Wednesday, 20 November 2013

The water beetles' sad end as a teenage heron's snack

The predatory beetles of the last two entries look spectacularly fearsome through a hand lens; pointed jaws which pierce prey and along which the digested innards of their victims are sucked out, clusters of simple eyes spots which suggest a very different perspective on their world. However if you are a 1 metre tall Grey Heron, Ardea cinerea, these beetles probably pass for a light snack. The experimental ponds at Hauxley are a frequent hang out for the young herons that have left the nest. They loiter, preen and wander round with no obvious purpose in a perfect echo of teenagers. They also throw up pellets, like the one in the photo above. Heron pellet contents vary greatly with time and place, as they seem to hone their hunting to fine grained variations in the available prey. Mammal remains such as voles and moles occur frequently, birds too, especially chicks, and the tough wing cases (elytra) of water beetles. Three large, ridged elytra, edged with a distinct yellow border are in this pellet, the remains of Great Diving beetles (Dytiscus) based on the size and colour. The victims were female beetles, the ridges a give-away of gender compared to the smooth wing cases of the males. The rarity of fish bones, even their apparent absence, from heron pellets has attracted comment in many more detailed studies. Why they should be missing is not clear, since plenty of other fish eating birds regurgitate bones and otter spraints are essentially fish bones in Earl Grey tea scented oil. The beetles’ wing cases are tough. Beetles body parts found in ancient soils and peat can be so intact that precise species identification is still possible allowing local habitat conditions to be worked out based on where the species live these days. Quite where these two female Dytiscus came from I cannot tell. Dytiscus beetles have never been found in the experimental ponds, so the herons must have travelled further afield. In true teen fashion they last leave the debris lying around when they’ve finished.

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