Dariusz JAKUBAS

University of Gdańsk, Department of Vertebrate Ecology and Zoology

Legionów 9, 80–441 Gdańsk, Poland, e-mail: biodj@univ.gda.pl

THE INFLUENCE OF CLIMATE CONDITIONS ON BREEDING

PHENOLOGY OF THE GREY HERON ARDEA CINEREA L.

IN NORTHERN POLAND

POLISH JOURNAL OF ECOLOGY

(Pol. J. Ecol.)

59 1 179–192 2011

Regular research paper

ABSTRACT: The breeding phenology of the Grey Heron Ardea cinerea L. was investigated in three colonies situated on the Baltic Sea coast in Poland in 1999–2002. The heronries differ in localization (inland vs coastal) and local climate conditions. In inland location, air temperatures in spring were lower by 1.1–1.5°C and ice cover was present on foraging grounds longer by 19–29 days comparing to coastal ones. Herons occupied all colonies in similar dates (multi-year median date for all colonies: 5 March). However, birds from one coastal heronry tended to start breed-ing 9–10 days earlier (multi-year median date: 28 February) than birds from other colonies, what might be possible due to early presence of herons in the vicinity. Accessibility of foraging grounds (lack of ice cover) in spring was an important factor affecting the onset of breeding as dates of colony occupation, egg laying and hatching were positively correlated with the dates of last ice cov-er occurrence on the foraging grounds (r> 0.75, P <0.05). Intercolony differences in hatching dy-namics might result from various frequencies of replacement clutches, and nests of late breeding subadults. In one season (2002), the number of medium-aged chicks per nest and fledging success were negatively correlated with the hatching date. Since chick mortality rate during last three weeks before fledging was not correlated with hatching date (suggesting similar food conditions in the course of the season), worse breeding parameters late in the season could be reflected in the lower

clutch size or/and hatching success/chick survival rate during the first three weeks in nests of late breeders (i.e. subadults and replacement breed-ers).

KEY WORDS: timing of breeding, foraging ground accessability, breeding success, hatching dynamics, Grey Heron

1. INTRODUCTION

Birds as other animals have to adjust re-productive decisions to environmental sea-sonality. In temperate zones, decision to start breeding is influenced by a combination of fixed (photoperiod) and variable (environ-mental and other) cues. Photoperiod is con-sidered as fixed cue triggering the initiation of the seasonal sexual recrudescence of the gonads (Gwiner 1986, Dawson et al. 2001, Wingf ie ld and Hunt 2002, Sharp 2005). The action of photoperiod supplemented by additional environmental (ambient tem-perature, phenology of the vegetation, food abundance) and other (previous reproduc-tive experience, and social and stress fac-tors) cues (Moore 1983, Gr ieco et al. 2002, Breuner and Hahn 2003, Sa lvante and Wil l iams 2003) allow the animal to fine-tune the seasonal activation of the

Dariusz Jakubas180

hypothalamo-pituitary-gonadal axis to the local environment (Wingf ie ld and Farner 1993, Per f ito et al. 2008).

The right time of breeding onset is essen-tial for matching the time of maximum food requirement of the offspring with the brief moment when food is most abundant (L ack 1968, Murton and Westwood 1977, Ba lthazar t 1983). Mistiming of reproduc-tion with maximal food availability leads to decline in the breeding success (Thomas et al. 2001, Durant et al. 2005, Visser et al. 2006). Thus, the timing of breeding is often considered as one of the key factors for suc-cessful reproduction in birds (Dunn 2004, Reed et al. 2009). As most birds invest energy for egg production from resources acquired on the breeding areas, onset of breeding may be also strongly influenced by the food avail-ability during the prebreeding period (Per-r ins 1970, Drent and Daan 1980). Timing of breeding may differ considerably, even at a very local scale what reflects the adaptation of birds to local characteristics of their envi-ronment (Møl ler 2008).

Breeding phenolo gy of Grey Heron is relatively well documented in West Europe (O wen 1960, Mi lste in at al. 1970, van Vessem and Draulans 1986a, Prosper and Hafner 1996). However, the data from Central and East Europe are often anecdotal

and still scarce (Dement iev and Gladkov 1951, Noskiewicz 1964, S okołowski 1972, Czapulak and Adamski 2002, Wojczulanis et al. 2005).

