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Red Crossbill eating oak apples

Crossbills are known to feed primarily on the seeds of pines and other conifers. They use their tongue and peculiarly shaped bill to remove the seeds from the cones and this makes them pretty unique.

On 12 May 2020, I came across a small flock of Red Crossbills Loxia curvirostra (type C) near Bergen in North Holland. Among them were several juveniles. As they seemed to be foraging high up in a couple of oak trees, I did my best to find out what exactly they were after. I then came to the conclusion that the juveniles were eating from oak apples and leaf buds. 

That oak apples are produced by gall wasps, was about all that I knew about them. After a quick search on the internet, however, I learned that this particular species is called Oak Potato Gall Wasp Biorhiza pallida. The oak apples are formed after the gall wasp lays eggs inside the leaf buds and the plant tissues swell as the larvae develop inside. The gall provides a nutritious and protective environment for as many as 30 larvae.

I am quite certain that I will never become an expert on gall wasps or oak apples, but to get out and learn new things is what makes birding so interesting to me. Besides, watching young crossbills in action is very entertaining!


Red Crossbill eating from oak apple

Red Crossbill eating from oak apple

Red Crossbill eating from oak apple

Juvenile Red Crossbill eating from oak apple of Oak Potato Gall Wasp near Bergen, Noord-Holland, on 12 May 2020.

Red Crossbill eating from leaf buds

Juvenile Red Crossbill eating from leaf buds of European Oak Quercus robur near Bergen, Noord-Holland, on 12 May 2020.

Red Crossbill

Adult male Red Crossbill near Bergen, Noord-Holland, on 12 May 2020.

Biorhiza pallida oak apple

Biorhiza pallida oak apple

Oak apples of Oak Potato Gall Wasp.


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Published 14 May 2020