Thrush Eggshells Get Thinner
RSPB press release, 21 April 1998

The eggshells of some of Britain's favourite thrushes have been getting thinner for the last 150 years, according to a study by the Royal Society for the Protection of Birds.

The thickness of the eggshells of Blackbird, Song Thrush, Mistle Thrush and Ring Ousel, collected from England, Scotland and Wales, were measured from museum specimens to look for any changes over the period from 1850 to the present day.

Samples were taken from more than 1,000 eggs and the thickness was both estimated and measured directly. All four species showed a clear thinning since 1850 with Blackbird and Song Thrush recovering slightly since 1960. The cause of the thinning is as yet unknown but must be linked to a decline in available calcium for eggshell formation. Acid rain and agricultural changes are two potential reasons why Thrushes might find calcium difficult to obtain.

Recent studies on Great Tits in the Netherlands revealed areas where acid soil has resulted in loss of snails, essential for providing calcium for eggshells, and this has caused a high proportion of thin-shelled or deformed eggs.

Dr. Mark Avery, RSPB head of conservation science, said: "This thinning is a cause for concern. It appears that eggshells have been thinning ever since the industrial revolution. We seem to have discovered a new ecological footprint".

Additional Notes:

1. 'Long-term decline in the thickness of eggshells of thrushes Turdus spp. in Britain' by R E Green is published in the Proceedings of the Royal Society this week.
2. The changes in eggshell thickness were measured using an Eggshell Thickness Index, calculated using egg volume, surface area and mass, and also a direct measurement of thickness using a digital micrometer.
3. The eggs were measured in the collections housed in the Royal Museum of Scotland (Edinburgh), the Natural History Museum (Tring) and the Liverpool Museum.
4. RSPB plans to do further research in collaboration with the Natural History Museum and University College London to establish how many other species are affected, how widespread is this phenomenon across Europe and to try to establish the cause of the eggshell thinning.
5. In the 1950s and 1960s the use of pesticides, especially DDT, directly caused thinning of eggshells of many birds of prey and fish-eating birds and led to poor breeding success as eggs were easily broken in the nest.

For further information contact:

Chris Harbard
RSPB press officer
01767 681577

Dr. Mark Avery,
RSPB head of conservation science
01767 680551