VARIATION IN EGG DIMENSIONS IN A HOODED CROW
37
BIOL. LETT. 2003, 40(1): 3742 Available online at http://www.biollett.amu.edu.pl
Repeatability and within-clutch variation in egg dimensions in a Hooded Crow Corvus corone cornix population PIOTR ZDUNIAK1 and MARCIN ANTCZAK2 1
Department of Avian Biology and Ecology, Institute of Environmental Biology, Adam Mickiewicz University, Fredry 10, PL-61701 Poznañ, Poland. E -mail:
[email protected] 2 Department of Zoology, University of South Bohemia, Braniovská 31, CZ-370-05 Èeské Budejovicé, Czech Republic. E-mail:
[email protected]
(Received on 24th June 2002; Accepted on 24th September 2002)
Abstract: Variation in egg size and shape of the Hooded Crow Corvus corone cornix was investigated in the National Park Ujcie Warty, western Poland, in 2000 and 2001. Mean clutch size was 4.55±0.79 (n=40). Coefficients of variation for clutch means ranged from 2.00±1.16 (breadth) to 3.35±1.76 (elongation index). Mean repeatability estimates were 0.69, 0.71, 0.73, and 0.62 for length, breadth, volume, and elongation index, respectively. The results suggest that in the population studied, one should expect a relatively high heritability of egg dimensions and thereby a low influence of environmental conditions. This can be connected with the food storage ability in corvids. Key words: breeding biology, clutch size, Corvus corone cornix, egg size, repeatability
INTRODUCTION
Egg dimensions affect the size of nestlings and thus may have an influence on their survival (SCHIFFERLI 1973, BOLTON 1991, SMITH & BRUUN 1998, RISCH & ROHWER 2000). Hence, the knowledge of the degree of genetic determination and heritability of egg dimensions is very important in describing the impact of environmental conditions on these features. A relatively easy way to evaluate this phenomenon is to estimate repeatability (e.g. BOAG & VAN NOORDWIJK 1987, BAÑBURA & ZIELIÑSKI 1990, FALCONER & MACKAY 1995). High repeatability often indicates a significant heritable component of the phenotypic variance (BOAG & VAN NOORDWIJK 1987). The aim of this study was to investigate the repeatability of egg dimensions of the Hooded Crow in habitats where food availability is presumably highly variable, which should affect repeatability of egg dimensions.
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Piotr Zduniak and Marcin Antczak
MATERIALS AND METHODS
The study was carried out in the National Park Ujcie Warty in western Poland (N52°34, E14°43) during two breeding seasons, in 2000 and 2001. The area is under protection of the RAMSAR convention, which protects wetlands of global importance. Details of the study area have been described elsewhere (CHMIEL et al. 2000, CHOIÑSKI 2000). We started to search for the nests at the beginning of April, when the trees were still without foliage. In the case of pairs that started nesting later, the nests were detected on the basis of presence of birds near to potential nesting site. Each nest was bearing an insert into the GPS (Global Positioning System) receiver. We examined eggs from 40 nests of Corvus corone cornix Linnaeus, 1758 (17 in 2000 and 23 in 2001). Only eggs from complete clutches (when all eggs were laid in the clutch) were chosen for analysis. Egg length and breadth were measured by one person (P. Z.) to the nearest 0.1 mm with the sliding calliper. Egg volume index was calculated from the length and breadth by using the formula V = 0.5 * L * B2 / 1000 (HOYT 1979), where V is volume (in cm3), L is length (in mm), and B is breadth (in mm). An index of egg elongation was calculated by dividing L by B. To avoid the problem of pseudoreplication, egg size was defined as the mean volume of all eggs laid in each nest (LESSELLS & BOAG 1987, SOLER & SOLER 1996). Data from the two years were pooled. Within-clutch coefficient of variation (CV) of egg measurements was calculated according to the formula: s *100 CV = Y where s is the standard deviation (square root of within-clutch mean square divided by within-clutch degrees of freedom) and Y is the mean of the egg measurement within the clutch. The coefficient of variation calculated with the above formula is known to be biased, especially for small sample sizes (S OKAL & ROHLF 1995). For this reason, a correction was applied:
1 CV* = 1 + * CV 4n where CV* is the bias-adjusted estimate of coefficient of variation, and n is clutch size. One-way analysis of variance (ANOVA) was carried out in order to obtain variance components. Repeatability was calculated as intra-class correlation coefficient (SOKAL & ROHLF 1995), by applying the formula: r = (MSA − MSW )/ [MSA + (n 0 − 1)MSW ] where MS A is the between-group (i.e. between-clutch) mean square, MS W is the within-group mean square, and n0 is a coefficient related to sample size per group in ANOVA, given by: 1 n0 = (a − 1)* Ón i − Ón i2 Ón i
[
(
)]
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VARIATION IN EGG DIMENSIONS IN A HOODED CROW
where ni is the size in the i-th group and is the number of groups (LESSELLS & BOAG 1987, FALCONER & MACKAY 1995). Standard errors for repeatability values were calculated as described by BECKER (1992). Mean values are given with standard deviations. RESULTS
A total of 180 eggs from 40 clutches of the Hooded Crow were used in the analysis (Table 1). Clutch size ranged from 3 to 6. Mean clutch size was 4.55±0.79, modal clutch size was 5 (Fig. 1). Correlation coefficient between mean egg length and mean egg breadth was significant (r=0.47, n=40, p=0.002; Fig. 2). Table 1. Ranges, means and within-clutch coefficients of variation (CV *) for Hooded Crow egg measurements from National Park Ujcie Warty, W Poland Trait
Clutch mean (SD)
CV* (SD)
Range (clutch means)
Range (all eggs)
Length [mm]
41.24 (2.11)
3.27 (1.59)
35.98–44.25
34.70–46.70
Breadth [mm]
29.09 (1.00)
2.00 (1.16)
26.78–31.44
26.20–32.30
Volume [cm3]
17.51 (1.78)
5.82 (2.80)
12.91–21.85
11.91–23.84
1.42 (0.07)
3.35 (1.76)
1.29–1.57
1.27–1.68
40
40
180
Elongation n
40
24 21
No. of clutches
18 15 12 9 6 3 0
3
4
5
6
Clutch size Fig. 1. Frequency distribution of clutch size in Hooded Crow nests
The greatest within-clutch coefficient of variation was recorded for egg volume, then for the elongation index, egg length and egg breadth (Table 1). Mean
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Piotr Zduniak and Marcin Antczak
Mean egg breadth [mm]
32
y = 0.221*x+19.981
31 30 29 28 27 37
38
39
40
41
42
43
44
45
Mean egg length [mm] Fig. 2. Correlation between within-clutch mean values of egg length and egg breadth in the Hooded Crow
repeatability estimates were 0.69, 0.71, 0.73, and 0.62 for length, breadth, volume and elongation index, respectively (Table 2). To test for presence of a trade-off between egg size and clutch size, one-way ANOVA was performed. No significant differences in egg dimensions were found Table 2. Repeatability (r) of egg length, breadth, volume and elongation index in the Hooded Crow Trait
F-ratio
d.f.
r
95% Cl
Length
10.89
39
0.69
0.56-0.80
Breadth
12.21
39
0.71
0.60-0.82
Volume
13.05
39
0.73
0.61-0.83
8.30
39
0.62
0.48-0.75
Elongation
All F ratios (one-way ANOVA) are significant at p
