Bird Study Breeding biology of the Cattle Egret ...

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Breeding biology of the Cattle Egret Bubulcus ibis in southwest Spain D. Parejo , J.M. Sánchez-Guzmán & J.M. Avilés Published online: 29 Mar 2010.

To cite this article: D. Parejo , J.M. Sánchez-Guzmán & J.M. Avilés (2001) Breeding biology of the Cattle Egret Bubulcus ibis in southwest Spain, Bird Study, 48:3, 367-372, DOI: 10.1080/00063650109461236 To link to this article: http://dx.doi.org/10.1080/00063650109461236

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Bird Study (2001) 48, 367–372

SHORT REPORT

Breeding biology of the Cattle Egret Bubulcus ibis in southwest Spain DESEADA PAREJO1,2*, JUAN M. SÁNCHEZ-GUZMÁN2 and JESÚS M. AVILÉS1,2 1Laboratoire

d’Ecologie CNRS-UMR 7625, Université Pierre et Marie Curie, 7 quai Saint Bernard, Bâtiment A, 7ême étage, Case 237, F-75252 Paris Cedex 05, France and 2Grupo de Investigación en Conservación, Área

Downloaded by [91.214.200.45] at 11:45 24 March 2014

de Biología Animal, Departamento de Ciencias Morfológicas, Biología Celular y Animal, Facultad de Ciencias, Universidad de Extremadura, Avenida de Elvas s/n, E-06071 Badajoz, Spain

The Cattle Egret has expanded its breeding range worldwide since the 19th century, although the explosive phase occurred principally between 1950 and 1970. The species is not a new colonist to Europe and was a common breeder in Andalucía (southern Spain) during the last century. It is known to have colonized later in other areas of Spain (Alicante-Murcia, later Extremadura and after that Cataluña) and the south of France (Voisin 1991). Although Extremadura has only recently been colonized by Cattle Egrets, the similarity and proximity between Andalucía and Extremadura suggests that they have not needed to adapt their breeding to completely new environmental conditions there. The breeding biology of this species is welldocumented for most of its traditional range in Africa and Asia (Hellebrekers & Hoogerwerf 1967, Siegfried 1972), as well as in its extended range in America (Lowe-McConnell 1967, Jenni 1969) and Australia (McKilligan 1997). However, in the western Palearctic, only data from the Camargue (H. Hafner unpubl. data), the Ebro Delta (Ruiz et al. 1981) and the Valencia Albufera (Prósper & Hafner 1996) are available. Factors affecting the breeding success of the species have been widely studied in these recently colonized areas (Baxter 1994, McKilligan 1997). However, few data exist from those areas with well-established colonies (see however Ruiz et al. (1981) and Maddock & Baxter (1991)). Here we describe the breeding biology of Cattle Egrets in mixed colonies in southwest *Correspondence author. Email: [email protected] © 2001 British Trust for Ornithology

Spain (Extremadura) and we analyse the implications of nest location within the colony (nest height and density) on breeding success. Because nest height is essential for success in temporal marshes, where it confers more protection from terrestrial predators (Perennou et al. 1996), we expected a higher breeding success in these sites. For many colonial birds, density-dependent effects on reproduction have been detected (see review in Wittenberger & Hunt 1985). Here we test for breeding success in relation to nest density and we compare breeding parameters in southwest Spain with others from the western Palearctic to test for any latitudinal patterns. The study was carried out in three mixed colonies of Extremadura (southwest Spain): Montijo (38°55′N–6°23′W), Badajoz (38°53′N– 6°58′W) and Morante (39°03′N–6°41′E). The Badajoz and Montijo colonies are in highly modified habitats, while the Morante colony has more traditional habitats with Holm-oak Quercus rotundifolia dehesas and pasturelands. The Badajoz and Montijo colonies are on islands on the Guadiana river covered with Tamarix sp., Phragmites sp., Populus sp. and Eucalyptus sp. Both colonies contained Cattle Egrets, Little Egrets Egretta garzetta and Blackcrowned Night-herons Nycticorax nycticorax. The trees found in the Morante colony were mainly Fraxinus sp. and Quercus rotundifolia, and only Cattle and Little Egrets nested there. Data were collected during the 1996 and 1997 breeding seasons (March–August). Before birds began nesting, we installed wooden stakes of known height within the colonies as reference points for measurements of length and height. Large-scale photographs of the colony sites


