The summer Blackcaps

At the end of April, the arrival of migrant Blackcaps is marked by their beautiful fluty song. Males arrive a few days earlier than females and settle in their breeding territories. I heard the first one in the park on the 19th of April. Blackcaps usually sing from cover in trees and bushes, and the rapidly sprouting leaves will make it more difficult to spot them singing as the spring avances. Today I found a female in the territory where the male had been singing. She was very busy in a tall Sycamore tree, Acer pseudoplatanus, picking aphids from under the leaves, stretching and balancing on the fine branches. The male wasn't far, singing and feeding on aphids like her.

The female feeding.

Note the two aphids on the main veins of the leaf near the male's bill and...

...a few seconds later they are gone.

A short clip of the male feeding.

Summer and winter Blackcaps and evolution

During April, Blackcaps return to thickets and woodland, where the male's beautiful song joins the resident birds. These Blackcaps have just arrived from their winter quarters in Spain and North and coastal West Africa. We tend to think of migration behaviour as something fixed, but recent research shows that many birds have recently changed their migration routes. One of these is the Blackcap.
In the last 50 years or so, a small contingent of Blackcaps have started to winter in the UK. About 30% of gardens have a wintering Blackcap in recent years. Is it because these birds are staying here year round? Surprisingly not, they are migrants from the continent, mostly from Germany, where they breed. This migration behaviour has evolved in a few generations, with these birds taking advantage of milder winters, increased food availability in gardens and a shorter migration route. They usually migrate to Britain and Ireland as winter berry and winter temperatures drop at the beginning of the year. The direction in which blackcaps prefer to migrate is innate, strongly influenced by one or a few genes: the offspring of blackcaps migrating NW prefers to migrate NW, and the offspring of hybrids between NW and SE migrants have an intermediate migratory direction. Birds with both migration strategies coexist in South Germany and Austria. The different migration routes mean that they arrive at their breeding grounds at different times, and therefore tend to pair with birds with a matching migration strategy. The birds involved in the shorter Germany-UK migration trips have evolved shorter, more rounded wings than the Mediterranean migrants. In addition, they have slightly different plumage colour, with might also help them find the right mate. Bill shape is also different, possibly reflecting a different diet: birds wintering in the Mediterranean feed mostly on fruit, while British wintering birds rely mostly on fat and seeds in gardens. These differences are reflected in incipient genetic differentiation between blackcaps with different migration routes present in the same area during the breeding season, which could mean that both populations eventually become different species.

A wintering female Blackcap feeding in my garden. 26/1/2013.

A summer migrant male singing today.

More information

Rolshausen, G., Segelbacher, G., Hobson, K., and Schaefer, H. (2009). Contemporary Evolution of Reproductive Isolation and Phenotypic Divergence in Sympatry along a Migratory Divide Current Biology, 19 (24), 2097-2101 DOI: 10.1016/j.cub.2009.10.061
Fiedler, W. (2003). Recent changes in migratory behaviour of birds: A compilation of field observations and ringing data Avian migration, Springer Berlin Heidelberg., 21-38 DOI: 10.1007/978-3-662-05957-9_2

Nectar feeding Blackcap

Yesterday I heard a male Blackcap singing on my way to work. Although it was high on a tree, luckily, I had a small pair of binoculars with me and I found him on a Sycamore in bloom. While I watched it, the Blackcap alternated singing and clearly drinking from the Sycamore flowers, in one occasion clinging upside down from a branch like a tit to reach them. It is well documented that several European birds, especially warblers, regularly drink nectar (I have posted on Blue Tits feeding on Mahonia nectar before in this blog). I could only take some distant shots of this behaviour, and when I got back, I did some research on the nectar feeding habits of the Blackcap.
 The Blackcap is actually one of the most nectarivorous European warblers. Nectar feeding in warblers is most common during spring migration, and has been documented in many stopover sites. Blackcaps often have stained faces due to their flower feeding habits (you can see some examples here and here. This can result on the formation of a 'pollen horn' a matted mass of nectar, pollen and feathers around their bills. Given that their long, thin bill leaves flowers intact after feeding, that they visit many flowers and the presence of pollen on their faces indicating regular contact with the flower's anthers, it is not surprising that Blackcaps can also act as pollinators.
 Experiments on birds trapped during migration indicates that migrating warblers seem to prefer nectar to mealworms when both are offered simultaneously. Nectar feeding is not a last food resort for emaciated birds, but a valuable source of easily digested carbohydrates when birds have reduced digestive systems and high energy demands.
 Blackcaps feed on nectar from a large diversity of flowers, both native and introduced. From the references below, I have compiled the following list of visited plants, which also includes some Mediterranean plants that they encounter during migration, and the Sycamore too. I note the native status in the UK.

1. Goat Willow, Salix capraea. Native.
2. Sycamore, Acer pseudoplatanus. Native.
3. Oregon Grape. Mahonia sp. Ornamental.
4. Citrus trees, Citrus. Mediterranean.
5. Hawthorn. Native
6. Damsons. Here. Native.
7. Almond blossom. here. Mediterranean.
8. Scrophularia. Here. Four large flowered species. Mediterranean, Canaries.
9. Aloe arborescens This female has so much orange pollen on its chest that almost looks like a robin. South African plant widely introduced in the Mediterranean.
10. Anagyris foetida. The only documented native European plant that is exclusively pollinated by birds. Mediterranean.
11. Giant fennel Ferula communis. Mediterranean.
12. Brassica oleracea-group.

More information

Ortega-Olivencia, A., Rodríguez-Riaño, T., Valtueña, F., López, J., & Devesa, J. (2005). First confirmation of a native bird-pollinated plant in Europe Oikos, 110 (3), 578-590 DOI: 10.1111/j.0030-1299.2005.13877.x

Ford, H. (1985). Nectarivory and Pollination by Birds in Southern Australia and Europe Oikos, 44 (1) DOI: 10.2307/3544053

Schwilch, R., Mantovani, R., Spina, F., & Jenni, L. (2001). Nectar consumption of warblers after long-distance flights during spring migration Ibis, 143 (1), 24-32 DOI: 10.1111/j.1474-919X.2001.tb04166.x

Jacopo G.Cecere, Costanza Matricardi, Beatrice Frank, Simona Imperio, Fernando Spina, Gabriel Gargallo, Christos Barboutis & Luigi Boitani. (2010). Nectar exploitation by songbirds at Mediterranean stopover sites. Ardeola, 57(1), 143-157.

Ortega-Olivencia, A., Rodriguez-Riano, T., Perez-Bote, J., Lopez, J., Mayo, C., Valtuena, F., & Navarro-Perez, M. (2011). Insects, birds and lizards as pollinators of the largest-flowered Scrophularia of Europe and Macaronesia Annals of Botany, 109 (1), 153-167 DOI: 10.1093/aob/mcr255