Contamination by chemical residues, when resulting in eggshell change, takes the place of a 'natural' adverse factor, but the outcome is more detrimental to the species if reduced breeding success is maintained. (7) Parent egg-breaking may be a normal, adaptive response to various adverse conditions which arise between ovulation and hatching, and militate against successful breeding. (6) Interspecific differences in eggshell change probably reflect a combination of differences in exposure to persistent residue contamination, differential uptake and variations in biochemical response to a particular level of a residue. While the well-known population 'crashes' of peregrine and sparrowhawk after 1955 evidently involved unusually heavy adult mortality, the declines in breeding success may have contributed, and would certainly help to prevent recovery. The outcome is a reduction in breeding success.

Adverse effects on breeding behaviour may also occur. (5) Organochlorine contamination evidently triggers a causal chain in some raptors decrease in eggshell thickness (producing reduced mechanical strength) leads to more frequent accidental damage to eggs, and hence to more frequent parental egg destruction (since birds have an innate tendency to destroy their damaged eggs). (4) Within Britain, there is a strong correlation between geographical variations in level of organochlorine contamination, degree of change in eggshell weight, and frequency of egg breakage, in peregrine, sparrowhawk and golden eagle. The post-1955 use of dieldrintype insecticides contributed additively to organochlorine contamination and probably reinforced the initial eggshell change the industrial pollutant PCBs evidently showed a gradual post-war increase as an environmental contaminant, and could also have contributed to the eggshell change. The general introduction of DDT and ОіBHC, first into domestic, veterinary and horticultural use, and then into agriculture, during 1946-48, coincided exactly with the onset of the eggshell change, and these chemicals are suspect as initiators of this change. In particular, persistent organochlorine compounds accumulate in the tissues of wild raptors, and have been shown experimentally to disturb physiological mechanisms affecting calcium in birds. (3) The only known environmental change which parallels the eggshell change in timing and geographical pattern, and has a likely physiological connection, is the wide-spread post-1945 contamination of the ecosystem by residues of synthetic organic chemicals used as pesticides and in industry. No significant change was found in eggshells of common buzzard, raven, guillemot, razorbill, kittiwake, black-headed gull, golden plover and greenshank. In the golden eagle it probably began not later than 1951. In the first two species, this eggshell change began in 1946-47, reached a climax in 1948-50, and has persisted up to 1969. (2) One proximate cause of this phenomenon appears to have been a widespread and substantial decrease in relative weight (largely thickness) of eggshells of peregrines, sparrowhawks and golden eagles (19.1, 17.2 and 9.9% respectively). The majority of these egg breakages are believed to have been caused by the parent birds themselves, usually by the eating of both shell and contents. (1) Since 1950, egg breakage has occurred with unprecedented frequency in the nests of British peregrines, sparrowhawks and golden eagles.

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