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.
- Goat Willow, Salix capraea. Native.
- Sycamore, Acer pseudoplatanus. Native.
- Oregon Grape. Mahonia sp. Ornamental.
- Citrus trees, Citrus. Mediterranean.
- Hawthorn. Native
- Damsons. Here. Native.
- Almond blossom. here. Mediterranean.
- Scrophularia. Here. Four large flowered species. Mediterranean, Canaries.
- 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.
- Anagyris foetida. The only documented native European plant that is exclusively pollinated by birds. Mediterranean.
- Giant fennel Ferula communis. Mediterranean.
- Brassica oleracea-group.
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.