Thursday, 5 June 2014

Predation of Greenshank eggs by crows

An adult greenshank fitted with a geolocator on a leg ring

While studying Greenshank in northern Scotland recently, it was again apparent that some of these birds are being predated by Hooded Crows eating their eggs. This was also noticed in the past two years of the study, a joint project between Ron Summers, Brian Etheridge, Nick Christian and myself, which is focused on marking the birds with individual colour-ring combinations and geolocator tags to discover where the birds which breed in Sutherland migrate to in winter and on passage.

A greenshank egg, sucked by a  Hooded Crow and the remains of another on the edge of the nest

Two clutches of eggs, from eight found, were definitely lost to crows, as evident from the distinct holes in the side of the eggshells left at the scenes. Two other clutches might have been taken away by crows, they can lift and carry them away from the nest to eat elsewhere, often next to a pool of water, but as there was no evidence at the nest sites, they could possibly have been taken by egg-collectors. Not that that is any better news for the study.
The large hole in the side of the egg, with angled notches left by the crow's bill are indicative of the predator type

Once the birds have lost their eggs they are likely to leave the area to perhaps nest somewhere else, or abandon their breeding attempt for the year. The crows do not nest on the moors where the greenshank nest, they come up from nearby croftland where they find food all year round. Food is scarce for them on the moors in spring, but they probably know there are eggs to be found there and they have been seen walking over the ground searching for any food they can find, including eggs.

A typical clutch of four eggs in a Greenshank nest 

There is not much known on the effect of this predation on the greenshank population, and it is only during intensive study that such information is gained. Perhaps, as the greenshank live for several years or more, they only need to raise chicks successfully once to maintain the population. But then, we do not know what other impacts there are on the birds in their wintering area, and that is why we consider it needful to know where they go.

Greenshank chicks hatching, two have already left the nest. The hatched eggshell is typically holed around the broad end and there are red blood-marks on the inside membrane

Greenshank are not the only birds of the moor which the crows take eggs from. Meadow pipits and skylark are the most abundant species there, and they even find secretive nests like that of Teal, one nest of which I found deserted after crows had taken one egg from the nest and sucked it. They had also rolled another egg out of the nest, but as there were still five eggs in the duck's nest, it seems that she manged to scare off the crows. However, she abandoned the nest after that, probably from fear of future attack.

A Teal's egg, sucked by a crow, with yolk spilled on the edge of the hole

Teal nests a can be placed almost anywhere on a moor, hidden under heather, and I would not have found this nest if the eggs had not been uncovered and obviously abandoned.

The abandoned Teal's nest and eggs, cold and uncovered.
A duck always covers her eggs when leaving the nest to feed

No comments:

Post a Comment