Abstract A Peregrine Falcon egg, collected from a nest in North Wales in 1980 after it failed to hatch, was found to contain two fully developed embryos. This rare case of twin embryos in a wild bird is discussed in the context of a wider study, which looked at the levels of organochlorines in wild birds’ eggs.


Published data on the development of embryos of birds in the wild are scarce and the topic has only recently become of interest within the scientific community. This account details a case of abnormal development observed in a Peregrine Falcon Falco peregrinus egg, namely the first, and to date only, recorded case of embryonic twinning in a raptor in the UK (Pattee et al. 1984).

Between 1964 and 1982, more than 1,400 wild birds’ eggs were examined prior to chemical analysis under the former Nature Conservancy’s Monks Wood pesticide monitoring programme. The function of the long-running survey was to ascertain and continually monitor the levels of organochlorine pesticides and PCBs in the tissues and eggs of a select number of predatory species thought or known to be especially at risk from these compounds. The eggs analysed were collected annually under licence, obtained opportunistically from clutches that had totally or partially failed. 

The eggs collected included those of Peregrines, Golden Eagles Aquila chrysaetos, Ospreys Pandion haliaetus, Eurasian Sparrowhawks Accipiter nisus and Grey Herons Ardea cinerea. Fresh, unincubated eggs of Grey Herons, and those of some seabirds, such as Northern Gannet Morus bassanus, Sandwich Tern Sterna sandvicensis and Common Guillemot Uria aalge, were also collected periodically for continual monitoring of the marine and freshwater environments.

All eggs were first measured, weighed and then blown, or broken open and the contents weighed and examined to confirm the degree of embryonic development. The shells were then air-dried to obtain the dry weight and, from that, the eggshell index (Ratcliffe 1967). The scheme amassed a large series of data on wild birds’ eggs during a period when many avian species were vulnerable to the new and ongoing environmental hazard presented by these organochlorine compounds (Cooke et al. 1982).

Results and discussion
A high proportion of eggs that had failed to hatch showed no signs of embryotic development, perhaps through infertility, erratic or a total lack of incubation, or from desertion at some point during the incubation stage. Others contained embryos that had died in the egg at varying stages, possibly from chilling or desertion but, particularly in the earlier years of the programme, some of these failures may have been due to the higher levels of the three main organochlorines (DDE, HEOD and PCBs) that were often found in the eggs. Although these chemicals were formerly present to some degree in all avian specimens, no developmental anomalies or mutations were detected in embryos during this period. Residues of such chemicals generally began to decline through the 1960s and 1970s, following the gradual introduction of controls on pesticides.

A failed Peregrine egg, collected from a nest in North Wales in June 1980, was found to be outwardly normal, with typical rich shell coloration, a total weight of 35.16 g and shell measurements of 52.30 mm x 39.75 mm (average comparable data from 31 failed Peregrine eggs from 1975 to 1981 were 38.00 g, with shells of 50.96 mm x 40.40 mm (table 1)). Drilling revealed development within and conventional blowing was therefore impossible, so the egg was broken open. It was found to contain two apparently full-term embryos, both covered in the usual white down, with death likely happening just one to two days from hatching. Both chicks were connected abdominally by a single, almost fully absorbed common yolk sac (plate 272). The smaller of the two chicks weighed 12.2 g and had presumably died from cardiac failure, being intensely haemorrhaged overall with the entire skin discolored deep purple-red. The larger chick, which weighed 13.4 g, and the yolk sac were apparently unaffected by this. The larger embryo appeared normal, with nothing external to indicate its cause of death, although chilling and constriction of movement through proximity to its dead sibling in the latter stages are likely causes. Together with remaining yolk, allantois and residual fluids, the combined weight was 30.69 g. The typical weight of a single Peregrine chick at hatching is around 35–40 g. Had both pulli survived longer, been able to exit the shell alive and then been fed by the parents, they obviously would have soon succumbed owing to still being connected abdominally by the diminishing and finally fully absorbed yolk sac. 

peregrine 001.jpg

272. Twin-embryo Peregrine Falcon Falco peregrinus chicksFrom an egg collected in North Wales in June 1980. 

P. Ainsworth

The chemical-analysis results for this failed Peregrine egg from North Wales are recorded as follows, expressed as ppm wet weight: DDE 4.44, HEOD 0.98, PCBs 3.63, with a shell index of 1.55.

