Shooting Times & Country Magazine
Country Diary
We tend to blame the usual suspects for egg predation — but it may be that sheep, cattle and even deer are not as innocent as they appear...
If you were to jot down a list of British mammals that might predate the eggs or chicks of a ground-nesting bird, I doubt that you would include sheep, cattle or deer, but some will do just that, probably to alleviate deficiencies in essential minerals, such as calcium.
According to Defra, there are around 23.1million sheep and 9.8million cattle in the UK. Wild deer populations are at their highest for more than 1,000 years and numbers continue to rise. Getting an accurate assessment of deer numbers is challenging because deer are secretive and highly mobile, but the Deer Initiative estimates around 2million deer in the UK. If you tot that lot up, it is around 35million “potential” nest and chick predators, some of which probably aren’t even on the radar of those involved with ground-nesting bird conservation projects.
Ungulates have evolved to munch plants and not other animals. But though they lack the refined hunting skills of natural-born killers like foxes, they are equipped with acute olfactory and auditory senses, and are perhaps better at locating ground-nests and barely mobile chicks than we realise.
Scope for conflict
In the lowlands, cattle and sheep are used to graze and/or poach-up natural and semi-natural grasslands and wetlands, to help maintain optimum nesting and brood-rearing habitats. Though stocking densities can be carefully managed, there is clearly scope for conflict, through trampling and predation. This summer, I heard two reports of people monitoring lapwing nests having watched grazing cattle and sheep eat lapwing eggs. For some ground-nesting bird species, in certain landscapes, under certain management regimes, nest and chick losses to ungulates could be a problem, albeit one that is poorly understood.
Sheep were a problem for seabirds breeding on Foula in the Shetland Isles. The annual discovery of partially mutilated and headless Arctic tern chicks was attributed to attacks by hedgehogs or otters. It wasn’t until a local crofter watched a grazing ewe mooch into the tern colony, scoop up a chick and bite its head off, that locals and seasonal bird-ringers rumbled what was going on. Between 1973 and 1980 sheep are reckoned to have eaten the body parts of at least 680 live Arctic tern chicks, and eventually terns began to desert the colony, to nest in areas away from sheep.
On Rhum in the Inner Hebrides, birdwatchers used to puzzle over the decapitated and limbless Manx shearwater chicks after they had emerged from their nest burrows. Then a stalker watched a red deer bite the head off a chick and consume its legs and wings. It has since been estimated that Rhum’s red deer consume 36g of shearwater bone each season, equating to three chicks per animal, most likely to excise calcium from their bones. But why should that surprise us? It is well known that deer will chew cast antlers.
In Wisconsin in the US, researchers using video nest-cameras to monitor passerine bird nests on grazed pastures recorded cattle eating the eggs of savannah sparrows and the nestlings of eastern meadowlarks. The researchers reported that without the video evidence, they would have attributed those nest losses to other traditional predators. A similar study in Pennsylvania, where 25 passerine nests were monitored with video cameras, revealed eight depredation events. Of those, three nests were predated by white-tailed deer; two by red fox; one by raccoon; one by (probably) weasel and one by an unknown predator.
Another study on grasslands in North Dakota showed that white-tailed deer removed nestlings quickly (5-19 seconds per nest) at night and left no evidence of predation. Unless you find clear evidence of hatching or predation having occurred, an empty nest is frustratingly ambiguous. Here, skylarks are in dramatic decline. We know that badgers destroy nests but I wonder how many are taken by grazing herbivores?
Current high levels of predation threaten many vulnerable ground-nesting bird species in the UK. We know that alleviating the risk posed by predators such as foxes, badgers and crows leads to better breeding success for some species. We shouldn’t take our eye off the ball there, but perhaps we also need to consider the risk posed by all those seemingly innocent sheep, cattle and deer, at least at the local level?
“Sheep had eaten the body parts of at least 680 live Arctic tern chicks”
Mike Short is an ecologist at the GWCT. He is a keen angler, deer stalker and forager and helps to run a wild bird rough shoot in Wiltshire.