biologia - Studia

PLATFORM INITIATION, CLUTCH INITIATION AND CLUTCH SIZE IN COLONIAL GREAT CRESTED GREBES
For all 4 years, seasonal changes in clutch sizes were analyzed with respect to
the dates of platform initiation and clutch initiation. Clutch sizes were also grouped
with respect to their degree of clumping. Clumping of nests was expressed by the
number of neighbouring platforms within 3 m (Konter, 2005). The measure of clumping
retained was taken at the moment when the maximum density for the colony was
reached, independently from clutch initiation dates of the different nests. It was
considered that even if clutch initiation was early when the degree of clumping was less,
the pressure from not yet settled grebes was proportional to later degrees of clumping.
Full clutches were classified into 3 categories depending on the date of
clutch initiation: first egg laid before 11 April, from 11-20 April and after 20 April.
During the night from 8 to 9 April 2005, a storm destroyed most of the existing
platforms, whether containing eggs or not. Therefore related data were analyzed
separately. Full clutch sizes were also classified into 4 categories depending on the
date of platform initiation: start of building in March, 1-10 April, 11-20 April, and
after 20 April. Only nests in the Reed zones of the colony were considered as the
numbers of nests in the Bulrush zone were too limited in each year (between 17
and 23), and therefore did not warrant analysis if the data were distributed into
different time or clumping categories.
Full clutches of below 3 and above 5 eggs were considered to be of abnormal
size. They were therefore identified and analyzed separately. The breeding parameters
of their owners (settlement date, clutch initiation date, time from settlement to clutch
initiation) were compared to the average figures for the colony as a whole.
While in 2003 and 2004, the content of each nest was recorded mostly only
once per observation weekend, in 2005 and 2006 nest contents were recorded up to
3 times per observation weekend. In order to detect anomalies in the laying patterns
of the grebes pointing to possible brood parasitism, I focused on the 2005 and 2006
data of nest contents. The egg-laying interval in the Great Crested Grebe ranges
from 1-2 days, but more often occurs every other day (Fjeldså, 2004; Henriksen,
1995, Onno, 1966; Simmons, 1974). Nests were classified as parasitized if an
average laying interval of less than 24 hours was detected in a clutch or if the
addition of late eggs occurred more than 48 hours after laying of the previous egg.
Statistical Analyses. I used a one-way ANOVA to test for differences in clutch size
among nesting-season periods (platform initiation and clutch initiation) and nestclumping
classes. Significant ANOVA results (P < 0.05) were followed by Tukey
post hoc tests for pair wise comparisons. ANOVA and Tukey tests were performed
using the VassarStats web site for statistical computation
(http://faculty.vassar.edu/lowry/VassarStats.html). All means are given ± SD.
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