Bird Study Recent changes in the diet and breeding ...

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Recent changes in the diet and breeding parameters of the Purple Heron Ardea purpurea in southern France C. Barbraud , M. Lepley , V. Lemoine & H. Hafner Published online: 29 Mar 2010.

To cite this article: C. Barbraud , M. Lepley , V. Lemoine & H. Hafner (2001) Recent changes in the diet and breeding parameters of the Purple Heron Ardea purpurea in southern France, Bird Study, 48:3, 308-316, DOI: 10.1080/00063650109461230 To link to this article: http://dx.doi.org/10.1080/00063650109461230

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Bird Study (2001) 48, 308–316

Recent changes in the diet and breeding parameters of the Purple Heron Ardea purpurea in southern France CHRISTOPHE BARBRAUD*, MICHEL LEPLEY, VINCENT LEMOINE and HEINZ HAFNER

Downloaded by [201.211.254.207] at 13:27 24 March 2014

Station Biologique Tour du Valat, Le Sambuc, 13200 Arles, France

The Purple Heron population in southern France suffered a 46% decline between 1981 and 1994. A study of breeding biology was instigated in 1997 in order to examine potential changes in the main breeding parameters between the early 1980s and the late 1990s. Here, we present data on the breeding biology and diet of breeding Purple Herons in the Camargue, southern France, collected from 1997 to 1999. These results were compared with those from a similar study conducted between 1979 and 1982. No difference in first spring arrival date on breeding grounds was noted between these two periods. Mean first egg date was 22 April during 1979–82, in contrast to 6 May during 1997–99. Mean colony size was c. 118 breeding pairs during 1979–82, but only c. 59 during 1997–99. For broods of three and four chicks, linear growth rates for rank A (first hatched) chicks were higher during 1979–82 than during 1997–99. For broods of four chicks linear growth rates were also higher for rank B (second hatched) chicks and tended to be higher for rank C (third hatched) but lower for rank D (fourth hatched) chicks. During 1979–82 nestling diet (determined from regurgitates) was dominated by fish (85.4% by biomass). There was an increase in the proportion of insects from 12.8% during 1979–82 to 21.7% in 1998 (from regurgitates), whereas the proportion of fish was 61.3% in 1998. Diet was determined from pellets in 1999 and indicated that 42.6% of the biomass was insects whereas 33.8% was fish. No difference was found between 1979–82 and 1997–99 for clutch size and hatchling mass. Results suggest that birds needed more time to initiate breeding during 1979–82 than during 1997–99. Possible causes for the observed differences in the Purple Heron breeding biology and diet between both study periods are discussed.

The Purple Heron Ardea purpurea is a widely distributed species which breeds very locally in wetlands (Cramp 1992, del Hoyo et al. 1992). Although the feeding habits and foraging habitats used by adults during the breeding season have been studied extensively in several countries (e.g. Owen & Phillips 1956, Tomlinson 1974, Amat & Herrera 1978, László 1986, Fasola 1994, Campos & Lekuona 1997), few describe the species laying patterns, clutch size or chick growth (Moser 1986, Gonzáles-Martín et al. *Correspondence author. Email: [email protected] © 2001 British Trust for Ornithology

1992). Although several studies have reported yearly variation in breeding numbers of Purple Herons (den Held 1981, Cavé 1982, van der Kooij 1991, Kayser et al. 1994, Bergerandi et al. 1995), no study has investigated changes in breeding parameters or diet. Breeding numbers of Purple Herons in the Camargue represent about 30–60% of the breeding population in France. Some of the largest breeding colonies of this species in Europe are found in the Camargue (Moser 1984, Kayser et al. 1994). From the early 1980s to the early 1990s breeding numbers declined by about 46% (Kayser et al. 1994). In the mid-1990s

Purple Heron diet and breeding research was initiated to determine the causes and consequences of this decline. Here we present details of breeding biology and diet of Purple Herons from data collected during the breeding seasons of 1997, 1998 and 1999 in the Camargue, southern France, and compare these data with those obtained from the same location in 1979, 1980 and 1982 (Moser 1986). METHODS


