biologia - Studia

A. KONTER
average clutch size more in early than in later clutches and potential parasitism was
not detected in any late clutch. It seems also clear that early clutches were more
prone to the reception of foreign eggs simply because they existed early when settlement
pressure built up. Brood parasitism was proved in 69% of the exceptionally large
clutches laid in 2005 and 2006 and it could not be excluded in the remaining 31%.
Parasitic egg laying was thus a major component in extraordinary large clutches
which in turn contributed to high average clutch sizes. This is in agreement with
Vlug (1983) who found the modal clutch of Great Crested Grebes in the Netherlands to
hold four eggs and clutches of seven and more eggs are laid most times by two
females. Blinov et al. (1981) and Goc (1986) support this thesis. Other arguments
in favour of the limitation in normal clutch size are the maximum numbers of
fledglings of 3 to 4 in the species and the findings of Lyon and Everding (1996)
predicting that a non-parasitized full clutch of Eared Grebes P. nigricollis holds 3
or 4 eggs and never exceeds 4 eggs while, similarly to Great Crested Grebes, larger
clutches occur. We conclude that, at least for experienced Great Crested Grebes, a
not parasitized clutch should range between 3 and 5 eggs.
Lyon and Everding (1996) found nest density to influence brood parasitism
in the Eared Grebe; high levels of aggression related to dense nesting may provide
more opportunities for a parasitic female to sneak onto a momentarily undefended
nest. We then could expect more parasitic eggs in denser nesting parts of a colony
and therefore generally increasing clutch sizes with increased clumping. This was
indeed the case in the colony investigated here in 2006 and for all years combined,
but the pattern for the 4 years taken separately was not uniform. Nesting density
might therefore only be one out of several factors affecting brood parasitism and
may not play a predominant role. Considering only extraordinary large clutches
(that had a good chance of having been parasitized), we found no homogenous
attributes other than generally early platform and clutch initiation (prior to the
mean dates for the colony). It appeared however that adverse weather conditions
leading to the destruction of a more or less important part of the platforms might
push brood parasitism at least momentarily to higher levels. At Lake Braband,
Henriksen (1996) found that 30% of the females lost their clutch before completion
through nest destruction and he expected platform loss to be responsible for a more
or less important part of parasitic egg laying.
Vlug (1998) argued that fierce nest defence by colonial grebes is possibly
an attempt to prevent brood parasitism on their nests. Behavioural tactics so far
evolved to avoid parasitism may then still be insufficient and would only limit the
degree of conspecific brood parasitism in the species.
Analyzing small clutches more in detail, we observed that their platform
initiation dates were mostly late, their waiting times until clutch initiation generally
above average (80%) and their start of egg laying occurred either on the average date
for the colony as a whole (27%) or later (53%). This pattern could of course be
explained by the differences in migratory decisions and in body conditions of the
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