2011 2nd International Conference on Environmental Science and Technology IPCBEE vol.6 (2011) © (2011) IACSIT Press, Singapore
VARIABILITY IN THE WING AND TAIL FEATHERS OF THE ROCK PIGEON (Columba livia) Carlo Stephen O. Moneva
Cesar G. Demayo
Mark Anthony J. Torres
Department of Biological Sciences College of Science and Mathematics MSU-Iligan Institute of Technology Iligan City, Philippines e-mail:
[email protected]
Department of Biological Sciences College of Science and Mathematics MSU-Iligan Institute of Technology Iligan City, Philippines e-mail:
[email protected]
Department of Biological Sciences College of Science and Mathematics MSU-Iligan Institute of Technology Iligan City, Philippines
[email protected]
ABSTRACT—Aside from differences in body size, studying the detailed structure of a bird feathers poses great importance in the determination of sexual dimorphism among birds. This study was conducted to examine 107 feather samples of Rock dove subjected to Geometric Morphometric Analysis, to evaluate sexual dimorphism. The mean shape and variance values in the feathers (primary left wing, primary right wing and tail) of a male and female pair of Columba livia were analyzed using the Principal component analysis (PCA). The results showed that Rock doves, as member of monogamous bird species, exhibit similar feather mean shapes between sexes. However, variance values show a high variation within and between sexes in different location of the feathers. Procrustes superimposition was also used to analyze the overlapping contours of the feathers as forms of shape feather variability. Three patterns were observed for both sexes of C livia in both the right and left primary wing feather, variations were observed in the base, a point where the feather and shaft meet. In addition, tail feather observed to possess a single form however, variability depends on the size. In female, tail feathers are broader and wider compared to male. The degree of sexual dimorphism was higher for both primary left wing and tail feathers with a discriminant function 100% correctly classified and 96.016% for the primary right wing feather. This could be attributed to sexual selection (social mating system) and natural selection (parental care and territorial defense) occurring in the evolution of the species being studied. Further experimentations were recommended to separate the influences of these selective processes in the evolution of the wing and tail feathers in Columba livia. Keywords- Columba livia, Geometric Morphometric Analysis, Principal component analysis, Procrustes superimposition, Discriminat function analysis.
I. INTRODUCTION The Rock Pigeon (Columba livia) is a monogamous bird species belonging to family Columbidae. It is a small, brown-grayish dove that inhabits open areas. In the field, a trained observer can differentiate a male from a female by the crown and neck, which is more intensely gray in males. However, all these assertions rely on human perception. Bird sexing based on human perception can thus lead to erroneous conclusions. Recent studies showed that sexes within species can be compared in biometric studies using
increasingly comprehensive morphometric analysis, showing sexual dimorphism. Sexual dimorphism is the determining feature of organisms in which male and female differ. It is described by the existence of physical differences between the sexes other than those differences in the sex organs. Andersson (1994) and as cited by Johnsen and colleagues (2003) reported that sexual dimorphism is common in nature and often attributable to sexual selection, which indirectly results in the ornament expression in both sexes as a consequence of genetic correlation. In birds, it is commonly regarded as a result of sexual selection favoring large males through the size-advantage in combats over females (Bjorklund, 1990). This explains that variation in the extent of sexual dimorphism among bird species is traditionally attributed to differences in social mating system. Information about sexual dimorphism is essential for understanding the ecology, behavior, and life history of a species. In addition, sexual dimorphism will measure on how much influence sexual selection plays a fundamental role on the species. Many studies have shown sexual dimorphism among birds in terms of dichromatism (Badyaev and Hill, 2003), body size (Bjoorklund, 1990) and plumage size (Miller et al., 1999). However, studies involving sexual dimorphism in terms of bird’s feather morphometric analysis associated with flight and mating system have not yet been accounted. It has been accounted that individual feathers in the wings and tail play important roles in controlling flight and balance. Furthermore, feathers are considered as secondary sexual characteristics. In some species, feathers are often used in courtship rituals and mating. With regard to their taxonomic importance, feathers have been employed in a number of studies treating various topics of variation among species and population, both by traditional morphometrics (Bjorklund (1990); Cuervo, 1999; and Kis and Szekely, 2003), and more recently by geometric morphometrics. Morphometric analysis of feathers has been employed in pigmentation development analysis and sexual ornamentation (Badyaev et al., 2001, Badyaev and Landeen, 2007). Geometric Morphometrics is the statistical study of biological shape and shape changes which involves quantifying visual data and extracting biological meaning
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from that data (Silao et al., 200). Furthermore, the use of image analysis particularly Elliptic Fourier Analysis (EFA) has been found to be a more sophisticated tool, which employs quantitative analysis of shape and of its changes while providing a more ready account of size. Using these techniques synthesized from statistics and geometry in measuring, comparing and categorizing shape of biological objects, such as feathers, could help researchers arrive a more quantitative and reliable conclusions. In this study, the patterns of sexual dimorphism in the shape of feather (primary wing and tail) in a sample of Rock Pigeon were analyzed using geometric morphometric analysis. The study was conducted to evaluate the amount of shape variation, with particular reference to sexual differences of the Rock Pigeon. II. MATERIALS AND METHODS A. Feather Samples The present study was based on analysis of 107 feathers from a pair of male and female Columba livia captured in Iligan City. The feathers were further subdivided based on its location (right primary wing feather (42), left primary wing feather (41), and tail feather (24)), and compared between sexes. Images of C. livia and the feathers and its location were graphically presented in Figure 1.