Study area Fieldwork was carried out in 1997, 1998 and 1999 in eight of the 23 known breeding colonies of Purple Herons in the Rhône river delta (Camargue): Charnier, Couvin, Tour du Valat, Bélugue, Vigueirat, Landre, Meyranne and Bourguidou. In these locations colony size varied from four to 263 breeding pairs; colonies were situated in reed beds of Phragmites australis. For comparison purposes (see below) we tried to select the same colonies as those selected by Moser (1984). However, some colonies had disappeared and only two colonies were common to both periods. A description of the study area can be found in Moser (1986) and Thomas et al. (1999). The other ardeid species which nest in the area are the Grey Heron Ardea cinerea, the Little Egret Egretta garzetta, the Cattle Egret Bubulcus ibis, the Night Heron Nycticorax nycticorax and the Squacco Heron Ardeola ralloides. Breeding parameters We collected data on the timing of breeding, clutch size, hatchling weight, nestling growth, nestling mortality, diet and colony size during the 1997, 1998 and 1999 breeding seasons. Dates of return to colonies were extracted from data logs from 1965–83 and 1991–94. Methods used for data collection and analysis were identical to those of Moser (1986) so that results from both studies could be compared. Colonies were located and the number of breeding pairs counted using a single-engine aircraft (see also Moser 1984, Kayser et al. 1994). Colonies were visited once a week during the incubation and chick rearing periods and nests were located by walking transects. The nests were marked with numbered tags (tied to the reeds below the nest). Visits were limited to one

309

hour between 06:00 and 09:00 hours in order to limit disturbance. On each visit, nest contents were recorded, nestlings were weighed and measured (bill length and tarso-metatarsus length). As nests were not visited daily, the first egg dates of most clutches were unknown. Most first eggs dates were therefore back-calculated from the age of the oldest nestling (from the stage of feather development) based on tarsus length (Moser 1984) and using an average incubation period of 26 days (Kral & Figala 1966). Clutch size was determined when the same number of eggs was recorded on two consecutive visits, or when the number of chicks present on the second visit did not exceed the number of eggs recorded on the first. Because Purple Heron nestlings leave their nests well before fledging, survival was measured only until the oldest chick was 16 days old (Moser 1986). Egg and nestling survival were estimated and compared after Mayfield (1961) and Johnson (1979). Biometrics Chicks were weighed to the nearest 5 g. Hatchling mass was estimated from the mass of freshly hatched chicks identified by their wet down. Mean linear growth rates were calculated from day four until day 24 as weight gain was approximately linear during this period (Moser 1986). Nestlings were assigned a rank (A–D) on the basis of their order of hatching. Diet Food items regurgitated by chicks were collected in 1998. The prey samples were stored in 70% alcohol for examination in the laboratory. Prey items found in regurgitates were identified, counted and their length measured to the nearest millimetre. Fish were measured from the tip of the snout to the fork of the tail. Prey items found in regurgitates were generally poorly digested and easy to identify. Samples of food items found in regurgitates were collected from the feeding grounds and the items used to estimate the dry mass of each prey type. Dry mass was measured by weighing food items dried in an electric oven at 70°C until steady mass was attained. For large prey © 2001 British Trust for Ornithology, Bird Study, 48, 308–316


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such as fish and snake, dry weights were read from length/dry weight calibrations calculated from samples of prey captured in the field (Moser 1984). In 1999, pellets were collected on the nests during visits to colonies. Prey items were sorted, identified (Bertrand 1954, Richoux 1982) and counted. Counting problems were encountered for mammals and birds as only hairs and feathers, respectively, were found in pellets. We thus used the minimum possible number of individuals for each pellet after hair and feather identification. Biomass estimates from pellets were obtained as for regurgitates. As pellets were collected on nests we could not determine whether they were from chicks or adults. Data analysis As Moser (1986) presented results obtained from pooled data from the 1979, 1980 and 1982 breeding seasons (hereafter PER1), we pooled data collected during the 1997, 1998 and 1999 breeding seasons (hereafter PER2) when comparing data. We checked the distribution of quantitative characters for normality with the one-sample Kolmogorov–Smirnov test (Siegel & Castellan 1988). Data that were not normally distributed were log-transformed before analysis. Appropriate non-parametric tests were used when normality was not obtained following log-transformations. Statistical analyses were performed using Systat (1997). RESULTS Breeding Median first spring observation date was 22 March for PER2 versus 15 March for PER1, but no significant trend was detected in first spring observation date (r26 = 0.20, P = 0.30; mean ± sd = 18.4 ± 6.3 March (9–31 March)). Mean colony size was lower during PER2 than during PER1 (Table 1), and did not vary between years during PER2 (F2,20 = 0.266, P = 0.769). Median colony size was 106 nests during PER1 and 27 nests during PER2. First egg date was not significantly related to colony size during PER2 (rs = –0.19, n = 8 colonies, P > 0.05) or during PER1 (rs = –0.40, n = 4 colonies, P > 0.05). Major differences © 2001 British Trust for Ornithology, Bird Study, 48, 308–316