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D. Parejo, J.M. Sánchez-Guzmán and J.M. Avilés

were used to facilitate individual identification and monitoring, and it allowed us to estimate distances between nests and estimate height. We visited the colonies weekly and observed them from the same place from where we had taken photographs (about 50 m from the colonies and high enough to observe the highest nests). Nests were visited between seven and 11 times. Only nests with hatchlings present on the first visit were used in the analysis. First-egg date for each clutch was estimated by subtracting the mean incubation period (Voisin 1991) from hatching dates, taking into account the intervals between the laying (Voisin 1991). Only successful clutches (where at least a chick hatched) were monitored. To estimate the breeding success we recorded: (a) number of hatchlings per successful clutch (brood size per successful clutch); (b) number of fledglings per successful clutch (we considered that a chick fledged when it survived until 25–30 days old); (c) mortality rate as the percentage of the total number of chicks hatched per successful clutch that did not fledge, assuming that a chick died when it was not in the nest on the last day that we observed their family group. To test for associations between nests and vegetation height classes we estimated the availability of vegetation in each height class. The amount of suitable vegetation for nesting at each height class in each colony was estimated using photographs. Photographs were divided into areas of 2 × 2 m. When more than 75% of a 2 × 2 m frame were filled with vegetation, we assigned it a value of 4 m2, assigning fractions of this value according to the surface of the 2 × 2 m area covered by vegetation. We assumed that this measure of vegetation height in an area was directly related to the available area to place nests. Only those nests situated in the foreground, as seen from our sampling point, were used in this analysis to avoid the effect of depth of the colony on the assignation of nests to the discrete height classes (Parejo et al. 1999). When nests were observed in a 2 × 2 m area, we counted all heron/egret nests and the number of Cattle Egret nests to estimate the total nest density and the Cattle Egret nest density per 2 × 2 m area. Nest density in each 2 × 2 m area was first considered at the time the first pair in the area reproduced and again after 60 days. © 2001 British Trust for Ornithology, Bird Study, 48, 367–372

The number of pairs breeding at each colony during the two breeding seasons was obtained by direct counts of nests. Laying date, nest height and density, brood size and number of fledglings per successful nests were logarithmically transformed and mortality rate was arcsine transformed (Zar 1996). Two-way ANOVAs were used to compare mean values among colonies and seasons, with the Scheffé test to compare pairs. The association between distribution of nests at different height classes, colonies and years was tested by a log-linear analysis. The expected values were calculated taking into account the estimated available surface to nesting. Because of the low number of Cattle Egret nests in the highest height class (see below) we had to pool the two higher height classes. We used partial correlations to examine independently the effect of mean laying date, mean nest height and mean density of nests in each 2 × 2 m area on mean reproductive rates in all colonies. All data were pooled and year and colony were considered as factors in the models, as dummy variables with two and three levels respectively, because data from both years and different colonies were significantly different. Since colony is a categorical variable with three levels, we had to generate two dummies for this factor to perform the model (Zar 1996).a The number of breeding pairs in all colonies across both years is shown in Table 1. First-egg date ranged from 12 March to 29 April over the two years. The mean laying dates differed between years (F1,612 = 230.9, P < 0.02), as well as across colonies (F2,612 = 83.2, P < 0.03). The yearly variation of laying date was different in relation to colonies (Interaction year × Colony: F2,612 = 6.7, P < 0.05), though in all colonies breeding started earlier in 1997 (Table 1). The mean brood size per clutch differed between years (F1,683 = 7.1, P < 0.05), though there were variations only in Montijo (Scheffé test: Montijo: P < 0.05; Badajoz: P = 0.8; Morante: P = 0.7), as well as across colonies (F2,683 = 9.4, P < 0.03). The yearly variation was different among colonies too (Interaction year × Colony: F2,683 = 6.2, P < 0.05) (Table 1). There was no difference in the mean number of fledglings per clutch across years (F1,672 = 3.2, P = 0.07), but there was across colonies (F2,672 =

