The palatability of the eggs of birds: illustrated by experiments on the food preferences of the Hedgehog (Erinaceus europaeus).
Authors: COTT HUGH B. 
Published: August 2010

The paper describes the results of an investigation of the relative acceptability of the eggs of twenty‐five species of birds, belonging to ten orders, as illustrated by the feeding preferences of the Hedgehog (Erinaceus europaeus). In each of 332 experiments, one of four animals used as tasters was presented with a choice as between two egg‐samples. Possible disturbing factors such as differences in egg‐size or shell‐coloration, and those due to spatial arrangement of the samples, were eliminated by the use of raw egg, offered in equal quantities, and by the successive reversal in position of the egg‐species matched within a particular group of experiments, or as offered to a particular experimental animal.

The reliability of the experimental method and the degree of consistency of hedgehog discrimination was checked by repetition experiments (between the same species‐pair), both for the same and for different individuals; and by linking and overlapping experiments (between pairs of egg‐species more or less widely separated in the edibility series). In general, the results show the hedgehog to be sensitive to differences in the relative palatability of the eggs offered, and to be consistent in its preferences and aversions, which show a broad agreement with those of man for the egg‐species considered. Separate experimental results are tabulated; the evidential value of the groups of experiments is discussed; and the edibility status of the several species considered.

The experiments, which fall into groups carried out with sixty species‐pairs. afford a basis for the arrangement of the several species in a provisional hierarchy of preference: to each has been assigned a numerical edibility rating. The species fall broadly into three grades–here shown in descending order of acceptability–as follows: (a) Palatable.–Kittiwake (Rissa t. tridactyla), Eider (Somateria m. mollissima), Razorbill (Alca torda britannica), Gannet (Sula bassana), Domestic Fowl (Gallus gallus). Lapwing (Vanellus vanellus), Common Tern (Sterna h. hirundo), Coot (Fulica a. atra): (b) Intermediate.–Wood‐Pigeon (Columba p. palumbus). Little Owl (Athene noctua vidalii). Rook (Corvus f. frugilegus), Puffin (Fratercula arctica grabae), Magpie (Pica p. pica). Jackdaw (Corvus monedula spermologus). Hobby (Falco s. subbuteo). Kestre) (Falco t. tinnunculus). Little Grebe (Podiceps r. ruficollis), Buzzard (Buteo b. buteo), Arctie Tern (Sterna macrura), Ringed Plover (Charadrius h. hiaticula); (c) Distasteful.–Blackbird (Turdus m. merula), Chaffinch (Fringilla coelebs gengleri), Sedge‐Warbler (Acrocephalus schoenobaenus), Whitethroat (Sylvia c. communis), Linnet (Carduelis c. cannabina).

Grounds are given for the belief that the preferences shown by hedgehogs are likely to correspond with those of other egg‐eating animals, and that, within the limits of the series examined, they afford a satisfactory basis for consideration of the data in general terms. The wide range in specitie edibility rating found to occur within certain orders. suggests that relative acceptability may be largely independent of systematic status: however, the low mean edibility rating for Passeriform species indicates a trend towards nauseousness within this order. No close or obvious connexion has been traced between relative acceptability of the egg and feeding habits of the parent: nor is the quality of the egg at all constantly related to that of the flesh in the species examined.

On the other hand, the experimental results point towards correlations between relative acceptability and various other ecological factors likely to influence susceptibility to attack by predators. Firstly, there is a broad direct relationship between palatability of the egg and size of the parent. Secondly, the distribution, in terms of acceptability, of species having colonial and solitary nesting habits, respectively, shows a marked tendency for eggs of the former to be more, and those of the latter to be less palatable. Thirdly, consideration of egg‐palatability in relation to nest‐site, indicates that eggs laid in open nests in moderate growth or low herbage may be relatively distasteful as compared with those laid in various other situations. Fourthly, a general correlation is shown to exist between coloration of the shell and palatability of its contents. Broadly, cryptic eggs tend to belong to the higher edibility grades, and distinctively marked eggs to the lower edibility grades.

The several species have been assessed in terms of relative vulnerability, account being taken of such factors as size, weapons of defence, powers of offence, availability of the clutch, general habits, sociability and coloration. A general inverse correlation is shown to exist between the degree of vulnerability and that of acceptability–the most palatable eggs being those laid by non‐vulnerable species, the most nauseous by those that are (otherwise) highly vulnerable. It is suggested that the facts point towards an adaptive interpretation of distastcfulness in eggs, coasidered as a deterrent to attack; and that the distinctive coloration of nauseous but otherwise vulnerable eggs may have a warning or recognitional value in relation to potential predators.