Knowledge of the arrival dates and breed-ing dates of birds is important for study-ing long-term trends of changes in timing of breeding in the ongoing climate changes (Parmesan and Yohe 2003, Root et al. 2003). Therefore the phenology data of Grey Heron from three colonies in northern Po-land presented in this study may serve as a reference for possible future studies on cli-mate change consequences for Grey Heron.

The main aim of this study was to com-pare the dates of heronry occupation in spring, and hatching dynamics among three breeding colonies of Grey Heron differing in local climate conditions. Asuming that ice melting (permitting acces to the foraging grounds and possibly prey availability) influ-ence timing of breeding, I hypothesized that the earliest onset of breeding should occur in the coastal colonies with expected the mild-est climate conditions. Colony of more inland character, due to expected severer conditions in early spring, should be occupied later, only when foraging grounds are free of ice.

Since many species with short breeding seasons and low nest-predation rates typical-ly exhibited seasonal declines in reproductive

Fig. 1. Localization of the studied breeding colonies of Grey Heron (dots, bolded names) and main for-aging grounds. Colonies: MT (locality – Mosty), KR (locality – Kąty Rybackie), KS (locality – Kiersity).

Climate effects on breeding phenology of Grey Heron 181

success (e.g. S lagsvold 1984, Moreno 1998, Hansson et al. 2000, Farnsworth and Si-mons 2001), I also analyzed brood size and breeding success in all colonies in the course of breeding season.

2. STUDY AREA

The study was conducted in three her-onries situated in the northern Poland (Fig. 1) in 1999–2002. The colony at Mosty (MT) (54°37´N, 18°29´E) is situated 0.7 km from the Puck Gulf coast (the Baltic Sea) and is the most northern among studied. Herons nest on common alder Alnus glutinosa (L.) in a small wood (238–412 occupied nests in 1999–2002). The colony adjoins buildings of the village. Feeding areas are situated 0–8 km from the heronry (the Puck Gulf coast and

adjoining meadows; Jakubas 2004, 2005) (Fig. 1).

The colony at Kąty Rybackie (KR) (mixed with Great Cormorant Phalacrocorax carbo (L.)) is situated at the „Kąty Rybackie” reserve (54°21´N, 19°14´E), 100 meters from the Bal-tic Sea coast. Herons nest on Scot’s pine Pinus sylvestris L. in the coastal pine forest. This heronry is the biggest of the three studied (716–879 nests in 1999–2002). The Vistula Lagoon (2 km from the colony) is the main feeding area (Jakubas 2005) (Fig. 1).

The colony at Kiersity (KS) (53°57´N, 19°28´E) is the most inland (45 km from the Baltic Sea coast). There were 215–277 occu-pied nests in 1999–2002 on 140 years old pe-dunculate oak Quercus robur (L.) in a small wood on the Korsuń Lake shore (22.5 ha). This lake and „Drużno Lake” reserve (3021

Table 1. Mean air temperatures (°C) recorded in February and March in meteorological posts situated close to studied colonies (MT, KS, KR – see Fig. 1) of Grey Heron; data from Fi l ipiak et al. (2001a, b, 2004) and O wczarek and Fi l ipiak (2003).

Year MonthHeronry / adequate meteorological post

MT / Gdynia KS / Elbląg KR / Port Północny

Mean for 1971–2000

February 0.6 –0.9 –0.5a

March 1.4 0.6 0.6a

1999February 0.4 –1.4 0.4

March 3.7 4.6 3.8

2000February 3.6 2.5 3.5

March 3.7 3.2 3.6

2001February 1.2 –0.7 0.4

March 2.2 1.5 1.9

2002February 4.3 3.7 b 4.4 b

March 4.6 4.2 b 4.5 b

Median for 1999-2002

February 2.4 0.9 2.0

March 3.7 3.7 3.7

a – multi-year data available from meteorological post in Gdańsk-Świbno situated 16 km east off Port Północny; the post in Port Północny was established in 1987.b – data from the website http://www.TuTiempo.net.

Table 2. Choosen climatical features of studied colonies – MT, KS and KR (Fig. 1) (derived from maps in Woś 1999).

Mean values for 1951–1980 MT KS KR

Date (March) of the winter end 5 th 10 th 5 th

Date (March) of snow cover disappearance 20 th 31st 31st

Duration of winter (days) 60 80 60

No of days with ground frost per year 65 70 65

No of days with snow cover 60 70 60

Dariusz Jakubas182

ha) situated 20 km from the colony are forag-ing areas (Jakubas 2005) (Fig.1).