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369

Table 1. Breeding variables of the Cattle Egret in the study area. Number of Little Egret and Night Heron pairs is also included. Day 1 = 1 January. Values are given as mean ± standard deviation (sample size). Colony Badajoz


Little Egret pairs Night Heron pairs Cattle Egret pairs Cattle Egret Nest height

Montijo

Morante

1996

1997

1996

1997

1996

1997

60 43 750

95 62 1000

30 14 750

30 2 700

20 0 525

20 0 550

2.0 ± 1.1 (211) 4.3 ± 2.3 (75) 4.6 ± 2.5 (75) 2.2 ± 0.5 (95) 2.0 ± 0.5 (95) 4.8 ± 16.6 (95) 17 March 91.1 ± 12.6 (106)

2.6 ± 1.5 (131) 4.3 ± 2.0 (35) 4.5 ± 2.2 (35) 1.9 ± 0.5 (95) 1.7 ± 0.6 (95) 10.0 ± 21.7 (95) 16 April 119.8 ± 7.9 (48)

2.8 ± 1.5 (191) 5.6 ± 3.1 (55) 5.8 ± 3.3 (55) 2.0 ± 0.6 (140) 1.7 ± 0.6

3.0 ± 1.4 3.0 ± 1.4 1.5 ± 0.8 (143) (195) (223) Nest density/2 × 2 m area 5.7 ± 3.6 6.8 ± 4.7 5.8 ± 3.3 (36) (68) (60) Total nest density/2 × 2 m area 6.9 ± 4.3 7.8 ± 4.9 6.0 ± 3.3 (36) (68) (60) Brood size/nest 2.2 ± 0.6 2.2 ± 0.6 1.8 ± 0.5 (101) (135) (118) Fledglings/nest 1.9 ± 0.7 2.0 ± 0.8 1.8 ± 0.5 (101) (135) (118) Mortality rate 11.7 ± 24.8 8.0 ± 21.1 2.5 ±12.8 (101) (135) (118) Onset of egg-laying 29 April 12 March 17 April Mean laying date 153.4 ± 19.6 122.6 ± 26.6 128.8 ± 26.6 (86) (183) (79)

17.7, P < 0.0167). The degree of difference between colonies varied between years (Interaction year × Colony: F2,672 = 4.4, P < 0.05) (Table 1). The mean mortality rate was not different across years (F1,683 = 0.04, P = 0.8), but varied among colonies (F2,683 = 8.6, P < 0.05). The degree of difference between colonies did not vary between years (Interaction year × Colony: F2,683 = 1.8, P = 0.2) (Table 1). The mean nest height of Cattle Egrets was different across colonies (F2,1082 = 107.96, P < 0.0167) and years (F1,1082 = 13.43, P < 0.03), with higher nests in Badajoz than in Morante (Scheffé test: P < 0.05) and higher in these two colonies than in Montijo (Scheffé test: BadajozMontijo: P < 0.05; Morante-Montijo: P < 0.05) (Table 1). The yearly variation of nest height was different among colonies (Interaction year × Colony: F2,1082 = 5.45, P < 0.05) (Table 1). There was a significant association between number of nests and height classes (log-linear analysis: Height class × Observed – Expected: P < 0.001), with less nests than expected in the highest class (Fig. 1). There was no effect of

12.5 ± 22.6 (140) 18 March 101.1 ± 12.8 (113)

colony, nor of year, or any interaction (loglinear analysis: P > 0.05). However, there was a combined effect of colony and height class (loglinear analysis: Colony × Height class × Observed – Expected: P < 0.001), as well as year and the height class (log-linear analysis: Year × Height class × Observed – Expected: P < 0.001). However, the interaction between these three factors did not affect the distribution of nests (log-linear analysis: P > 0.05). Nest density of Cattle Egrets per 2 × 2 m area did not differ between years (F1,323 = 0.46, P = 0.5), but across colonies (F2,323 = 4.7, P < 0.05), and the inter-colony variation varied in relation to years (Interaction year × Colony: F2,323 = 5.9, P < 0.025). Similarly, mean nest density of all species per 2 × 2 m area with Cattle Egrets nests was not different between years (F1,323 = 0.4, P = 0.5), though it was different across colonies (F2,323 = 11.7, P < 0.0167) and the intercolony variation was different in relation to years (Interaction year × Colony: F2,323 = 4.7, P < 0.05). The mean brood size per clutch was inversely related to laying date and affected by colony, © 2001 British Trust for Ornithology, Bird Study, 48, 367–372

