Mimas et al. • SPATIAL PATTERNS IN A COLONY OF WHISKERED TERNS 
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cosmopolitan species characterized by fluctuating 
numbers and scattered distribution (Cramp and 
Simmons 1997). It is classified as a species of 
unfavorable Pan-European conservation status 
and as a conservation priority by the European 
Union Wild Birds Directive (BirdLife Interna¬ 
tional 2004). There are few reliable data on the 
breeding ecology of Whiskered Terns, which 
precludes application of appropriate conservation 
measures (Bakaria et al. 2002, Paillisson et al. 
2006). The goal of our paper is to investigate 
spatial variation in clutch and egg size within a 
colony of Whiskered Tems in central Poland. 
Whiskered Terns nest on beds of floating 
vegetation (Paillisson et al. 2006), which is a 
highly homogenous habitat and its quality is 
expected to have a negligible role in choice of 
nest sites within the colony. Therefore, we 
expected central-periphery distribution of nesting 
pairs resulting in earlier breeding and higher 
clutch/egg size in central parts of the colony or 
particular nest clusters. 
METHODS 
Study Area.—The research was conducted at 
Jeziorsko Reservoir in central Poland (51'40' N, 
18° 40' E). Jeziorsko is one of the largest 
reservoirs in Poland with respect to water surface, 
which oscillates between 17.6 km 2 in autumn to 
42.3 km 2 during highest water level in spring. 
Suitable habitats for Whiskered Terns do not 
appear at the reservoir before the second half of 
June, due to water management policies resulting 
in high water levels earlier in the season. The 
colony of Whiskered Terns studied was al the 
western shore of the reservoir, in the proximity of 
the village Tomislawicc. Nests were on floating 
vegetation dominated by amphibious bistort 
(Polygonum amphibium). The distribution of 
vegetation within the colony w'as patchy and four 
separate beds of floating vegetation were distin¬ 
guished (Fig. 1). Beds of floating vegetation were 
separated by much deeper open-water channels 
and we distinguished four separate nest clusters 
(A-D, Fig. 1). The mean distance between centers 
of neighboring nest clusters was 125 m. There 
were 21 to 44 nests in each cluster. 
Field Procedures.—Viz visited the colony 
every 10 days from 20 June 2009, when the first 
nests were initiated, throughout the entire breed¬ 
ing season. The last clutches were initiated 
between 20 July and 1 August. We assigned 
initiation of each clutch to particular 10-day 
FIG. I. Nest clusters distinguished within the colony of 
Whiskered Tems at Jeziorsko Reservoir, central Poland in 
2009. Solid lines delimit distribution of floating vegetation 
within the colony. 
periods. We recorded 125 active nests, all of 
which were individually marked. The length (L) 
and breadth (B) of all 312 eggs found in the 
colony were measured with calipers to the nearest 
0.1 mm by the same person. Each egg was marked 
with indelible ink, Egg volume was calculated 
using the formula: V = 4.866 X 10 4 X L X B 2 
(Coulson 1968b). Mean within-clutch values of all 
egg size characteristics were used in the analyses 
to avoid pseudo-replication (Bahbura and Zie¬ 
linski 1990). One clutch with an atypically small 
egg was excluded from the data set, following 
suggestions of Zar (1996) on sample homogene¬ 
ity. The within-clutch variation in length, breadth, 
and volume of eggs was expressed as the within- 
clutch coefficients of variation (CV) calculated 
with the formula: CV = SD X 100/Y, where SD 
is the standard deviation and Y is the mean 
within-clutch value of egg measurements. Calcu¬ 
lations of the coefficients of variation are known 
to be biased for small sample sizes (Sokal and 
Rohlf 1995) and we applied the correction: 
CVadjusted = <l+l/4n) X CV, where n is clutch 
size. Location of all nests within the colony (n = 
125) was mapped using a hand-held Global 
Positioning System (GPS) unit (Garmin GpsMap 
60Cx, Olathe. KS, USA) with European Geosta¬ 
tionary Navigation Overlay Service (EGNOS) 
