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3 D e p ar tm en t o f T e a c h e r T ra in in g an d B io d iv e rsity S tu d ie s, U n iv ersity o f Ł ó d ź , 1/3 B a n a ch a S tr., 9 0 - 2 3 7 Ł ó d ź
VARIABILITY OF CLUTCH SIZE IN CORM ORANT
(.PH AL AC RO C O R AX CARBO S IN E N S IS ) AT THE JEZIORSKO
RESERVOIR (CENTRAL POLAND) IN 2004
A b s tra c t: The analysis o f the clutch size variability in C orm orant (P. carbo sinensis) was based on study results in colony at the “Jeziorsko Reservoir” (central Poland) in 2004. T here were used 328 broods controlled 3 -5 tim es in the season. Num ber o f eggs in a single clutch ranged from 1 to 7, although Corm orants m ostly laid 3 to 5 eggs. M ean clutch size was 3.80 (SD = 1.0 0). The size o f clutch depended on time o f laying eggs and the part o f colony where the nest with brood was localised. The clutch size was bigger for pairs that started laying eggs earlier and lower for birds bred in the part of colony where the num ber of nests and their density were low. Key w ord s : clutch size, Corm orant, central Poland
1. IN T R O D U C T IO N
Clutch size is one of the primary parameters studied in bird breeding ecology. Studies focused on its variability and factors shaping this variability are numerous and multi-dimensional (e.g. La c k 1954; Pe r r i n s 1965; Pr i c e, Li o u 1989 and many others).
Recently, there is a constant increase in interest shown in the issues concerning Cormorant (Phalacrocorax carbo), a species which considerably interfere with human’s fishing industry (e.g. v a n Ee r d e n et al. 1995; Ca r s s
et al. 2003). With regard to this matter, researches allowing to objectively estimate the influence exerted by this rapidly expanding species on the
ecosys-tern, seem to be particularly vital. However, if intended to be reliable, such investigations have to reveal sufficient complexity and include several aspects of Corm orant’s population ecology. Study of elementary breeding parameters is of utmost importance among them.
D u ring the last 50 years, the s iz e o f C orm oran t’s breeding pop ulation , its trends and d y na m ics w ere sub jecte d to a thorough exa m in ation (e .g . L i n d e l l et al. 1995; M e l l i n e t al. 1997; PRZYBYSZ 1997; PRZYBYSZ et al. 1997; M e l l i n , M i r o w s k a - I b r o n 2003). D esp ite the p ostu late o f perform ing such detaile d in v estiga tio n s a lso in Poland (STEMPNIEWICZ et al. 1998), data c o n -cernin g b reeding parameters o f Corm orant is still not abundant. T h is sh ortage o f in for m ation is straightly con n ecte d w ith frequent in a cc es sib ility o f nests, w hich in c a s e o f Corm orant su bs p ec ie s P. carbo sinensis b ree din g in Poland, are u sua lly loc ated high in the tree crow n s. In co n se q u en ce , obtain ing any e x te n siv e data about clu tch size presents a sp ecia l d ifficu lty . In Poland, such broad material w a s c o llec te d on ly in o n e m aritim e c o lo n y in Kąty R yb ack ie at the G u lf o f G dańsk (KOPCIEWICZ et al. 2003). T h e o ccurren ce o f Cormorant breeding c o lo n y in th e m idd le part o f the coun try is a re lativ ely recent p he nom en on . Its b reed in g attempts in this reg ion w er e o b serv ed on ly sin ce the 80s o f X X century ( J a n i s z e w s k i et al. 1991; T o m i a ł o j ć , S t a w a r c z y k 2003). N e v erth eless , the m ajority o f recorded c o lo n ie s occurred to be im perm anent. T he on ly b ig and stab le c olo n y is located at the Jeziorsko reservoir (JANISZEWSKI et al. 1998; OSIŃSKA, JANISZEWSKI 2006). T h e aim o f th is paper is to describ e variation o f clu tch s iz e in C ormorant (P. carbo sinensis), b reeding in this c o lo n y in 2004.
