Changes Attributable to Pesticides in Egg Breakage Frequency and Eggshell Thickness in Some British Birds

Abstract

Before the late 1940s it was a rare event for broken eggs to be found in the nests of peregrine (Falco peregrinus) and sparrowhawk (Accipiter nisus), but from 1951 onwards, both species have shown a high frequency of clutch depletions in which eggs were either found broken, or disappeared in circumstances where human interference was not indicated as the cause. In most instances it appeared that the parent birds destroyed their own eggs, either by eating or by breaking and/or ejecting them (Ratcliffe 1958, 1960). Scottish golden eagles (Aquila chrysaetos) also showed an unusually high frequency of egg breakage, again attributable to parental destruction, during 1960-63 (Lockie & Ratcliffe 1964). Although golden eagle egg breakage evidently became frequent before 1960 (Ratcliffe 1960), lack of earlier observations made accurate dating of the change difficult. An examination of the eggshells of peregrine and sparrowhawk has recently disclosed that a highly significant decrease in weight (thickness) occurred in both during 1946-50, and has been maintained ever since; golden eagle eggshells showed a less marked but still highly significant decrease in weight (thickness), dating from around 1951 and continuing up to 1966 (Ratcliffe 1967a). The present paper sets out in greater detail the evidence for egg destruction and change in eggshell weight, and discusses the correlation between these phenomena; relationships to the well-established decline in population status and breeding success* in these three species are examined, together with the environmental changes believed to be responsible. The study has been extended to other raptors and a wider range of other bird types.