D. Parejo, J.M. Sánchez-Guzmán and J.M. Avilés

120

Badajoz

Morante

200 120

100

200 200

80

150 150

100

40

60

200

100 100

150

40 50

Number of nests

250

80

60

20

50

100

50

20

0

0

80

50

0

0–2 2–4 4–6 6–8

0

0

0–2 2–4 4–6 6–8 120

60

100

50

350 300

100 150

60


Montijo

0 0–2 2–4 4–6

200 120

300

100

250

80

200

60

150

40

100

20

50

150

40

80

40

60

30

40

20

100

20

Vegetation surface (m2)

370

50

0 0–2 2–4 4–6 6–8

20

10

0

0

0 0–2

2–4

4–6

0

0 0–2 2–4 4–6

Height class (m) Figure 1. Number of Cattle Egret nests in each height class (■ ■ ) and surface of available vegetation (■) in each colony. The 1996 season is in the upper row and the 1997 season in the lower row. See text for results of statistical tests.

while no other effects were detected (Table 2). The number of fledglings per clutch was related to mean brood size per nest and colony, while it was not affected by nest height, laying date, nest density of Cattle Egrets, total nest density, or year (Table 2). The mortality rate was not affected by any of the factors studied (Table 2). The onset of egg-laying observed in the present study shows that the earliest breeding Cattle Egrets in the western Palearctic are in southwest Spain (see revision in Prósper & Hafner 1996). Earlier breeding in southern areas has been shown for other bird species, and has been related to the earlier onset of spring and production of food (Järvinen 1989, Kazantzidis et al. 1996). In this study, the earlier laying of Cattle Egrets was in a year with higher mean winter temperatures and lower mean winter rainfall (18.7°C and 1.5 mm in 1997 versus 14.8°C and 2.8 mm in 1996). However, only data for two seasons are presented here and a long-time series of data would be needed to further validate this point. Though we expected an increase in reproductive © 2001 British Trust for Ornithology, Bird Study, 48, 367–372

rates when birds began to breed earlier, the earlier laying in 1997 did not show this pattern. We found the lowest fledging success of Cattle Egret in comparison with other localities in the western Palearctic (Table 3). We believe that different parameters explained the varying success across colonies. For example at Badajoz and Montijo irrigation practices may have led to greater foraging areas, while the Morante colony has more natural habitats. The differences in the number of Cattle Egret nests and the total number of nests per 2 × 2 m area among colonies are most likely explained by differences in vegetation of colonies as has been shown in other ardeid colonies (Kazantzidis et al. 1997). Although other authors have shown the advantages of nesting high in the heronry vegetation (Fasola & Alieri 1992), we did not find any effect of nest height on the Cattle Egret reproductive success. Similarly, we did not detect density-dependent effects on the species breeding, as in other colonial birds (Jehl 1994). Finally, we found a relation between breeding success and the time of the onset

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371

Table 2. Results of the partial correlations between the reproductive rates of Cattle Egrets and the independent factors analysed. The values of R (partial correlation coefficient) and P (level of significance) are given for each independent variable. Factors individually significant after the application of the sequential Bonferroni test are in bold type. The independent variables called Colony 1 and 2 are referred to the dummy variables created for taking into account the colony as a factor. Dependent variables Mean brood size/ clutch


Independent variables Mean nest height/2 × 2 m area Mean laying date/2 × 2 m area Density of Cattle Egrets/2 × 2 m area Total density/2 × 2 m area Mean brood size/clutch Colony 1 Colony 2 Year

Mean no. of fledglings/clutch

Mean mortality rate/clutch

R

P

R

P

R

P

0.082 –0.133 0.028 –0.030 – –0.118 –0.118 –0.001

0.121