Figure 1. Pictures of male (A) and female (B) Columba livia, (C) left primary wing (in left) and right primary wing feathers (in right) and (D) tail feathers.
B. Image analysis and quantitative evaluation of the feather outlines The whole process can be summarized in four steps namely: (1) image acquisition, (2) feather outlining and landmarking, (3) derivation of the elliptic Fourier analysis descriptors and lastly (4) multivariate statistical analysis of the descriptors. First, a 600 dpi colored images of the feathers were generated using a CanonScan LiDE 100 scanner. Obtained digitized images of the feathers were then outlined with 100 sample points around its contour in order to get the x and y coordinates. This was made possible using an image analysis and processing software Tps Dig freeware 2.12 (Rholf, 2008). Tps Dig facilitates the statistical analysis of landmark data in morphometrics by making it easier to collect and maintain landmark data from digitized images.
These coordinates were then transferred to Microsoft Excel application for organization of the data into groups (Male and Female). The collective coordinates of all individuals were then subjected to different multivariate analyses, which include Principal Component Analysis (PCA), Elliptic Fourier Analysis (EFA), and Discriminant Function Analysis (DFA), using a geometric morphometric computer application. The computer program was used to calculate the shape variables following procedure unique to a specific multivariate analysis. In this study, the software PAST version 1.91 (Hammer, et al. 2009) was used as platform for the analyses. 1) Principal Component Analysis To summarize the information of the variations and mean shapes contained in the coefficients of landmark descriptors, Principal Component Analysis (PCA) was performed using the Paleontological Statistics (PAST) software developed by Hammer et al. (2009). Major sources of variations were identified by principal components, which accounts for as much of the variance in the multidimensional data. 2) Procrustes Analysis The patterns of shape variability were shown as overlapping contours constructed using the transformation of coefficients of landmark descriptors via the Procrustes superimposition. Procrustes Analysis is a multivariate exploratory technique that involves transformations, such as translation, rotation, reflection and isotropic rescaling of individual data matrices to provide optimal comparability. This way, the whole process would eliminate the scale, orientation and position biases of the samples. To analyze the overlapping contours of the feathers, the transformed outline descriptors were then plotted into landmarks. 3) Elliptic Fourier Analysis In this study, the shapes of Columba livia feathers of both sexes were compared using the method Elliptic Fourier Analysis (EFA). Four elliptic Fourier coefficients represent one harmonic. EFA from Fourier harmonics were obtained. These coefficients were derived from the x and y coordinates of the sample points that were outlined around the contour of the feathers. 4) Discriminant Function Analysis The Fourier coefficients were used as variables for another multivariate method of statistical analysis in the form of Discriminant Function Analysis (DFA), to calculate the percentage of sexual dimorphism in the feathers of C. livia. The results were then interpreted and given their biological implications with reference to feathers anatomy and function. III. RESULTS AND DISCUSSION The landmark coefficients of Columba livia feathers were calculated. Figure 2 shows the mean feather shapes of C. livia. The average shapes of the left and right primary wing feather (fig. 2a and fig. 2b) are highly similar in both sexes, which is obovate in shape. The tail feathers (fig.2c), on the other hand, vary between male and female. It was observed that the tail feathers in female were wider and compared to that of the male.
