 40% and was probably relatively unimportant in limiting subsequent increase to pest density. Winter temperatures in Southern England do not harm eggs, except perhaps at hatching time. The two main groups of predators were anthocorid bugs and birds, especially blue tits.

Research by Cowie & Hinsley (1988) on the feeding ecology of great tits (Parus major) and blue tits (Parus caeruleus) breeding in suburban gardens also revealed the importance of aphids in the diet. Male and female blue tits brought similar types of prey to their nestlings, and blue tits brought more aphids than great tits. Proportionately more spiders were fed to young nestlings (3-9 days), whilst older nestlings (14-15 days) received more aphids and artificial food. There were significant differences between the bird species: great tits brought more spiders (Araneae) and flies (Diptera), blue tits brought more aphids (Hemiptera). In blue tits, the tendency to bring prey in 'runs' was most pronounced for aphids, and for artificial food - reflecting the aggregated nature of both aphids and food on a bird table.

How to increase blue tit numbers

There are various ways to increase the number of blue tits using a garden:

1. Provide supplementary feeding through the winter and well into summer.

   For example in gardens which had a bird feeder, Orros & Fellows (2012) found significantly fewer aphids, and shorter colony survival times, in pea aphid colonies exposed to predation compared to protected 'control' aphid colonies. Conversely, in gardens that did not feed birds, they found no difference between exposed colonies and protected colonies.

2. Clean (unfrozen) water is also important, both for drinking and bathing. Water should be about 2.5-5 cm deep if you want to encourage blue tits to bathe.

3. Blue tits are woodland birds, and they like woody vegetation - mainly because they will search for small caterpillars, and other insect life such as aphids. Hence hedges of hawthorn, pyracantha or dog rose will encourage these birds. Don't worry that they will spend all their time feeding on insects in the hedge - if you get a lot of aphids on your roses or beans, the blue tits will find them as well. If you only have a few aphids on your flowers or crops, they will probably cause insufficient damage to merit control. Most species of aphids cause no measurable harm, and are almost never noticed - other than by the occasional scientist.

   Note: if blue tits are foraging in the vicinity, their chances of finding aphid colonies is much higher because birds often identify feeding places early morning to revisit and exploit later that day.

4. Encourage blue tits to nest in your garden by providing nest boxes and nesting material. This is especially important for aphid control since blue tits feed aphids preferentially to older nestlings.

Nest box with blue tit, image copyright David Friel under a Creative Commons Attribution 2.0 Generic License.

Acknowledgements We especially thank Marsha Levine, Africa Gomez (The Rattling Crow), Roy & Marie Battell (Moorhen) and Graham Hall for permission to use their photographs. We also thank Middle Farm, East Sussex and the UK Forestry Commission Bedgebury Pinetum for their kind assistance, and permission to sample. For bird identification and diet characteristics we have used British Bird Lovers and BTO Bird identification videos 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. In the great majority of cases, identifications have been confirmed by microscopic examination of preserved specimens. 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 Cowie, R.J. & Hinsley, S.A. (1988). Feeding ecology of great tits (Parus major) and blue tits (Parus caeruleus), breeding in suburban gardens. Journal of Animal Ecology 57(2), 611. Full text Kennedy, J.S. & Crawley, L. (1967). Spaced-out gregariousness in sycamore aphids Drepanosiphum platanoides (Schrank) (Hemiptera, Callaphididae): With a statistical appendix. Journal of Animal Ecology 36(1), 147-170.  Full text Orros, M.E. & Fellows, D.E. (2012). Supplementary feeding of wild birds indirectly affects the local abundance of arthropod prey. Basic and Applied Ecology 13(3), 286-293. Abstract Smith, B.D. (1966). Effects of parasites and predators on a natural population of the aphid Acyrthosiphon spartii (Koch.) on broom (Sarothamnus scoparius. Journal of Animal Ecology 35(2), 255-267. Full text Way, M.J. & Banks, C.J. (1964). Natural mortality of eggs of the black bean aphid, Aphis fabae Scop., on the spindle tree, Euonymus europaeus. Annals of Applied Biology 54(2), 255-267. Abstract