Climate conditions in two coastal colo-nies (MT and KT) were milder (considering mean air temperature in February and March, dates of the winter end and snow cover disap-pearance, duration of winter, number of days with ground frost and with snow cover; re-viewed in Woś 1999) than in the inland one (KT) (Tables 1 and 2) .

Also the ice conditions in spring differed among the main foraging grounds of herons from the particular colonies. The ice cover melted the earliest in the feeding area of birds from MT colony (the Puck Gulf, mean date of the last ice occurrence for 1961–1990: 12 March; Stanis ławczyk 2002). Last ice oc-currence on Druzno Lake (foraging ground of herons from KS colony; Korsuń Lake is not considered due to lack of multi-year data) was recorded usually a few days later than in the Puck Gulf (mean date of the last ice oc-currence for 1956–2005: 14 March; Skow-ron 2009). In spite of coastal localization, ice cover on the Vistula Lagoon (foraging grounds for birds from KR) was usually pres-ent longer than on the Puck Gulf and similar-ly long as on Druzno Lake (mean date of the last ice occurrence for 1961–1990: 26 March; Stanis ławczyk 2002).

3. MATERIALS AND METHODS

Dates of all three colonies occupation in spring and the first egg hatching were noted in four seasons (1999–2002). Data on hatch-ing dynamics were collected at MT colony in 1999–2001 and at KS and KR colonies – in 2000–2002. Data on breeding success in re-lation to egg laying date are presented for all three colonies in three seasons (2000–2002).

Dates of colony occupation by herons were determined by visits in each of the heronries 1–2 weeks before expected time and also by in-formation from researchers performing other studies in the same area. Colony was consid-ered as occupied when group of at least 20–30 individuals sitting on the nests was observed during the control. Dates of colony occupa-tion were verified by back-calculation from the estimated dates of egg laying commence-ment (see below) seven days (assuming those several days for pair formation, copulation and

nest re-building). In three occasions, dates of colony occupation were verified and corrected (advanced) by 1 day, two times by 2 days and once by 4 days.

Dates of egg laying commencement were back-calculated from the dates of first finding of the post-hatching egg shells and the length of the incubation period (25–26 days; as in-cubation begins with the first egg; Cramp 1998). Thereby, some results of analyses were presented for eggs laying/hatching jointly. For comparison with other colonies overall median dates of colony occupation and com-mencement of eggs laying/hatching were cal-culated (as median dates of those phases in all colonies and seasons combined). To show variation in dates of colony occupation and commencement of eggs laying and hatching, deviations from overall median dates were calculated for the particular colonies and sea-sons.

For comparison among studied colonies four-year median dates were calculated (as median dates in particular colonies in studied seasons 1999–2002 combined).

To compare the hatching dynamics (on the whole colony scale) between heronries, all post-hatching eggshells (removed from nests by parent birds shortly after hatching; Milstein et al. 1970) were collected on the ground be-low the nests every 7–10 days in each colony. A failure eggs (destroyed by predators, fallen down) were also collected but not included in further analysis. At KS and MT colonies, the eggshells were collected in the whole colony area, at KR colony – in the sample plot of 170 nests. Hatching dynamics was showed in 10 days periods (further called ‘decades’; dates of the decade beginning were adopted from pentads; division of year into pentads accord-ing to Busse (2000). Due to different dates of the visits in the studied colonies the num-ber of hatched eggs (found eggshells) was cal-culated per decade. To standardize different control dates and periods between consecu-tive controls in four years and three colonies, number of eggs hatched in given decade (N) was estimated basing partially on eggshells collected during the visit in the given decade and the first visit in the consecutive decade in the following way:

Nd= (a × n

d) + (b × n

d+1) (1)

Climate effects on breeding phenology of Grey Heron 183

where (N) number of hatched eggs (found eggshells) per decade d, (a) number of days between beginning of the decade d and the given control date/number of days between actual collection date and the previous col-lection date in the previous decade d-1, (n

d)

number of eggshells collected during the con-trol in the actual decade d, (b) number of days between the actual control date and the end of the decade d/number of days between the next collection date in the consecutive decade d+1 and the actual collection date, (n

d+1) –

number of eggshells collected during the first control in the consecutive decade d+1.

Intercolonial and interseasonal differenc-es in hatching dynamics were analyzed with Kruskal-Wallis test.

