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Abnormal eggs of the Common Guillemot Uria aalge: the role of stress

Tim R. Birkhead1* ORCID logo, D. R. Russell2 and J. E. Thompson1 ORCID logo

1School of Biosciences, University of Sheffield, Sheffield, S10 2TN, UK;

2 Bird Group, Department of Life Sciences, Natural History Museum, Akeman Street, Tring, HP23 6AP, UK.

Full paper


This study documents several types of abnormal eggs of the Common Guillemot Uria aalge collected from Bempton Cliffs, Yorkshire, UK, during the 1900s and currently housed in the Natural History Museum at Tring, Hertfordshire, UK. These eggs are abnormal in terms of size, shape, colour and surface topography. We suggest that the incidence of these abnormal eggs was relatively high because the manner in which they were collected — repeated visits to the breeding colony — caused considerable disturbance and stress that in turn disrupted the normal process of egg formation. We discuss the possibility of using abnormalities in avian eggs as a measurement of environmental stress.


There has always been considerable popular interest in abnormal eggs. In the past, the mystery of their origin evoked a folklore of fantastic explanations, but superstition has now been largely dispelled by the findings of science” (Romanoff & Romanoff 1949).

From at least the 1500s onwards, people descended the 60–120 m vertical chalk cliffs at Bempton on the Flamborough Headland, Yorkshire (54°8’N 0°9’W) to obtain eggs of the Common Guillemot Uria aalge (hereafter ‘Guillemot’) and other seabirds. Initially, these eggs were taken for human consumption, but from the mid-late 1800s and early 1900s they were also taken to be sold to egg collectors.

The eggs were taken by ‘climmers’ (climbers) who were labourers from farms adjacent to the cliffs (Vaughan 1998). Harvesting was systematic with each gang of climmers visiting their own stretch of cliff every three days (weather permitting) between May and June. Several tens of thousands of eggs were taken each year, until 1954 when egg collecting became illegal (Vaughan 1998; Birkhead 2016).

As a result of this vast harvest of Guillemot eggs, almost every natural history museum in the UK, and many elsewhere, contains one or more Guillemot eggs from Bempton. The Natural History Museum at Tring, Hertfordshire, holds around 6,000 Guillemot eggs, many of which are from the collection of F. G. Lupton (1881–1970), who obtained eggs from Bempton in the 1920s and 1930s. Lupton specialised in collecting (i) ‘sets’ of eggs from what were assumed to be the same females within and between breeding seasons, (ii) unusually coloured and patterned eggs (e.g. red eggs; Birkhead & Montgomerie 2018), and (iii) eggs of abnormal size, surface texture and shape (Birkhead 2016).

Abnormal eggs of the domestic fowl Gallus gallus domesticus have been recorded from the earliest times (Tiedemann 1814), and Romanoff & Romanoff (1949) classified these in terms of their internal and external appearance. Eggs with abnormal external appearance were of unusual (i) size (larger or smaller than average), (ii) shape (bent, elongate, twisted), (iii) surface texture, often with additional calcareous deposits, or wrinkled, and (iv) pigmentation, including ‘banded eggs’ with a band of pale pigmentation around the egg’s short axis. The shape category also includes stalked eggs, truncated eggs and those with additional shell or tissue attachments, and two eggs bound together, usually by a narrow waist, that occasionally occur in domestic fowl (see illustrations in Romanoff & Romanoff 1949).

Among non-domesticated birds, the most frequently encountered type of abnormal egg is the runt or dwarf egg (Koenig 1980a, b; Mulvihill 1987; Crick 1995; see also Romanoff & Romanoff 1949), arbitrarily defined by Koenig (1980a) as having less than 75% of the volume of ‘normal’ eggs. Other types of abnormality are rare and rarely encountered in sufficient numbers for systematic study. Among the Guillemot eggs collected from Bempton Cliffs during the 1900s there are several abnormal eggs.

The aim of this study is to document the different types of abnormality in the eggs of the Guillemot and to speculate about their causes and possible value in monitoring stress in populations of other wild bird species.


We are especially grateful to Bergur Olsen and Aevar Petersen and their contacts, in the Faroes, Óla Jákup Hentze and Meinhard Hentze, and in Iceland, respectively, for providing information on abnormal Guillemot eggs. Thanks also to Ruedi Nager for providing details and an image of abnormal Herring Gull eggs, to B. Olsen, M. P. Harris, and M. Huebeck for information, to John Hart for telling us about Guillemot eggs on the seabed, and to Karl Schulze-Hagen for help with references. We thank N. Hemmings, R. Montgomerie, Sarah Wanless and an anonymous referee for helpful comments on the manuscript. This research was funded by a grant (to TRB) from the Leverhulme Trust (RPG-2015–221).


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