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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123, No. 3. September 2011 
ensuring accuracy of 1-1.5 m. Distances between 
all nests within the colony were calculated, which 
allowed calculations of the nearest neighbor 
distance (m), nest density (number of nests within 
the radius of 15 m), distance to the colony center 
(m), and distance to the center of the nest cluster 
(m). Colony and cluster centers were calculated as 
the mean coordinates of all nests within the 
colony and particular clusters, respectively. Nest 
location characteristics were described for all 
nests in the colony, as there was no time-gap 
between breeding activities of the earliest and the 
latest pairs of terns. All nest location character¬ 
istics were divided into separate intervals. Nest 
density was divided into three categories: low 
(—4 nests within a radius of 15 m), intermediate 
(between 4 and 11 nests within a radius of 15 m), 
and high (>11 nests within a radius of 15 m). 
Nests were grouped as near (<1.5 m), interme¬ 
diate (between 2 and 5.5 m), and far (>5.5 m) to 
the nearest neighbors. Nests were also classified 
by distance to the colony/cluster center as central 
(distance to the colony center <70 m; distance to 
the cluster center <10 m), intermediate (distance 
to the colony center between 70 and 150 m; 
distance to the cluster center between 10 and 
45 m). and edge (distance to the colony center 
~ 150 m; distance to the cluster center >45 m) 
nests. The intervals of all categories were 
established ad hoc to produce similar sample 
sizes. We avoided the confounding effect of 
clutch size on the size of eggs by investigating 
spatial patterns of egg size only for three-eee 
clutches. 66 
Statistical Analyses. —Main-effects and factori¬ 
al analysis of variance (ANOVA) were used to 
investigate influence of different spatial charac¬ 
teristics on clutch and egg size. Post-hoc analyses 
were performed with the Tukey HSD procedure 
Significance of linear and quadratic trends among 
different groups of categorical variables was 
checked with trend analysis. All statistical proce¬ 
dures followed Zar (1996). 
RESULTS 
Nest location within the colony depended on 
time of breeding; pairs breeding in different 
periods of the season differed in distance to the 
cluster center and were in areas of different nest 
fT y , nK* F , = 289 - df = <■' = o. 03 1! 
Nests of the Earliest bra h 0 001 ’ res P ec(ive ly). 
centers of nesTc'Ttl Were closer to the 
nest clusters m comparison to nests of 
FIG. 2. Within-cluster location of nests of Whiskered 
Terns breeding in different periods of the season (central 
point = mean, error bars = 1.96 X SE) at Jeziorsko 
Reservoir, central Poland. Numbers above indicate 
sample sizes. 
pairs which bred in the following 10-day penod 
(Tukey test: P = 0.035, Fig. 2). Terns built nests 
in areas of highest nest density at the beginning of 
the breeding season and occupied less dense areas 
as the season progressed (trend analysis: F = 
20.47, df = I, P < 0.001). There was no 
relationship between timing of breeding and either 
nearest neighbor distance or distance to the colony 
center (all P > 0.15). 
The mean (± SD) clutch size in the colony was 
2.47 ± 0.76 eggs (n = 125). Three-egg clutches 
were recorded most frequently (55.2%), followed 
by clutches of two eggs (27.2%), and one egg 
(14.4%). Four-egg clutches were recorded spo¬ 
radically (3.2%). Clutch size did not differ 
between nest clusters (ANOVA: F = 2.26, df = 
3, F = 0.09). Clutch size was significantly related 
to the distance to cluster center and nearest 
neighbor distance after accounting for timing of 
breeding (ANOVA: F = 3.19, df = 2,P = 0.045; 
F = 3 - 36 ’ df = 2 , P = 0.038, respectively), but 
was not affected by the distance to the colony 
center (ANOVA: F = 0.38. df = 2, P = 0.69). 
Pairs which nested closer to the centers of clusters 
and closer to their nearest neighbor had signifi¬ 
cantly higher clutch size (trend analysis: F - 
5.70. df = 1. P = 0.019. Fig. 3; F = 3.93; df = I: 
P = 0.049, respectively). 
The mean (± SD) egg length was 38.70 - 
1.54 mm, the mean breadth was 27.64 ± 0.76 mm. 
and the mean volume was 14.40 ± 1.05 cm*(#i = 
309). We found no trade-off between egg volume 
and clutch size (ANOVA: F = 2.93. df = 2, P = 
0.06), but there were significant differences in egg 