Due to lack of data on clutch size and hatching success, and difficulty in counting small chicks, the number of chicks per nest was analysed in two age categories: medium-aged chicks (21–27 days old) and fledglings (39–51 days old). Number of fledglings per nest was considered as an estimate of fledg-ling success. Chicks were counted in 21–42 nests in each of the three colonies in each sea-son (sample sizes depended on the visibility related mainly to stage of leaves development) during all-day observations (from dawn to dusk) performed four to eight times in each colony and season. The age of chicks was established on the base of their appearance according to Marion (1979). Date of eggs hatching in each nest was estimated by back-calculation from the estimated age of chicks (middle value from the range of age catego-ries was considered for each age category – e.g. 24 days for category 21–27 days) and the length of the incubation period (25–26 days; Cramp 1998). The percentage of chicks which died in the observed nests between the two age categories: medium-aged chicks and fledglings was considered as mortality rate during last three weeks before fledging.

Relationships between the date of hatch-ing and the number of medium-aged chicks per nest, between the date of hatching and fledging success, and between the date of hatching and chick mortality rate during last three weeks before fledging were analyzed with Spearman rank correlation coefficient (r). Range of days between the earliest and the latest hatched nests among the observed

was 13–63 (MT: 2000 – 63, 2001 and 2002 – 17, KS: 2000 – 27, 2001 – 31, 2002 – 34, KR: 2000 and 2001 – 13, 2002 – 41).

To investigate influence of local climate on Grey Herons timing of breeding, I used available data from studied seasons on mean monthly air temperature in February and March from meteorological posts situated close to the colonies (Gdynia – 5.5 km from MT colony, Port Północny – 34 km from KR colony, Elbląg – 25 km from KS colony) and dates of the last ice cover occurrence on foraging grounds of herons from particu-lar colonies (the Puck Gulf, Korsuń Lake and the Vistula Lagoon for birds from MT, KS and KR colonies, respectively). Mean air temperatures for February and March 1999–2001 were taken from Fi l ipiak et al. (2001a, b, 2004). For 2002, data for Elbląg and Port Północny were taken from the website http//www.TuTiempo.net and for Gdynia from O wczarek and Fi l ipiak (2003). Dates of the last ice cover occurrence on the Puck Gulf and the Vistula Lagoon were taken from Stanis ławczyk and Sztobr yn (2001), and Stanis ławczyk (2001, 2002, 2004). Ice con-ditions on the Korsuń Lake were recorded by the author during visits at the end of February and at the beginning of March. Relationships between the mentioned spring climate con-ditions and dates of colony occupation were analyzed with Spearman rank correlation coefficient (r). All tests were calculated using STATISTICA 8.0.

4. RESULTS

4.1. Spring weather condition and dates of colony occupation

In 1999–2002, monthly air temperatures were similar in all climatological posts situ-ated close to the studied colonies both in Feb-ruary (Kruskal-Wallis test, H = 1.09, df = 2, 12, P = 0.58) and March (K-W test, H = 0.04, df = 2, 12, P = 0.98) (Table 2). Also dates of the last ice cover occurrence were similar on foraging grounds of herons from all stud-ied colonies (four-year median dates, MT: 4 February, KS: 5 March, KR: 17 February; K-W test, H = 2.36, df = 2, 11, P = 0.31).

Herons appeared in the colonies usually in the first week of March (overall median

Dariusz Jakubas184

date – 5 March; Fig. 2). Birds from MT col-ony generally tended to occupy the site ear-lier than overall median date (four-year me-dian date: 28 February), and birds from KS and KR colonies usually occupied their sites later (four-year median dates, KS: 6 March, KR: 9 March; Table 3). In 2001 at MT colony, a group of 30 herons temporarly occupied (at least for 4 days) the colony even earlier on 10 February. However, those birds deserted the colony during the cold snap (snow and

freeze) in the following weeks and finally oc-cupied the heronry in March. Dates of colony occupation in spring were similar in all stud-ied heronries (K-W test, H = 4.08, df = 2, 12, P = 0.13) and seasons (H = 5.70, df = 3, 12, P = 0.13).

Dates of colony occupation were not cor-related with mean monthly air temperature neither in February (Spearman rank cor-relation coefficient, r = 0.007, P = 0.98, N = 12) nor in March (r = –0.007, P = 0.98, N =

Table 3. Deviations from overall median dates (for all colonies and seasons combined) of colony occu-pation in spring and egg laying in particular seasons and colonies -MT, KR and KT (see Fig. 1) of Grey Heron.

