Fledgling production and population trends in Finnish common eiders (Somateria mollissima mollissima) — evidence for density dependence

Abstract

We present a 57-year time series of a common eider, Somateria mollissima mollissima (L., 1758), population from one of the core monitoring areas in the Baltic Sea, the Söderskär bird sanctuary, Gulf of Finland. We applied permutation tests to inspect the relationships between breeding parameters and population density. Of the parameters studied, only fledging rate (during a 34-year period) showed a significant negative relationship with population size, indicating density dependence. Furthermore, the fledging rate responded strongly to the population growth rate and to the rate of recruitment. Clutch size and duckling (downy young entering the water) rate did not show negative density dependence. Thus, losses during brood rearing seem to be the regulatory factor. The population decline at Söderskär is similar to those recorded in many other monitoring sites around the southern coast of Finland. Compared with data from more productive sea districts in northwestern Europe (Dutch Wadden Sea and Scottish North Sea), the Finnish fledging rates do not seem excessively bad. There are indications of viral infections playing an increasingly central role in duckling mortality, whereas adult female mortality has not been affected.Résumé : Nous présentons une série chronologique de 57 années de données sur une population de l'eider à duvet, Somateria mollissima mollissima (L., 1758), dans un des sites principaux de monitorage de la Baltique, le sanctuaire d'oiseaux de Söderskär, dans le golfe de Finlande. Des tests de permutation nous ont permis d'examiner les relations entre les variables de la reproduction et la densité de la population. Des variables examinées, seul le taux d'envol (au cours d'une période de 34 ans) est en relation négative significative avec la taille de la population, ce qui montre qu'il y a une densitédépendance. De plus, le taux d'envol réagit fortement au taux de croissance de la population et au taux de recrutement. La taille des portées et le taux de production de canetons (jeunes oiseaux duveteux sur l'eau) ne montrent aucune densitédépendance négative. Ainsi, les pertes durant l'élevage de la portée semblent être le facteur de régulation de la population. Le déclin de la population à Söderskär ressemble à ceux observés dans plusieurs autres stations de monitorage le long de la côte sud de la Finlande. Par comparaison aux données obtenues dans les zones plus productives du nord-ouest de l'Europe (la mer des Wadden en Hollande et la mer du Nord en É cosse), les taux d'envol en Finlande ne semblent pas exagérément bas. Il y a des indications que les infections virales jouent un rôle de plus en plus important dans la mortalité des canetons, mais la mortalité des femelles n'est pas affectée.[Traduit par la Rédaction]

Survival of female common eiders Somateria m. mollissima in a declining population of the northern Baltic Sea

“…As reported by Hario and Rintala (2006), the fledging rate at Söderskär tended to decrease with time. This was principally an outcome of the extremely low fledging success during the steeply decreasing phase of the population size in 1986-1996, when the per capita fledging rate was 0.052 ± 0.013 (SE).…”

“…Finally, whatever the causality behind the relatively small range of variation in apparent annual female survival rates, results suggest that female survival is not the main driving force behind the population changes. In a previous study, Hario and Rintala (2006) showed that annual reproductive rate (fledglings/female) at Söderskär was strongly related to population growth rate. The proportion of explainable variance was 61%, whereas the corresponding figure is only 2% in the current survival data (Fig.…”

“…We have previously shown that fledgling production is the trait bearing the highest population regulatory effect among the breeding parameters of this population and found significant direct density dependence (Hario and Rintala 2006). Here, we aim to investigate the role of adult female survival in population dynamics, and whether or not survival responds to large-scale climatic variations.…”

Habitat-specific clutch size and cost of incubation in eiders reconsidered

“…For example, in eiders, females rarely have the same clutch size in consecutive years (Erikstad et al 1993), which could be caused by uncontrolled partial predation on Wrst-laid eggs. To take another example, it may prove to be futile to detect density dependence in the fecundity of female eiders (Hario and Rintala 2006), since uncontrolled and potentially elevated partial clutch predation may mask any positive eVect of decreased breeding density on female fecundity. The need to avoid predation of Wrst-laid eggs may even shape the expression of behaviours such as conspeciWc brood parasitism and its counterpart, egg adoption, in eiders (Robertson 1998).…”

Trends and drivers of change in diving ducks

“…The reasons for fluctuating numbers of breeding Common Eiders are not known. ), and Hario & Rintala (2006 have convincingly argued that the decline of the population of Common Eiders on Söderskär, Gulf of Finland, primarily was driven by very low fledging success, while female mortality remained on a low level throughout the period of decline. However, they also argued that productivity was generally depressed all over the core breeding range in the south-western archipelagoes.…”