biologia - Studia

PLATFORM INITIATION, CLUTCH INITIATION AND CLUTCH SIZE IN COLONIAL GREAT CRESTED GREBES
Deducting detected egg parasitism from clutch sizes, mean values were
reduced by 0.43 to 4.81 eggs in early April 2005 and by 0.45 to 3.45 eggs in mid-
April 2005. In 2006 the respective decreases were of 0.29 and 0.39 to 4.29 and 4.05
eggs. It is also noteworthy that in 2005 for 6 large clutches out of 9, parasitic egg
reception was proved whereas in 2006 this was the case for 5 out of 7. From the 17
large clutches in total, 11 were initiated in the first decade of April, 6 in the second
and none in the third.
Discussion
The asynchronous platform and clutch initiation by Great Crested Grebes
revealed in this study appears to be rather common, even in colonial nesting of the
species (Berthoud, 1963; Blinov et al., 1981; Henriksen, 1992; Koshelev, 1977;
Koshelev and Chernichko, 1985; Leys et al., 1969; Van IJzendoorn, 1967). At least
for the Netherlands, the asynchronies could be largely triggered by the migratory
behaviour of the grebes and their arrival at the breeding grounds in distinct periods
whereby late birds are migrants and may be more subject to extended pre-laying
periods, depending on their body conditions upon arrival (Ulenaers and Dhondt 1991).
Concerning clutch size evolution, I found only 2 studies that observed
seasonal increases of mean clutch sizes. Bukacinska et al. (1993) differentiated between
colonial and non-colonial grebes on the same lake. While seasonal clutch size
increased by ±0.4 eggs in colonial birds, it declined by ±0.7 eggs in non-colonial
birds. Including all nests in a single calculation however resulted in a slight seasonal
decline of ±0.15 eggs. Leys et al. (1969) studied the Great Crested Grebes at a site
directly adjacent to Lake IJssel and found clutch sizes first to increase and to
decrease later. The habitat originated only towards the mid-1960s and therefore could
not receive from the start grebes returning to a breeding site known from previous
years. It recorded extraordinarily high losses of clutches and platforms (152 nests out
of 263 were destroyed in 1967), mostly due to wave action. Direct food availability
was poor and the adults were forced to fly to the nearby IJsselmeer to feed. This
suggests that the habitat was problematic, especially early in the season, and it
should therefore have been avoided to a large extent by experienced grebes or more
generally by grebes physically in condition to settle at better sites. We conclude
that under normal conditions, average full clutch sizes of Great Crested Grebes
should decrease seasonally.
Ulenaers and Dhondt (1991) explained the decreases of nest contents not
by age differences of the grebes, but by differences in body conditions of locally
wintering and late arriving grebes: females in good condition immediately start
laying, otherwise they wait and possibly lay smaller clutches. This study found that the
seasonal decline in final nest content was not only triggered by seasonally
increasing numbers of small clutches (1 or 2 eggs). In parallel, numbers of exceptionally
large clutches (over 5 eggs) decreased seasonally. Also, parasitic egg reception increased
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