Colony Year

Deviation from overall median dates of (d)

colony occupation(5 March)

commencementof eggs laying / hatching

(15 March / 10 April)

MT

1999 –8 –11

2000 –21 –24

2001 1 –3

2002 –3 –6

KS

1999 –1 –1

2000 –3 1

2001 2 2

2002 5 2

KR

1999 4 8

2000 –5 –8

2001 5 2

2002 5 2

Fig. 2. Dates of the last occurrence of ice cover on the foraging grounds (stars), colony occupation (squares) and the estimated beginning of the egg laying (back-calculated from the first egg hatching) (triangles) of Grey Herons from particular colonies -MT, KR, KS (see Fig. 1) (KS in 2002 – ice not re-corded in March, lack of data from February).

Climate effects on breeding phenology of Grey Heron 185

12). Analysis of relationship between dates of colony occupation and dates of ice melting on the main foraging grounds did not reveal significant correlation (r = 0.51, P = 0.10, N = 11). However, after excluding from analyzes 2002 (with the earliest recorded ice melting dates), correlation was significant (r = 0.84, P = 0.004, N = 9) – the earlier ice melting on the foraging grounds, the earlier date of the colony occupation.

4.2. Eggs laying and hatching

Overall median date of egg laying com-mencement in the studied colonies was 15 March. Egg laying started the earliest at MT

colony (four-year median date: 7 March; Fig. 2, Table 3) – at the end of February (2000) and at the beginning of March (other seasons). At KS and KR colonies females usually started to lay eggs later – in the second half of March (four-year median dates, KS: 16 March, KR: 17 March; Fig. 3).

Overall median date of the eggs hatching commencement was 10 April. The earliest start of hatching was recorded at MT colony – at the beginning of April (four-year median date: 2 April; 3–11 days earlier than overall median dates) and in 2000 even earlier, in mid March (24 days before overall median date) (Fig. 2, Table 3). At KR and KS colonies eggs usually started to hatch later – in the second

Fig. 3. Hatching dynamics of Grey Heron at MT, KS and KR colonies (Fig. 1) in 2000–2002. Lines above the graph indicate percentiles (25–75%) and medians.

Dariusz Jakubas186

Fig. 4. Relationship between the hatching date and the number of medium-aged chicks in observed nests and fledgling success (number of fledglings per nest) in the three studied colonies – MT, KR and KS (Fig. 1) of Grey Heron in 2002.

Table 4. Relationship between date of hatching and the number of medium-aged chicks per nest, fledg-ing success (number of fledglings per nest) and chick mortality rate before fledging (during last three weeks before fledging) in the colonies – MT, KR, KT (see Fig. 1) of Grey Heron in 2000–2002 (Spear-man rank correlation coefficient).

Colony Year

No of medium-aged chicks per nest

Fledging successChick mortality rate before

fledging

rs P N rs P N rs P N

MT

2000 –0.15 0.65 12 0.35 0.32 10 0.38 0.24 11

2001 0.43 0.09 17 –0.13 0.72 10 0.16 0.53 17

2002 –0.75 0.0002 19 –0.65 0.007 16 –0.03 0.92 19

KS

2000 –0.22 0.43 15 –0.52 0.18 8 –0.10 0.72 15

2001 –0.28 0.20 23 –0.36 0.16 17 –0.18 0.40 23

2002 –0.41 0.04 25 –0.22 0.29 25 –0.13 0.53 25

KR

2000 –0.06 0.83 17 0.12 0.65 16 –0.31 0.22 17

2001 –0.05 0.79 27 –0.23 0.27 25 0.20 0.31 27

2002 –0.46 0.004 37 –0.53 0.0008 36 0.25 0.14 37

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Climate effects on breeding phenology of Grey Heron 187

decade of April (four-year median dates: KS: 11 April, KR: 12 April; Fig. 3).

The intercolonial differences in dates of eggs laying/hatching commencement were marginally significant (for distribution of dates – K-W test, H = 5.90, df = 2, 12, P = 0.052, for median dates – median test, χ2 = 6.0, df = 2, P = 0.0498). Interseasonal differ-ences were not significant (H = 2.79, df = 3, 12, P = 0.42).

