Spangle Galls and Bullfinches

This autumn saw a very heavy production of spangle galls on the oaks in the woodlands of the Blyth valley and nearby areas.  These are the little disc-shaped galls found on the undersides of oak leaves.  They are produced by oak gall wasps Neuroterus, of which there are several species, and they seem to have no adverse effect on the oaks – parasites that have got their relationship with their host just right!  Come the autumn, many of the galls detach from the leaves and fall to the ground, in years like this covering the ground with red, green and tawny ‘spangles’.   Given that each gall has a nice fleshy little proteinaceous titbit in the centre you would think they would be gobbled up by predators, but I have yet to see this for myself. Pheasants and wood pigeons are said to eat them, but I wonder if the gall wasps have found a way of concentrating the tannins from the oak leaves in the flesh of their galls that protects them against many insectivorous birds and mammals.

Last week I watched two gorgeous cock bullfinches picking spangles from the undersides of withered brown oak leaves still attached to the tree. Supposedly spangles only remain attached to leaves if the grub inside has died (or the grub dies if its gall doesn’t detach).  I checked this out for myself this morning and sure enough each of the fallen galls when cut open was as fleshy as a tiny pepper and examination under the binocular microscope revealed a blob of white jellyish matter in a spacious central cavity, which I presume meant that the grub was already well into the process of metamorphosing into an adult wasp.  By contrast, galls pulled from leaves that were still on the tree were more leathery, and the cavity was flattened and contained only a little clear jelly.  Was this a little morsel of decaying protein that was still worth a bullfinch’s bother?

Because of their propensity for causing serious commercial damage by eating the buds of fruit trees, the diets of bullfinches have been well-studied, but I have found no mention of spangle galls as food items even in Newton (1967) whose study areas included the oak woodlands of Wytham, near Oxford.  Perhaps my birds were just experimenting (the light was not great and I can’t be sure they were actually eating the galls they were picking).  It seems that ash seeds are the single most important item of winter diet for bullfinches (but that they are not so keen on those that have high tannin levels).  In this area at least the ash seed crop, unlike that of the spangle galls, seems very light this year.  Could that mean this is going to be a hard winter for bullfinches, especially if we get a snowy spell, and that their numbers will be low next year?  And if the fungal disease Chalara Ash Dieback has as drastic an effect on numbers of ash trees as is feared, could bullfinches end up as scarce as hawfinches?

References:

Askew, RR (1962) The distribution of galls of Neuroterus (Hym: Cynipidae) on oak.  J. Anim. Ecol. 31(3): 439-455

Greig-Smith, PW and Wilson, MF (1985) Influences of seed size, nutrient composition and phenolic content on the preferences of bullfinches feeding in ash trees.  Oikos 44: 47-54

Newton, I (1967) The feeding ecology of the bullfinch (Pyrrhula pyrrhula L.) in southern England.  J. Anim. Ecol. 36(3): 721-744

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