biologia - Studia

PLATFORM INITIATION, CLUTCH INITIATION AND CLUTCH SIZE IN COLONIAL GREAT CRESTED GREBES
grebes. Other explanations are however possible. Early reservation of a prime nesting
space coupled with delayed clutch initiation means prolonged exposure to
aggressiveness of neighbours and new settlers and high energetic costs for platform
defence. It is rather unlikely that grebes in poor physical conditions would then invest
in early site occupation. They would possibly be better off by first refilling their
energetic reserves outside the nesting habitat to move to the colony later. We may also
ask to what extent birds in poor physical conditions would be able to defend a platform
in a highly competitive context. If we consider in addition that experienced grebes can
be expected not to unnecessarily prolong their presence inside a stressful colony and to
initiate their clutches with minimal delay, habitat and weather conditions permitting,
early settlement coupled with delayed clutch initiation would better fit with birds
lacking experience. In line with the preceding, Vlug (2002 for the Red-necked Grebe)
speculated that young females lay later in the season, possibly also as a consequence of
delayed reproductive development of first-year birds or of their lack of experience for
coping appropriately with high levels of intraspecific aggression, and Goc (1986)
predicted that clutches of 3 and fewer eggs belong to young females.
In conclusion, Great Crested Grebe colonies are commonly composed of a
mix of birds from all age classes and seasonally decreasing average clutch sizes are
the norm for the species. This evolution relies on one hand on in time decreasing
numbers of extraordinary large clutches and on the other on increasing numbers of
small clutches. Not parasitized full clutches are expected not to exceed 5 eggs,
otherwise they are laid by more than one female. Conspecific brood parasitism
decreases gradually in time. The exceptionally small clutches are initiated later,
possibly in their majority by inexperienced females. Under abnormal conditions,
deviations from these general patterns are possible. Additional research should
analyze to what extent younger birds could settle early in the season and what
characteristics are then associated with their nesting.
ACKNOWLEDGEMENTS
Many thanks to Gilbert Laroche, Han Vlug and my wife Maria for their tireless
contribution to field work, and to the staff of the Compagnieshaven and the Museum of
Natural History in Luxembourg for their support. I am also grateful to Han Vlug for commenting
on the first draft of the paper and to Bruce Eichhorst whose most welcome suggestions
further improved this publication. Finally I thank the two anonymous reviewers from
Studia Biologia whose comments helped in clarifying some issues.
REFERENCES
Adriaensen F., Ulenaers P., Dhondt A.A. 1993. Ringing recoveries and the increase in
numbers of European Great Crested Grebes Podiceps cristatus. Ardea 81: 59-70.
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