Dates of eggs laying/hatching commence-ment were not correlated with mean monthly air temperature neither in February (Spear-man rank correlation coefficient, r = –0.16, P = 0.61, N = 12) nor in March (r = –0.06, P = 0.85, N = 12). Analysis of relationship between dates of eggs laying/hathing com-mencement and dates of ice melting on the main foraging grounds did not reveal signifi-cant correlation (r = 0.57, P = 0.07, N = 11). However, after excluding from analyses 2002 (exceptionally warm season, with the earliest recorded ice melting dates), correlation was significant (r = 0.76, P = 0.02, N = 9) – the earlier ice melting on the foraging grounds, the earlier eggs laying / hatching commence-ment.

Although the hatching median and 25 and 75 percentile dates were similar (Fig. 3), hatching dynamics differed significantly among all studied colonies (K-W test, 2000 – H = 72.06, df = 2, 1991, P <0.0001; 2001 – H = 34.72, df = 2, 1504, P <0.0001; 2002 – H = 135.48, df = 1, 1380, P <0.0001; Dunn test, P < 0.05). Interseasonal differences in hatching dynamics were noted at MT colony (K-W test, H = 56.34, df = 2, 1654, P <0.001; Dunn test, 2000 vs 2002, 2001 vs 2002, P <0.001) and KR colony (K-W test, H = 69.53, df = 2, 1890, P <0.001; Dunn test, 2000 vs 2001, 2001 vs 2002, P <0.001). At KS colony the hatching dynam-ics was less variable (K-W test, H = 7.35, df = 2, 1340, P = 0.02) and post-hoc test did not revealed significant differences (Dunn test, P> 0.05).

Generally, one distinct peak of hatch-ing was observed in all colonies. It occurred in the second and third decades of April (Fig. 3). Exception was season 2001 at MT colony, where two distinct peaks occurred – in the second decade of April (10–19 April) and in the first decade of May (30 April – 9 May). The latest hatching was recorded in

KS colony in 2002 – the last 5 fresh eggshells were found on 18 July. Those very late broods were probably unsuccessful – no birds were recorded in the heronry during the last visit on 12 August.

4.3. The influence of the breeding commencement for the breeding success

Significant correlations between the number of medium-aged chicks per nest and the date of hatching and between the fledging success and the date of hatching were found in all studied colonies only in 2002 (Table 4). A general decline in the number of medium-aged chicks per nest in the course of season was observed in all colonies (Fig. 4). Seasonal decline in fledging success was observed only at MT and KR colonies (Table 4, Fig. 4). Ana-lyzes did not reveal significant relationship between the mortality rate during last three weeks before fledging and date of hatching (Table 4).

5. DISCUSSION

The Grey Herons generally occupied the colonies in the first decade of March what is consistent with data from other Polish her-onries (Noskiewicz 1964, S okołowski 1972, Czapulak and Adamski 2002). Air temperature is believed to be important fac-tor affecting the onset of the breeding season in Grey Heron (Walmsley 1975, Marion 1989, Campos and Fernandez-Cruz 1991). However, it is unlikely that photope-riod or air temperature have a direct effect on timing of breeding as laying dates in Grey Heron (the present study and van Vessem’s 1991) and Great Blue Heron Ardea herodias L. (But ler 1993) are not synchronized among nearby colonies.

Studies on waterbirds (e.g. S chreiber 1980, But ler 1993) showed the prey avail-ability in the foraging grounds during the pre-breeding period is the key factor affect-ing the date of breeding commencement. In Great Blue Heron energy availability during the prebreeding period, not during the high chicks energy requirements period, deter-mines when females lay eggs (But ler 1993). In the present study dates of colony occupa-tion and eggs laying/hatching were correlated

Dariusz Jakubas188

with date of the last occurrence of ice cover on the foraging area. It suggests that lack of ice cover, reflecting accessibility of feeding grounds and prey availability is crucial for decision on onset of breeding.

Timing of colony occupation was similar in all studied colonies, as the spring weather conditions on the foraging grounds did not differ significantly. However, birds from the coastal heronry at MT site tended to occupy the colony and lay eggs earlier than those from other heronries. Differences in dates of the first hatching (at least marginally signifi-cant) could result from various dates of egg laying onset, as well as different duration of incubation in particular colonies (van Ves-sem and Draulans 1986b). Early colony occupation might have been possible there due to presence of birds early in spring in the vicinity (early migrants or/and wintering individuals, own observations). The extent of wintering Grey Herons influenced differ-ences in colony occupation dates in Belgium (van Vessem 1991).

