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Ewa Dziala-Szczepanczyk ©
Department of Zoology, Westpomeranian University of Technology (ZUT) 20 Doktora Judyma Street, 71-466 Szczecin, Poland Ewa.Dziala-Szczepanczyk@zut.edu.pl
BIOMETRICS OF GREATER SCAUP AYTHYA MARILA (AVES, ANSERIFORMES) WINTERING ON THE SZCZECIN BAY
Abstract
The object of the biometric characteristics were 90 specimens of Greater Scaup Aythya marila - 66 males (6 immature and 60 adult) and 24 females (6 immature and 18 adult). Biometrics included five parameters: body weight, body length, sternum length, tarsometatarsus length, and folded wing length. The average values of the above parameters were analyzed in regard to the age and the sex of the birds. The mutual relations between the biometric features of the investigated birds were also ascertained. Among all the measurements, the highest variability was noted in the body weight. A significant sexual dimorphism was indicated in the body weight of the examined Aythya marila group - all the analyzed parameters achieved higher values in the males than in the females. In both male and female group, the adult and the immature specimens differed significantly in merely one parameter, ie. the length of the folded wing. In both sexes the adult specimens achieved significantly higher values of this parameter as compared to the immature ones. All the analyzed parameters remained in a significant positive mutual correlation. The lowest value of the correlation coefficient was noted for the relation between the tarsometatarsus length with the lengths of the body and the sternum.
Key words: Greater Scaup , Aythya marila, biometry, body size
Introduction
Morphological diversification of birds, both interspecific and intraspecific becomes apparent in the values of biometrical parameters among others. These parameters are subject to seasonal variability, affected by e.g. migration, breeding, molting, and diet change (Dziubin 1959, Connell et al. 1960, Clark 1979, McLandress & Raveling 1981, Piersma & Davidson 1991, Winker 1998, McCarty 2001, Piersma et al. 2003). The avian biometrics is largely applied in the intra-population studies describing morphological variability of the species within the area of its occurrence as well as in the taxonomic research (Amadon 1943, Owen 1962, James 1970, Handford 1983, Setiawan et al. 2004). The biometric studies of birds have been conducted in Poland for several years. However, detailed studies of this type, on Great Scaup Aythya marila wintering on the Szczecin Bay have not yet been conducted.
The aim of this work is the characteristics and analysis of the selected biometric traits describing the body size of Greater Scaup in regard to the sex and the age of the birds.
© Ewa Dziala-Szczepanczyk, 2009
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Material and methods
The object of the study were 90 Aythya marila specimens - 88 males (6 immature and 60 adult) and 24 females (6 immature and 18 adult), collected during the fall and winter period of 2005-2007 on the Szczecin Bay area. Five traits characterizing the body size were determined, ie. the body weight BW (with precision of 10g), the body length BL, the sternum length SL, the tarsometatarsus length TL, and the folded wing length WL. The linear measurements of the body were performed with the precision of 1 mm. They were performed according to the method suggested by Dzubin & Cooch (1992). The basic statistics, ie. the arithmetic means and the standard deviation of the biometric traits were calculated for the entire group of the studied birds, for the male and female groups, and separately for the juvenile and adult birds within each sex. Due to an inadequate number of juvenile specimens the coefficient of measurement variability was counted for the entire group of Aythya marila and also for the male and the female groups. For the ascertainment of the significance of differences in the average measurements between the males and the females, as well as between the age groups (adult and juvenile) the t-Student significance test was used. The relations between the analyzed parameters counted for the entire group were also studied. For each relation the Pearson linear correlation coefficient was ascertained.