were found between the patterns of first egg dates for both periods (Fig. 1; two-sample Kolmogorov–Smirnov test: D = 0.474, P < 0.001) and was 14 days later during PER2 than during PER1 (Table 1). The variance in first egg date was higher during PER1 than during PER2 (F267, 215 = 1.917, P < 0.001). During PER2, first egg date varied according to year (F2,212 = 21.041, P < 0.001) with later laying in 1999 than in 1997 (Tukey: P = 0.001) and in 1998 (Tukey: P < 0.001), and a tendency for later laying in 1997 relative to 1998 (Tukey: P = 0.07). Neither clutch size nor colony size (P > 0.134) differed between study periods (Table 1), and clutches of three and four eggs were the most common. During 1997–99 (PER2) clutch size varied according to year (F2,231 = 7.14, P = 0.001) with larger clutches in 1999 than in 1998 (Tukey: P = 0.002). There was no decline in clutch size with laying date during PER2 (Table 1; F4,146 = 0.901, P = 0.465) as observed during PER1 (Moser 1986). This was not caused by the absence of early breeders during PER2 as when they were removed from analysis in PER1 (i.e. laying date < 10 April), the correlation between clutch size and laying date remained significant (Pearson: r207 = –0.220, P = 0.001). Chick growth Hatchling mass did not differ significantly between periods (Table 1). Chick growth patterns in relation to age for rank A chicks (first chicks hatched) are shown in Fig. 2 for both studies. For broods of three and four chicks, linear growth rates were lower during PER2 than during PER1 for rank A chicks (Table 1). For broods of four chicks, growth rates tended to be lower for rank B chicks but higher for rank D chicks (Table 1). During PER2, we detected marginally significant interannual variations in growth rates. For broods of four chicks, rank A and B chicks tended to grow faster in 1999 than in other years (F2,40 = 3.028, P = 0.06 and F2,40 = 3.168, P = 0.053, respectively), but no such variation was observed for rank C and D chicks (F2,40 = 0.539, P = 0.587 and F2,39 = 1.597, P = 0.215, respectively). For broods of three chicks, rank A and C chicks tended to grow faster in 1999 than in other years (F2,157 = 2.856, P = 0.06 and F2,156 = 3.169, P = 0.04), but rank B chicks did not (F2,157 = 1.675, P = 0.19).

Purple Heron diet and breeding

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Table 1. Comparison of the Purple Heron main breeding parameters between PER1 and PER2. PER1 n

mean ± sd

First egg date (days)

267

112.2 ± 14.4 [min–max 80–148]

215

Clutch size 16–31 March 1–15 April 16–30 April 1–15 May 16–31 May 1–15 June Totals

8 44 105 71 14 0 242

± ± ± ± ±

0 8 22 37 70 15 152

Variable


PER2

Hatchling mass (g)

30

Colony size

18

Growth rate (g/day) Brood 4 Rank A Rank B Rank C Rank D Brood 3 Rank A Rank B Rank C Egg survival rate (%) Nestling survival rate (%)

1Mann–Whitney

36 36 34 20 31 30 27

3.75 3.63 3.57 3.28 3.14

n

0.16 0.12 0.06 0.07 0.14

3.48 ± 0.66 [min–max 2–5] 36.3 ± 4.8 [min–max 21–46] 117.6 ± 104.9 [min–max 5 –381]

36.5 35.8 32.5 16.6

± ± ± ±

1.0 1.0 1.2 2.7

10 23

44 44 44 43

34.9 ± 1.1 34.0 ± 1.3 24.4 ± 1.5 79.0 (95% CI 73.6–84.8) 99.1 (95% CI 97.9–100.0)

161 161 160

mean ± sd

Test

P

126.3 ± 10.4 [min–max 104–156]

U = 12 6561