Goldfinches' diet is mainly composed of small seeds that they can deftly extract from seed heads using their long, thin bill. I have blogged before on their agility and skill manipulating seeds and stems with their feet. Loose flocks will move about in search of food. In spring they feed on dandelions, later in the summer on teasels and creeping thistles, whilst in the winter they feed on tree seeds including alder, birch and ash. They are an adaptable species, and have taken to feeding on garden feeders, especially on niger seeds and sunflower seeds. They have also learned to feed on the dangling fruits of plane trees, planted in city avenues. They are also seed predators of Rowan trees, feeding on the seeds and discarding the pulp of the rowan berries. Finally, in the winter they spend a lot of time pecking tree branches, and it is still a mystery to me what they are doing in there!
In a visit to my local cemetery I wasn't totally surprised when I found a group of Goldfinches feeding on leaf miners amongst Blue Tits. Using their feet to hold onto a leaf, they were extracting the miners, presumably pupae given the time of year (note the brown blotches on the leaves on the photos). The horse chestnut leaf miner, Cameraria ohridella, is a micromoth that has expanded its range throughout Europe from its native Balkans and is now widespread in cities and towns across Europe, responsible from the early browning of Horse Chestnut leaves in summer.
This is the first time I see Goldfinches feeding on invertebrates, but they are already known to be one of the few predators of this newly arrived species of moth (see Ralph Hancock's blog here). In a previous study, only tits, including Blue Tits, Great Tits and Marsh Tits were fed on the late larval or pupal stage of the miner. The bird species known to predate the miner are all small, dextrous and agile birds, and able to use their feet to hold onto leaves. This food resource is plentiful for a few months, as there are several generations of miner per year, so the miner provides a great resource of food in the summer months. The adult moth is also a source for food for other insect-eating birds.
Grabenweger, G., Kehrli, P., Schlick‐Steiner, B., Steiner, F., Stolz, M., & Bacher, S. (2005). Predator complex of the horse chestnut leafminer Cameraria ohridella: identification and impact assessment. Journal of Applied Entomology, 129(7), 353-362.