Results of study
The obtained results regarding the average biometric measurements were presented in Tables 1 and 2. The body weight appeared to be the most variable parameter among all the analyzed ones. The variability coefficient for this measurement, counted for the entire group achieved the value CV=9.5%. For the remaining measurements its value oscillated between 2.58% and 4.1%. The body weight is characterized by a high seasonal variability in a number of the avian species due to the molting or a seasonal fattening of the body (Baldwin & Kendeigh 1938, Folk et al. 1966, Ricklefs 1968, Nolan Jr. & Ketterson 1983, Piersma & Davidson 1991, Winker 1998, Twedt & linz 2002, Piersma et al. 2003). During the wintering period, significant changes in the body weight are noted in a number of avian species. Ryan (1972) studying four species of the ducks Aythya indicated a significant drop of their body weight in winter. He (Ryan 1972) suggests that that phenomenon results from a low temperature of the environment and a scarce food resources during that period. The studied Aythya marila specimens were collected in the period between November and March - some specimens in the beginning of the wintering period, and others by the end of that period. Due to that fact their body weight might have differed considerably. The intra-specific variability in the body weight might be conditioned by the age and the sex structure of the analyzed population of birds (Dzubin 1959, Reveling 1968, Koffijberg & Van Eerden 1995, McCarty 2001). The birds of different sex and age might react differently to the changing environmental conditions in winter. The studies of Raveling (1968) on Canada Goose Branta canadensis indicated that the juvenile specimens contrary to the adult ones did not lose weight during the wintering period. The studied group of Aythya marila males
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also indicated a difference in the average body weight in favor of the juvenile specimens. However it has not been statistically confirmed.
The sexual dimorphism was observed in all analyzed parameters in the studied Aythya marila group. The males achieved higher average values of those measurements in comparison to the females (Table 1). Similarly, in the case of Cormorant Phalacrocorax carbo sinensis, significant differences were observed between the males and the females in all six analyzed biometric measurements - the body weight, the body length, the wing length and the sternum length (Koffijberg & Van Eerden 1995). Also the sexual dimorphism was apparent in Musk Duck Biziura lobata in all 16 exterior body measurements (McCracken et al. 2000). However, in certain avian species the dimorphic differences in regard to the body size are not apparent in all biometric measurements. The studies of Freeman & Jackson (1990) indicated differences in the body size between the males and the females in three species: Red-winged Blackbirds Agelaius phoeniceus, Red Crossbills Loxia curvirostra, and Harris Sparrows Zonotrichia querula. However, those authors (Freeman & Jackson 1990) suggest that several biometric measurements should be applied simultaneously for the characteristics of the body size in birds of different sex belonging to this group. Also McGillivray (1985) studying Great Hornem Owls Bubo virginianus, concluded that the description of the dimorphic differences in this species should not be performed on the basis of singular exterior body measurements. In other species, on the other hand, certain biometric measurements should be used for the ascertainment of the avian sex (Clarke et al. 2003, Setiawian et al. 2004). The studies conducted on Yellow-eyed Penguin Megadyptes antipodes prove that such biometric measurements as the foot length and the head measurements can be applied for the ascertainment of the sex in this species (Setiawian et al. 2004). Table 1. Differences in absolute body parameters between males and females of the Greater Scaup Aythya marila, M - males, F - females, x - mean, SD - standard deviation, CV -coefficient of variation, t - value of t-Student's statistics, NS - statistically insignificant differences, p - level of significance, BW - body weight (g), BL - body length (mm), SL -
x ± SD x ± SD
Parameters CV % M vs. F CV %
M, n = 66 F, n = 24 M + F, n = 90
BW 1464.62±139.38 1371.87±85.03 t = 3.053, p< 0.01 1439.89±133.26
9.5 6.2 9.2
BL 431,36±11,38 424.17±8.42 t = 2.824, p<0.01 429.44±11.10
2.6 2.0 2.6
SL 98,59±3,92 95.42±2.68 t = 3.654, p<0.001 97.74±3.89
4.0 2.8 4.0
TL 38.81±1.09 37.87±2.38 t = 2.579, p<0.05 38.56±1.8
2.8 6.3 4.1
WL 224.45±6.64 219.25±5.41 t = 3.443, p<0.001 223.07±6.71
3.0 2,5 3.0
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Table 2. Differences in absolute body parameters between immature and adultus in males and females of the Greater Scaup Aythya marila, (M - males, F - females, imm - immaturus, ad - adultus, x - mean, SD - standard deviation), explanation to symbols - see table 1
Parameters M, n = 66 F, n = 24
M ad , n = 60 M imm , n = 6 F ad , n = 18 M imm , n = 6
x ± SD