A single chick had successfully hatched and fledged from a second egg at the same nest.

The DDE and HEOD residues, although still slightly elevated in the failed egg, were consistent with those of eggs from a coastal site, within the range of declining levels then extant in the UK Peregrine population (Cooke et al. 1982; Newton et al. 1986). This egg is the only egg collected in which mutation or abnormality of an embryo was detected during the investigation. The slightly elevated levels of DDE and HEOD are unlikely to have contributed to the partial failure or to the development anomaly, especially with a sibling’s successful development and fledging and no previous detection of such mutations when levels of these chemicals were considerably higher in eggs sampled during the 1960s and 1970s. 

Whether these 1980 Peregrine embryos were fraternal or identical twins was not established. Clearly, dual fertilisation and separate yolks would be essential for multiple embryos to survive the hatching process to become potentially viable pulli.

Coincidentally, a second, failed, twin-bearing Peregrine egg was observed in Greenland in 1981, just one year after the Welsh egg (Pattee et al. 1984). The pesticide residue content in this egg was not cited.

The dimensions of the 1980 Welsh egg were closely within the parameters of normal British Peregrine eggs of the period, whereas the failed Greenland egg from 1981 was appreciably larger than the norm in Greenland (table 1).

Romanoff & Romanoff (1972) suggested that twinning is as common in birds as other vertebrates and, while it is common reported in chickens, it has rarely been reported in wild species. They also suggested that stress may be a factor in the frequency of occurrence of double embryos in eggs. Despite this ambiguity, the incidence of stress in wild birds must be immense as they experience a continuum of potentially stressful events through interactions with man, predators and their own species as they undergo their routine of daily life. That stress is a cause of twinning in wild species thus seems unlikely; no instances of double yolks or twinning were found in the many Grey Heron eggs examined during the 1960s and 1970s, a time of high DDE contamination, when varying numbers of the birds were breaking their own thin-shelled eggs (Cooke et al. 1976) and which might then, logically, be deemed to be suffering elevated stress levels. Neither were any found in any of their repeat clutches, nor in the many failed raptor eggs until the current example in 1980.

Whatever the cause of avian twinning, nidicolous young that were able to hatch successfully would be at an obvious disadvantage through competition with potentially larger siblings from normal eggs. Twin nidifuges, however, of whatever size, might be more likely to survive through their ability to feed independently soon after hatching.

I thank A. S. Cooke for reading and commenting on the original manuscript; Ian Newton for access to the Peregrine residue data in the former Monks Wood files; and the late Peter Ainsworth for taking the photograph. Mike Storry steered me through the Internet.

Cooke, A. S., Bell, A. A., & Prestt, I. 1976. Eggshell characteristics and incidence of shell breakage for Grey Herons Ardea cinerea exposed to environmental pollutants. Environ. Pollut. 11: 59–84.

—, —, & Haas, M. B. 1982. Predatory Birds, Pesticides and Pollution. Institute of Terrestrial Ecology. Huntingdon.

Newton, I., Haas, M. B., & Leach, D. V. 1986. Organochlorines in Peregrine Eggs. Birds and Pollution (Part 5).NERC report to the NCC. Institute of Terrestrial Ecology, Abbots Ripton. 

Pattee, O. H., Mattox, W. G., & Seegar, W. S. 1984. Twin embryos in a Peregrine Falcon egg. The Condor 86: 352–353.

Ratcliffe, D. A. 1967. Decrease in eggshell weight in certain birds of prey. Nature 215: 208–210.

Romanoff, A. L., & Romanoff, A. J. 1972. Pathogenesis of the Avian Embryo. Wiley, New York.


A. A. (Tony) Bell, 34 Jubilee Drive, West Kirby, Wirral CH48 5EF; e-mail [email protected]

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