2. M A T E R IA L S AND M E T H O D S
Research was conducted in the colony of Cormorant, located in the ‘Jeziorsko Reservoir’ natural reserve, which forms the southern part of water body of the same name. Jeziorsko reservoir was constructed on the river W arta at the border between provinces of Łódź and Poznań (Fig. 1). Official exploitation of the reservoir started in the autumn of 1986. W ater surface area equals 42 km2 during the maximum water level, whereas during minimum level it does not exceed 18 km2. Reservoir is 16,5 km long and 1.8-3.5 km wide (Or ł o w s k i 1994),
“Jeziorsko Reservoir” natural reserve is not only an important breeding area for plenty of bird species, but it is also exploited by seasonal migrants as
a suitable foraging and stop-over site ( Ja n i s z e w s k i et al. 1998; Os i ń s k a, Ja n i s z e w s k i 2006). Cormorant’s colony at the reservoir was first recorded in
1991. In 2004, the colony was set in the proximity of Mikolajewice village (51°73’N 18°63’E), next to the former Warta riverbed. It was located in the willow thicket, mainly consisting of White Willow (Salix alba) and Grey Willow (Salix cinerea). W illows in the colony were predominantly of the shrub-type, only the minor fraction was comprised of medium high trees (up to 15 meters). The whole territory of the colony was severely sectioned by the web of channels and pools. In 2004 approximately 410 pairs o f Corm orant bred in the colony.
Fig. 1: Jeziorsko R eservoir with location o f C orm orant (P. carbo sinensis) colony, and its structure (four distinctive parts) as GPS view
Material was gathered in the colony of Cormorant at the “Jeziorsko Reser-voir” natural reserve during year 2004. Colony was controlled once a week in the period between the middle of March and July. Checks of the nest content were regularly performed during every control. Dates of nest checking were precisely fixed in order to maximize accuracy of egg laying onset and clutch size estimations. Therefore, nests were checked both during the building phase, as well as later, during the incubation and offspring upbringing period. Every nest was individually marked. Nest controls were carried out either directly from a ladder or with a mirror attached to the top o f 4 m eter high stick. The
most inaccessible nests were observed from neighbouring trees through bino-culars. Relatively low location of nests in the colony (ranging from 2 to 9 meters high) permitted successful controls of almost all of them.
Approximate dates o f onset of breeding were estim ated during a few subsequent checks. In many cases, it was possible to define exact day of laying the first egg on the basis of two consecutive controls, provided the clutch was incomplete during the first o f them. If such information was lacking, the evaluation was based on the size and the level of de-velopment of nestlings, assuming the incubation period to last for ca 4 weeks
( Sn o w, Pe r r i n s 1998). Eventually, broods were divided into three categories
(Table 1):
• early broods, when eggs were laid in the last decade of March (Term 1), • intermediate broods, when eggs were laid between l sl and 20th April (Term 2), • late broods, when eggs were laid after 20lh April (Term 3).
T ab le 1: The num ber o f broods which of 2004
started in different periods
T erm s N u m b e r of bro od s
Term 1 (Early broods) 49
Term 2 (Interm ediate broods) 109
T erm 3 (Late broods) 24
T o t a l 182
The colony was clearly divided into four parts. All the parts varied distin-ctively in general conditions, which thus were likely to affect breeding activities of birds in a different way. Inter-part variation was manifested in such parameters as thickness o f willow shrubs, height of shrubs and trees or simply the reciprocal location. Hence, particular parts were plausibly of different attractiveness for breeding pairs, which in turn resulted in different densities of nests (Fig. 1). Four distinctive parts of the colony were marked out:
1. A part with diverse height and low density of nests. Thickness of willow shrubs also low.
2. A part with maximum density of nests, which were located low in the trees. Thickness of willow shrubs medium, majority o f shrubs dried up.
3. A part with nests frequently placed high in the tree crowns. Such loca-tion precluded some of broods from regular controls. Thickness of shrubs medium.
4. A part separated by water channels from the rest of colony. Density of nests and thickness of shrubs diverse. Some of nests also not possible to be checked.
Comparisons between egg numbers laid in different parts of colony and during different periods were performed on the basis of variance analysis (ANOVA 1) and Tukey test.
3. R E SU L T S
Number of eggs in a single clutch of Cormorants breeding at Jeziorsko reservoir in 2004 ranged from 1 to 7 (Fig. 2, Table 2). Nevertheless, on average, a clutch consisted of 3 to 5 eggs and the cases of bigger clutches were extremely rare - only four ones recorded in 2004.
Fig. 2: Clutch size distribution in 2004
T ab le 2: The characteristics of distribution of eggs’ num ber per clutch in 2004
N M ea n SD Skew ness SE skew ness
We found significant differences in the mean egg numbers laid in the successive terms of the breeding season (F2;i79 = 40.234, P< 0.0001). Pairs which started laying eggs earlier had bigger clutches in comparison with those which started later (Fig. 3, Table 3).
