Eggshell thinning of osprey (Pandion haliaetus) breeding in Sweden and its significance for egg breakage and breeding outcome

Abstract

In the middle of the 1960s low reproductive rate was found in several osprey (Pandion haliaetus) nests in Sweden. Therefore a project was started to investigate the production and to collect unhatched eggs and shell fragments in different regions of Sweden during 1971–2008. Measurements of osprey eggs from museum collections from 1840 to 1970 were included to study long-term changes in shell thickness. In total, eggshell thickness of whole eggs from 666 clutches and of shell fragments from 693 nests was measured.The thinnest shell fragments were recorded in 1973 (mean for 71 clutches: 0.438mm), minus 15.0% compared to thickness of unaffected eggs before 1946 (0.515mm). After 1973, shell thickness increased to reach background levels in 2003 (0.515mm). Thus, it took 30years to reach full thickness again. From the start of the decrease it took more than 50years to reach unaffected conditions. The number of whole eggs remaining intact in the nest throughout incubation was reduced when shell thickness decreased. Great loss of eggs due to breakage seems to occur when the mean shell thickness in the clutch was below 0.40mm. In 1971–1973, when shell thinning was most pronounced, the average thickness of shell fragments was 0.366mm in nests with 0 intact eggs (minus 29% compared to the pre-1946 background value); 0.393mm (minus 24%) in nests with 1 intact egg; 0.431mm (minus 16%) in nests with 2 intact eggs and 0.450mm (minus 12%) in nests with 3 intact eggs. Differences in thickness were significant for 1 versus 2 intact eggs, 2 vs. 3 intact eggs and for 0+1 vs. 2+3 intact eggs. Actual data from the period 1971–73 shows a decline in the production of young (4–5weeks) of some 15% (0.25 young per nest).