Monday, July 13, 2015



Symposium: Biology of the American Oystercatcher.
Chair: T. Simons

Symposium: Aquatic Passerines: The Youngest Waterbirds.
Chair: K. Ruskin

Contributed Paper Session: Waterbird movements.
Chair: P. Jodice
Contributed Paper Session: Demography and foraging.
Chair: David Shealer
Contributed Paper Session: Conservation.
Chair: Scott Demers

Bruno Ens. Sovon Dutch Centre for Field Ornithology 

Abstract: The contribution of long-term studies of Oystercatchers to science and conservation.

The behavioural choices of individuals, like recruitment, risk-taking and feeding of nestlings, determine demographic rates and thus population processes. At the same time, the costs and benefits of particular behavioural choices depend on these population processes. It is from this Darwinian perspective that I have been studying the behavioural ecology and population dynamics of Eurasian Oystercatchers Haematopus ostralegus for the last 35 years. The Oystercatcher is a very good study species because it is a large bird living in an open habitat where it feeds on relatively large prey that can be monitored with little effort. The birds are easily marked and their high site fidelity allows the social career of individuals to be followed throughout their lives. However, the longevity of the species (it may live over 40 years) means that it requires perseverance to obtain the necessary measurements, i.e. keep the population study running. During winter, individuals compete for food. During summer, individuals compete for high quality territories and high quality mates. We understand much about the social career during the breeding season and the social career during the nonbreeding season, but we know very little about how these two careers are linked.

As I now see it, the aim is to unify three partial descriptions of Oystercatcher society in terms of competition, each accompanied by their own body of theory: Distribution theories describe the access of individual animals to limiting resources in space. Life-history theory describes the access of individual animals to limiting resources in the course of their life, and how these resources are  allocated to survival and reproduction Mating systems theory describes the access of individual animals to partners as a resource limiting reproduction. The queue model, which postulates a trade-off for nonbreeders between settling at an early age in a poor quality territory and at a later age in a high quality territory, links distribution theory to life-history theory. The initial queue model, where it was assumed that there were no major differences between individuals was proven wrong: chicks from high quality territories have a much higher chance to recruit into high quality territories compared to chicks from low quality territories. This silver-spoon effect has a long-term impact on fitness that increases over generations.

A major challenge to our current efforts to further improve the queue model is that the world is changing. The long-term population study on Schiermonnikoog shows that the expected increase in mean winter temperature will halt the current population decline, as does the expected decrease in variability in temperature. However, changing wind patterns have led to an increase in the risk of flooding during the breeding season, which offsets the expected benefit of warmer winters. Our long-term population is not the only one in decline: nearly everywhere in the Netherlands wintering and breeding populations are declining at an alarming rate since the late 1980s. A multitude of causes have been identified. In summer these include agricultural intensification (inland breeding populations), increased flooding during the breeding season (saltmarsh breeding populations) and increased predation (mainland saltmarshes). In winter, when all birds depend on tidal flats, these include: shellfish fishery, erosion of tidal flats (Oosterschelde) and colonization of mussel beds by the introduced Pacific oyster. Our aim is to construct a metapopulation model to assess the relative contribution and cumulative impact of all these factors on the population decline. For this, we rely heavily on citizen science, where we initiated volunteers to set up colour marking programs throughout the Netherlands, and developed a website where volunteer observers could input their observations of colour-marked individuals.

The current version of the Scientific Program for the meeting can be found here: DraftProgram07122015

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