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The graphical representation of the feather mean shape suggests that a variation on its architecture is minimal between sexes. This result somehow agreed with the findings of Bjorklund (1990) in which male polygynous species have a relatively short feathers than do females compared to males in monogamous species. Since, C. livia is a monogamous pigeon it was evident that feathers in both sexes may be related in terms of size and shape.
Figure 2. Variations of feather mean shape. (a) Primary left wing feather, (b) primary right wing feather and (c) tail feather.
To identify sources of variation within and between sexes, the coefficients of landmark descriptors were subjected to Principal Component Analysis (PCA). Table 1 provides a breakdown of all significant components and the proportion of variation explained by such components. The left primary wing feather of a female pigeon obtained five principal components and four for the male. These components were considered to be significant since their Eigenvalues are above the Jolliffe cut-off score. The significant components for both the female and male left wing feather accounted for 99.8% and 99.5% respectively. In addition, the primary wing feather obtained six principal components in female, and four principal components in male. A total variance of 99.9% and 99.6% were recorded for female and male right primary wing feather respectively. In the tail feather, five principal components and a total variance of 99.4% were observed in female, and on the other hand the male recorded six principal components and 99.5% of the total variance. Results showed variation in the shapes of the feathers in both samples of C. livia. More interestingly, high variations in different regions of the feathers within a single individual were also observed. Different authors such as, Hellmayr (1927) and Meyer de Schauensee (1970) and as cited by Bostwick and Zyskowski (2001) reported that in most birds, the primaries 4 and 5 are modified wing feather associated with specific function. Modifications in the feather morphology might be the cause of a high variation in the feather architecture within a single individual.
TABLE 1. PROPORTION OF VARIATION ASSOCIATED WITH THE SIGNIFICANT COMPONENTS OF THE LEFT, RIGHT AND TAIL FEATHERS. FEMALE Left primary wing feather PC1 PC 2 PC 3 PC 4 PC 5 Jollifee cut-off Right primary wing feather PC 1 PC 2 PC 3 PC 4 PC 5 PC 6 Jolliffe cut-off Tail feather PC 1 PC 2 PC 3 PC 4 PC 5 PC 6 Jolliffe cut-off
MALE
BOTH (male and female)
Eigenvalue
Variance (%)
Eigenvalue
Variance (%)
Eigenvalue
Variance (%)
1.86E+07 358098 244826 98507.6 84510.9
95.793 1.8418 1.2592 0.50665 0.43467
2.36E+07 508722 173115 125128
96.207 2.078 0.70715 0.51113
2.07E+07 395374 206343 135250 88276.7
95.961 1.8369 0.95869 0.62838 0.41014
68050 1.75E+07 334605 191627 103901 59126.5 20181.1
85683 96.016 1.8341 1.0504 0.56951 0.32409 0.11062
1.90E+07 491044 199693 75535.3
63854 1.52E+06 301193 73476.4 26453.1 22300.9
75332 95.744 2.4781 1.0078 0.3812
1.88E+07 405675 225738 111448 90083.7
69353 77.7 15.44 3.7666 1.356 1.1432
1.30E+06 236071 167420 75995.3 39725.7 7556.62
6827.7
The variations in feather shapes between female and male C. livia is high, as shown in table 1. In the primary
69034 70.872 12.829 9.0985 4.13 2.1589 0.41067
6440.3
95.49 2.0567 1.1445 0.56503 0.45672
2.13E+06 495604 148654 84626.1 55814.7 19273.2
72.013 16.772 5.0305 2.8638 1.8888 0.65222 10343
wing feather variations between sexes accounted for 99.8% and 99.78% of the total variance in the left and right
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respectively. Both the left and right primary wing feather recorded five principal components. On the other hand, the tail feather recorded six principal components and accounted for 99.2% variability of the total variance between sexes. The results suggest that there exist a variation in the feather shapes between the male and female C. livia.
To analyze the results given in table 1, the overlapping contours of the feathers as patterns of shape variability were constructed using Procrustes analysis. Landmarks of the feathers were shown in Figure 3.
Figure 3. Landmarks of feathers showing overlapping contours as patterns of shape variability. (a) Primary left wing feather, (b) Primary right wing feather and (c) tail feather.
