Too Many Sheep Spoil The Eggs

Too many sheep in Britain’s uplands could be responsible for the decline of some native birds according to research published today in the journal Biology Letters.

The research, led by Dr Darren Evans from the Centre for Ecology & Hydrology in Banchory, provides the first hard evidence of a link between increased sheep grazing and the breeding patterns of the meadow pipit, Britain’s most common upland bird.

The researchers examined the effects of sheep grazing on egg sizes of these ground-nesting birds. They varied sheep numbers in an upland field experiment and found that areas with high sheep numbers had meadow pipit nests with the smallest eggs, and that areas with low sheep numbers had nests with the largest eggs.

Dr Evans explained, “Egg size is important for many bird species as chicks that hatch from large eggs tend to grow faster and have a better chance of survival than those that hatch from small eggs. We believe that high sheep numbers reduce food supplies and with less food around, adult birds are unable to produce the large eggs necessary for well-nourished, healthy chicks.”

He added: “Livestock grazing is important for maintaining the character and biodiversity in upland areas. But sheep numbers in Britain have more than doubled in the last 50 years. Our results suggest that intensive livestock grazing appears to be bad for birds, whereas smaller numbers of sheep could be good for them. Eggs laid in ungrazed areas were also small, indicating that either too many or the complete removal of sheep in upland areas might have a detrimental effect on pipit breeding performance. We need to get the balance right.”

The study was carried out at Glen Finglas in Central Scotland during 2002 and 2003. It could help to explain the observed link between increased grazing and declines in grassland birds in both Europe and North America.