BW 1460.25±130.07 1508.33±224.53 1385.83±81.89 1333.00±87.46
BL 431.83±9.34 42667.±25.03 423.89±7.96 425.00±10.49
SL 98.72±3.70 97,25±6,03 95.92±2.05 93.92±3.88
TL 38.83±1.03 38.58±1.8 37.63±2.65 38.55±1.23
WL 225.05±5.65 218.50±12.2 220.78±4.98 214.67±4.08
Table3. Correlations among five variables in Greater Scaup Aythya marila. Shown are correlation coefficient, r, and level of significance (* - p< 0,05, **- p< 0,01, ***- p< 0,001, NS - non significant), explanation to symbols - see table 1
BL SL TL WL
BW 0 39*** 0 64*** 0 41*** 0 61***
BL - 0,39*** 0,33** 0,47***
SL - 0,29** 0,58***
TL - 0,40***
The dimorphism in regard to the age was apparent merely in one parameter, ie. the length of the folded wing (Table 2). The adult specimens indicated a significantly higher value of this parameter - both in the males (t = 2.385, p < 0.01), and the females (t = - 2.707, p < 0.01). The studies of Francis & Wood (1989) conducted on wood-warblers, collected in various seasons prove, that the wing length can relate directly to the bird age. Also in the case of Cormorant Phalacrocrorax carbo sinensis no differences in the biometric measurements were noted between the adult and the immature specimens, provided a simultaneous indication of such differences regarding the males and the females (Koffijberg & Van Eerden 1995). The lack of dimorphism related to the age in the studied Aythya marila group in the majority of analyzed measurements can result from a scarce quantitative representation of the immature specimens (13%).
The analysis of the relations between the body parameters of the studied Aythya marila ducks indicated that all the parameters correlated mutually (Table 3). The Pearson linear correlation coefficient for all the relations achieved a positive value. The weakest correlation was noted for two relations: the tarsometatarsus length with the body length (r = 0.33, p < 0.01) and the tarsometatarsus length with the sternum length (r = 0.29, p < 0.01). In the studied group of ducks the correlation coefficient of the body weight with the remaining measurements used to achieve high values (p < 0,001). Similarly, Handford (1983), studying Rufous-collared sparrow Zonotricha capensis, indicated that both in the males and the females of that species there were significant correlations among all the body parameters. However, for
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certain relations the correlation coefficient adopts a negative value. For the relation between the tarsometatarsus length with the wing length the linear correlation coefficient achieved the value - in the males r = 0.124 at p < 0.001, in the females r =
0.119.at p < 0.001 (Handford 1983). On the other hand, the studies conducted on 58 males and 109 females of Great Hornem Owls Bubo virginianus indicated the lack or an insignificant relation among seven analyzed body measurements of those birds (McGillivray 1985). In the male group of that species the correlation between the wing length with the body weight and body length was apparently insignificant. Whereas in the female group, the correlation coefficient for the relation between the wing length with the body weight achieved the value r = 0.23 at p < 0.05, and with the body length r = 020 at p < 0.05 (McGillivray 1985).
Conclusions
1.The sexual dimorphism was indicated in the body size of Aythya marila. In all
analyzed parameters the males achieved significantly higher average values than the
females.
2.The body weight indicated the highest variability among all parameters.
3.All parameters were in a significant positive correlation with one another.
Reference
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Summary
The object of biometric analysis were 90 specimens of Aythya marila - 66 males (6 immature and 60 adult) and 24 females (6 immature and 18 adult) collected on the area of Szczecin Bay (north-west of Poland). The biometrics included five parameters, ie. body weight (BW), body length (BL), sternum length (SL), tarsometatarsum length (TL), and folded wing length (WL). The analysis comprised the average values of the above parameters with regard to the age and sex of the birds. The mutual relations between the biometric traits of the studied birds were also ascertained. Of all measurements the highest variability was noted in the body weight
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(CV = 9.2%). An apparent sexual dimorphism was indicated in the body size of the studied Aythya marila group - all analyzed parameters achieved higher values in the males than the females. In both the male and the female groups the adult and immature specimens differed significantly in merely one parameter, ie. WL. The adult ducks achieved significantly higher values of this parameter than the immature ones. All analyzed parameters remained in a significant positive mutual correlation (p < 0.001). The lowest value of the linear correlation coefficient was noted for the relations TL with BL (r = 0.33, p <0.01) and TL with SL (r = 0.29, p <0.01).
Стаття надшшла до редакцИ 24.03.2009
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