Fig. 3: N um ber o f eggs in three groups show ing different tim es o f breeding onset in 2004. Circles are m eans, error bars are 1.96 SE
Tim e of laying e g g s
T a b le 3: T he m atrix for significance values o f differences between pairs o f m eans in Tukey test for clutch size o f Corm orants that started to breed in different tim e in 2004. Significant valu es are in bold
T e rm 1 T e rm 2 T e rm 3
Mean = 4.37 M ean = 3.96 M ean = 2.5
Term 1 _ 0.0488 0.0000
Term 2 - - 0.0000
The results of analysis of mean clutch size diversity in relationship with the part of colony in which the brood took place are presented in Fig. 4 and Table 4. It was shown that clutch size is dependent on the nest location (F3.324 = 7.705, P< 0.008), since Cormorants breeding in the Parts 2 and 3 laid significantly more eggs than those from Part 1. Birds from Part 4 on average laid intermediate number of eggs, which did not differ significantly from clutch sizes recorded in the other parts.
Fig. 4: Num bers of eggs recorded in the four parts of colony (see Fig. I). Circles are means, error bars are ± 1.96 SE
T ab le 4: T he m atrix for significance values o f differences betw een pairs o f m eans in Tukey test for C orm orant clutch size in different parts o f colony in 2004. Significant values are in bold
Part 1 Part 2 Part 3 Part 4
M ean = 3.26 M ean = 4.08 Mean = 3.94 M ean = 3.74
Part 1 _ 0.0006 0.0227 0.0973
Part 2 - - 0.9418 0.0586
Part 3 - - - 0.8391
4. D ISC U SSIO N
Clutch size is one of the major factors, which determ ine potential reproduc-tive capability of the species. Analysis of this param eter did not reveal any significant difference between investigated colony and most of other European colonies. Similarly to other researches, clutches of 3 to 5 eggs were principally predominant at Jeziorsko reservoir, with clutches consisting of 4 eggs recorded most frequently. Such distribution of clutch sizes is known to be typical of Cormorants. Consequently, the clutches of 1-2 or 6 -7 eggs were noted only sporadically ( De b o u t et al. 1995; van Da m, As b i r k 1996; Pr z y b y s z 1997;
KopciEWicz et al. 2003). Nevertheless, the clutches o f 1 or 2 eggs are exceptionally found to comprise a considerable fraction of all the broods in the colony. Such high proportion of small clutches was recorded by PAJKERT
and GÓ RSK I (1997) in Cormorant’s colony at Gardno Lake in the Słowiński
National Park. Authors explicated this situation with the relatively young age o f breeding birds and the presence of partial nest predation exerted by the Herring Gull. Due to KOPCIEWICZ et al. (2003) a clutch composed of 4 eggs appears to be o f the optimal size, as it promotes maximum reproductive output of a breeding pair with regard to the minimum cost o f eggs production. The authors found out that in case o f bigger clutches there is a tendency for increased losses, hence the increase in number of laid eggs has strictly limited impact on the num ber of fledged offspring.
Research in the colony at Jeziorsko reservoir showed a significant correlation between clutch size and the onset o f breeding. Sim ilar relationship was found in Kąty Rybackie at the G ulf of Gdańsk by KOPCIEWICZ et al. (2003). Due to the authors, April clutches were on average comprised o f 3.5-3.8 eggs, whereas those laid during May consisted of 3.1-3.4 eggs. Furthermore, not only time dependent differences, but also internal spatial diversification in clutch size was observed in the colony at Jeziorsko reservoir. Generally, the smallest clutches were found in Part 1, while the biggest ones in Parts 2 and 3. In Part 1 the number o f nests and their density were relatively low, which implies that this area was rather suboptimal for breeding. Similar diversification was observed in Kąty Rybackie, where the smallest clutches were laid in the newly established part o f colony, which was formed by relatively few nests (Kop-
c iE W ic z et al. 2003). On the one hand, such differences could be explained by different quality o f birds, which breed in certain parts o f colony. This could be manifested for example by various age, experience or condition of individuals. Beyond a doubt, there is a competition for breeding sites taking place within colony (B lR K H E A D, FURNESS 1985). Individuals of higher status are capable of
occupying optimal places for breeding, whereas birds o f lower status have to accept the suboptimal ones. Moreover, intrinsic features of bird mates, such as age, are likely to exert considerable influence on clutch size ( C l u t to n - B r o c k 1988). On the other hand, the differences can be caused by lower risk of predation in more densely inhabited parts of colony. Taking into consideration predator’s traces left on the shells of destroyed eggs, tree-dwelling mammals are plausibly responsible for significant fraction o f losses at the incubation stage in colony at Jeziorsko reservoir. Observations suggest that these are particularly Beech Martens (Martes foina), which were repeatedly noticed in the colony and even a litter was once found in the old Cormorant’s nests. It cannot be excluded that martens are unable to successfully cope with robbing eggs in case of close presence and aggressive behavior o f several adult Cor-morants.
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