Generally, the hatching dynamics were characterized by one peak. Only at MT col-ony in 2001 two peaks of hatching were ob-served. Decrease in the number of hatched eggs at the end of March probably resulted from the high rate of failures during cold snap month earlier (snow and freeze from 19 to 27 March, own observation). The sec-ond peak, following hatching decrease, may reflect replacement clutches. Deserting the heronry by adults during the cold snap has been also reported for other colonies (De-ment iev and Gladkov 1951, Mi lste in et al. 1970). In Spain early season losses led to bimodal laying peaks (Campos and Fer-nandez-Cruz 1991). Despite experienc-ing similar cold snap, bimodal hatching dy-namics was not recorded in other heronries. Birds from those colonies might foraged in alternative areas and/or food sources (e.g. at KR colony – on fish regurgitated by Great Cormorants; Wojczulanis et al. 2005) and were able to continue incubation. Extending hatching until July recorded in some seasons probably resulted from replacement clutches (recorded up to three times; Cramp 1998). However, observations from one nest at KS colony in 2002 may suggest second breeding attempt, what has been rarely noted (O wen

1960, Mi lste in et al. 1970). During one of the visits the fledgling sitting on the nest was chased away from the nest by adult birds sit-ting on the nest. Next visit incubating adult was observed there. Similar threatening be-havior toward chicks just before the second clutch commencement on the former nest has been reported by Mi lste in et al. (1970) after Ver wey (1930). Apart from the men-tioned reasons, intercolonial differences in eggs hatching dynamics might result from various frequencies of pairs consisting of sub-adults (which usually started breeding late in the season, Mi lste in et al. (1970) after Ver-wey (1930) and several waves of bird arrival observed especially in big colonies (Kushlan and Hancock 2005 after Marion 1988).

Decrease in the number of medium-aged chicks per nest in the course of season was observed in all colonies in 2002. Lack of re-lationship in other seasons could have been a result of smaller sample size or/and higher hatching synchronization among observed nests. Chick mortality rate during last three weeks before fledging was not correlated with the timing of breeding. It suggest that feeding conditions during this period were similar in the course of the season or/and adult birds were able to compensate potentially worse condition late in the season (as Common Tern Sterna hirundo L. in experimental study of Arnold et al. 2004). Lower number of me-dium-aged chicks and fledging success of late breeders could have reflected smaller clutches or/and lower hatching success/chick survvial rate during the first three weeks in the nests of late breeders. Decrease in the clutch size in the course of season has been reported for Grey Heron (Creutz 1975, van Vessem and Draulans 1986a, van Vessem 1991) and other heron species (e.g. Rodgers 1980, 1987, R anglack et al. 1991). A part of late broods consists of replacement broods and/or broods of inexperienced subadult. Such broods are usually smaller (Mock 1978, Rodgers 1978, Fernandez-Cruz and Campos 1993, Mart i 1994, S orci et al. 1997, Jakubas 2005). Clutches laid by sub-adults were of marginal significance in the studied nests because of low number. Only seven pairs among all observed in 2002 con-sisted of one subadult mate (in the second calendar year plumage).

Climate effects on breeding phenology of Grey Heron 189

Summing up, in migratory population of Grey Heron breeding in northern Poland photoperiod seems to be a general cue trig-gering the arrival at the breeding grounds. Food availability expressed in accessability of foraging grounds was the main determinant of the onset of breeding. Air temperature in-crease might act as a supplementary cue for individuals present in the vicinity of the colo-ny earlier (like at MT colony), but only when foraging grounds are accessible. As the chick mortality rate during last three weeks be-fore fledging was not correlated with hatch-ing date, observed decrease in the number of medium-aged chicks per nest and fledging success in the course of season probably re-flected smaller clutches of later breeders (i.e. subadults and replacement breeders).

ACKNOWLEDGMENTS: I would like to thank Prof. L. Stempniewicz and dr K. Wojczu-lanis-Jakubas for critical comments on the first version of the manuscript and to M. Czeczatka for access to IMGW reports. This project was sup-ported by grants of the State Committee for Scien-tific Research (6 PO4G 030 18) and the University of Gdańsk (BW 1140-5-005-0).

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Received after revision March 2010