10 fascinating facts about Skomer’s guillemots

Discover what a 42 year study by ornithologist Tim Birkhead and his students has revealed about Skomer’s guillemots. 


1. Breeding success is determined largely by the number of close neighbours. Guillemots breed at high density to deter predatory gulls and ravens that, given the chance, would eat their eggs and chicks.

2. High numbers and high-density breeding give guillemots the confidence to sit tight in the face of a predator. When the population was low in the 1970s, the birds were very nervous; they are much more confident now.

3. Guillemots on Skomer have a high survival rate. About 95 per cent of adults survive from one year to the next, and approximately 50 per cent of chicks that fledge manage to reach breeding age.

4. The Skomer guillemot population is largely self-contained. Young birds usually return to breed there, sometimes close to where they hatched. Immigration seems to be rare: one bird ringed as a chick in the Baltic moved to Skomer to breed, but that’s unusual.

5. Global warming resulted in progressively earlier breeding between the 1970s and about 2005 – an advance of roughly two weeks. More recently, the breeding seasons have flipped between the earliest and latest in 42 years. Could climate change be to blame?

6. The four major oil spills of the past 40 years, including three hundreds of kilometres from Skomer, resulted in a doubling of the annual adult mortality rate.

7. Climate change also affects the survival of adult birds. Warm, wet, windy winters are becoming more frequent, and are taking their toll. 

8. Using various tracking devices, TIm's team has been able to see the massive movements Skomer’s guillemots make outside the breeding season – from Portugal to the Faroes – in search of food.

9. Guillemot chicks on Skomer are fed mainly on energy-rich sprats; elsewhere in the UK they are fed sandeels. But in 2014 Skomer birds were catching more herring and cod-like fish, which may signal a change in the ocean, probably not for the better.

10. The increase in numbers since the mid-1980s is good news but fragile. Elsewhere in the UK, guillemot breeding success is generally very low and populations are decreasing.

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