Diet Detective (part 2)

No prizes for guessing what this bird has been eating.
No prizes for guessing what this bird has been eating.

Just interesting to compare the crop contents of a bird shot on Peak Naze in Derbyshire on Thursday with the “rush-seed-eating” bird I posted about previously. The Peak District bird has a crop full of readily identifiable heather shoots, including some flowers, demonstrating that the annual cycle of grouse food depends hugely upon what is available and where.

Peak Naze has a strongly heather dominant mixture which also includes some blaeberry and crowberry, but little in the way of white grass, sedges and rushes. By comparison, the Galloway bird was shot in an area where the balance of vegetation sits much further towards the grassy end of the spectrum.

At this time of year, grouse are often found away from the heather in “white” habitats made up of grasses and berries. While it would be a stretch to say that the Peak District bird was eating heather because there were no rush seeds available, it does give a good indication of the fact grouse will use a huge variety of different foodplants during this time of year. Interestingly, it occurs to me now that grouse obtain feed in “themes” i.e. even when a wide variety of food is available, they tend to stick to one at a time. Of all the grouse crops I have opened, it is normal for the huge majority of the contents to be made up of a single plant species. In this case, the bird had been feeding on heather but there were one or two blaeberry leaves in the crop. The Galloway bird had been feeding on rush seeds, but its crop also contained evidence of the odd blaeberry.

It reminds me of an article I read about pigeons foraging in an extremely single-minded fashion, meaning that they get “into the groove” of picking up, say, barley grains, feeding on nothing but this one foodstuff, seeming blind to other kinds of forage that they would usually enjoy. I only dimly remember the article, but the author suggested that pigeons feeding on a broadcast game mix were single-mindedly picking up tic beans, despite having to search for them at length along margins which were littered with clover and freshly drilled cereals. I daresay it is more efficient to focus on one single type of food at a time (particularly a finite one), rather than constantly being distracted like a child in a sweetshop.

Contributing to this is the fact that we human beings don’t look at grouse foodplants in the same way as they do. To us, the moor can be full of viable forage, while to a grouse the plants are all at different stages of ripeness and readiness, and focussing on one particular species at a time maximises the benefit that it can provide. This “seasonal availability” factor is slightly different in black grouse, but having watched greyhens and red grouse hens mowing through cottongrass flowers in March like sheep, it only makes sense that birds should focus their efforts on gathering one species at a time, rather than chaotically trying to pick and mix a selection. This is particularly relevant for plants like cottongrass, which reach a very fine peak of nutritional goodness that is very quickly passed.

According to Watson and Moss’s Grouse (which is “must read” material), the all important cottongrass flowers vary in goodness according to the weather, so while they may not show any outward change, the chemical content and subsequent value alters with the temperature. Being able to “cherry pick” the highest quality forage is an asset when life hangs on such a delicate thread.

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