Minias et al. • SPATIAL PATTERNS IN A COLONY OF WHISKERED TERNS 
489 
FIG. 3. Clutch size in relation to the within-cluster 
location of nests of Whiskered Terns (central point = mean, 
error bars = 1.96 X SE) at Jeziorsko Reservoir, central 
Poland. Numbers above indicate sample sizes. 
breadth between clutch size classes (ANOVA: F 
= 3.05, df = 2, P — 0.032). Egg volume did not 
differ between nest clusters (ANOVA: F - 2.02, 
df = 3, P = 0.13). There was significant temporal 
variation in volume, breadth, and length of eggs 
(ANOVA: F = 2.72, df = 3, P = 0.048; F = 
3.53, df = 3, P = 0.033; F = 3.87, df = 3. P = 
0.011, respectively). There was a constant de¬ 
crease in egg length over the course of the season 
(trend analysis: F = 11.20, df — 1, P - 0.001). 
Egg breadth and volume had no linear trends in 
time (trend analysis: all P > 0.1). Egg length and 
volume were related to nest density (ANOVA: F 
= 3.59, df = 2, P = 0.033; F = 3.19, df = 2, P = 
0.048, respectively). Eggs had the highest length 
and volume in the intermediate nest densities 
FIG. 4. Relationship between nest density and egg 
volume in 3-egg clutches of Whiskered Terns (central point 
= mean, error bars = 1.96 X SE) at Jeziorsko Reservoir, 
central Poland. Numbers above indicate sample sizes. 
FIG. 5. Within-clutch variation in egg volume of 
Whiskered Terns in relation to the within-cluster location 
of nests (central point = mean, error bars = 1.96 X SE) al 
Jeziorsko Reservoir, central Poland. Numbers above 
indicate sample sizes. 
(quadratic trend analysis: F — 7.17, df = 1, P = 
0.009; F = 6.35, df = 1, P = 0.014, Fig. 4, 
respectively). There was a significant relationship 
between distance to the cluster center and within- 
clutch variation in egg volume (ANOVA: F = 
3.32, df = 2, P ■= 0.043). Edge clutches had 
higher variation in egg volume in comparison to 
more central clutches (trend analysis: F = 5.07, df 
= 1, P = 0.028, Fig. 5). Similarly, within-clutch 
variation in egg breadth increased with increasing 
distance to the colony center (trend analysis: F = 
4.77, df = 1, P = 0.033). 
DISCUSSION 
We found specific spatial patterns in establish¬ 
ment of the colony of Whiskered Terns at 
Jeziorsko Reservoir. Nest sites in the central parts 
of clusters were occupied earlier in comparison to 
the edge sites. Pairs which bred at the edges of 
nest clusters and distant from the nearest neigh¬ 
bors had smaller clutches. The mean egg size of 
low-density pairs was smaller in comparison to 
the pairs nesting in denser areas. These findings 
indicate that quality of Whiskered Terns breeding 
in the sparse peripheral zones of the nest clusters 
was lower in comparison to quality of pairs 
nesting in more central or denser areas. This was 
further confirmed by higher within-clutch varia¬ 
tion in egg size recorded for edge pairs. Intra¬ 
clutch variation in egg size for the majority of 
studied land species was negatively correlated 
with fledging success and decreased with the age 
of parents (Sydem an and Emslie 1992). The 
nesting habitat of Whiskered Terns was highly 
