biologia - Studia

STUDIA UNIVERSITATIS BABEŞ – BOLYAI, BIOLOGIA, LV, 1, 2010, p. 45-59
PLATFORM INITIATION, CLUTCH INITIATION AND CLUTCH
SIZE IN COLONIAL GREAT CRESTED GREBES
PODICEPS CRISTATUS
ANDRÉ KONTER 1
SUMMARY. Data of platform initiation, clutch initiation and clutch size of
colonial Great Crested Grebes Podiceps cristatus were collected over a period of
four years and analyzed with respect to seasonal patterns and possible differences
between experienced birds and first-time breeders. Extraordinary large (above 5
eggs) and small (below 3 eggs) clutch sizes were analyzed separately and possible
cases of conspecific brood parasitism were documented. Average full clutch sizes
of Great Crested Grebes were seasonally decreasing and it was concluded that this
should be the norm. The seasonal decline in final nest content was triggered by
seasonally increasing numbers of small clutches and in parallel decreasing numbers of
exceptionally large clutches. Also, parasitic egg reception increased average clutch
size more in early than in later clutches. Brood parasitism was a major component
in exceptionally large clutches. At least for experienced birds, the not parasitized
clutch size should be comprised of 3 to 5 eggs. Differences in nesting density inside
the colony could possibly explain part of the parasitic egg laying. Adverse weather
conditions leading to the destruction of part of the platforms were found to play an
important role, too. The breeding parameters associated with small clutches
pointed at a high likelihood that these could have originated from inexperienced
females that could have laid later in the season and produced fewer eggs.
Keywords: brood parasitism, clumping of nests, clutch initiation, clutch size,
colonial breeding, experience, platform initiation, Podiceps cristatus.
Introduction
Clutch size in the Great Crested Grebe (Podiceps cristatus) is variable and
ranges from 1-2 to 8-9 eggs (Dittberner 1996, Fjeldså 2004, Ilicev and Flint 1985,
Vlug 1983), but more commonly is comprised between 3 and 5 (Dittberner 1996,
Ilicev and Flint 1985, Konter 2001, Melde 1973, Simmons 1989). Different studies
on the subject found seasonally decreasing clutch sizes, independently from whether
the grebes nested solitarily or colonially (Blinov et al. 1981, Fjeldså 2004, Goc
1986, Henriksen 1992, Ulenaers and Dhondt 1991). The most commonly accepted
explanation for this phenomenon is that experienced grebes breed earlier in the
1 Collaborator of the Museum of Natural History, L-2345 Luxembourg, E-mail: podiceps@pt.lu