Results have shown that in the left and right primary feather architecture observed in both sexes. These forms were evident on the base, a point where feather and shaft meet. This could explain a high variation, as shown in table 1, exist in the first principal component of the left and right primary wing feather in both the male and female. On the other hand, the tail feather (fig. 3c) in both sexes show only one form. Observed variation in the tail was based solely on
sizes, in female the tail feathers were wider and broader compared to male. To further support the claim that there exists a variation in the feathers of C. livia, harmonic coefficients from the Elliptic Fourier Analysis (EFA) of the outline coordinates data were subjected to Discriminant Function Analysis (DFA). Figure 4 graphically showed the frequency histogram of the degree of sexual dimorphism of C. livia.
Figure 4. Discriminant histogram of the EFA outline coordinates data of the left (a), right (b) and tail (c) primary feathers of a pair of male (in blue) and female (in red).
In all cases, the left (fig.4a), right (fig. 4b) and tail (fig. 4c) primary wing feathers and the discrimination of the female (in red) from male (in blue) are found to be strictly significant considering the fact that they were greatly separated along the discriminant axis. The male pigeon is
found to be on the negative side of the horizontal plane while the female is found on the positive side, with the right primary wing feather as an exception. However, only one portion of the male right primary wing feather overlaps with the female in the positive axis.
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. TABLE 2. PROPORTION OF VARIATION ASSOCIATED WITH THE DISCRIMINANT FUNCTION SCORES OF THE LEFT, RIGHT AND TAIL FEATHERS OF C. LIVIA.
Female Male
LEFT PRIMARY WING Female Male correctly classified (%) 21 0 100 0
20 Total
100 100
RIGHT PRIMARY WING Female Male correctly classified (%) 22 0 100 1
19 Total
Results in table 2 have shown that there were significantly 100% correctly classified discriminant function scores for both the left primary wing and tail feathers. In addition, the discrimant function correctly classified 97.62% of the right primary feather (100% of females, 95% of males). These percentages were significant since greater than or equal to 75% is considered to be a cut-off for variation in structures, leading to sexual dimorphism. The results obtained from the DFA somewhat agreed with the results obtained from the PCA. Both multivariate statistical analyses showed differences in the feather architecturebetween the male and female C. livia, suggesting sexually dimorphic individuals. It has been reported by Owens and Hartley (1997) that sexual dimorphism in plumage size and colour were correlated with different aspects of reproductive and social behaviour in birds. Rock pigeons (Columba livia) were strictly monogamous, and monogamous species exhibit low dimorphism (Bjorklund, 1990). This idea contradicts with the results of the present study. However, mating rituals could explain such morphological feather variation between male and female pigeons. During mating, the male follows the female with a pompous demeanor, its tail spread, wings drooping, body elevated and throat swelled, the male was observed to show a dance consisting of ascending flights and aerial glides with the tail expanded laterally (Saxena et al., 2008), males with the highest flight and most spread tails were favored by most females. Thus, sexual selection may act intensely on male pigeon favoring female choice. This claim was supported by Kis and Szekely (2003) in which sexual selection have contributed to sexual differences in most birds. According to the theory of sexual selection, sexual dimorphism should promote preference in either struggle for the female, or choice by the female (Geodakjan, 1985). Furthermore, natural selection might also have contributed to sexual differences in C. livia. This may be attributed to feathers’ functions. Feathers seem to be important in incubation and brooding of chicks. In C. livia both sexes take part in both incubation and brood-rearing (Saxena et al., 2008). Thus sexual selection may act on both males and females to develop long feathers. Furthermore, territorial defense by the use of wing feather may play an important role in sexual dimorphism for this species. A male pigeon defends a territory by holding its wings aloft, landing on its rival at times, or by clubbing the rivals head with bent wings (Saxena et al., 2003). Therefore, males with longer wing feathers may defend their territory more compared to
95 97.62
TAIL Female
Male
12
0
0
12 Total
correctly classified (%) 100 100 100
males with short wings. Thus, natural selection may be intense favoring males with longer wing feathers. IV. CONCLUSION The results of this study showed that the left and right primary wing and tail feathers were different between male and female, suggesting sexually dimorphic indivuals of Columba livia. This results could be attributed to sexual selection (social mating system) and natural selection (parental care and territorial defense) occurring in the evolution of the species being studied. However, distinction of sexual dimorphism will not guarantee since growth, age and environmental condition must also be accounted. Further experiment on mate choice, male-male competition and parental abilities in incubation and brooding in relation to feathers were recommended to explain the influences of such selective processes on the evolution of the plumage and tail feathers in the Rock pigeon. REFERENCES [1]
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