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Revista Brasileira de Ornitologia, 23(1), 18-24 March 2015 Sex determination by morphometry of adult White-crested Elaenia (Elaenia albiceps chilensis) 1, 3 1 2 2 Víctor R. Cueto , Susana P. Bravo , Natalia Trujillo-Arias and Gustavo S. Cabanne Centro de Investigación Esquel de Montaña y Estepa Patagónicas (CIEMEP), CONICET - Universidad Nacional de la Patagonia “San Juan Bosco”, Sarmiento 849, U9200 Esquel, Chubut, Argentina División Ornitología, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Av. Angel Gallardo 470, 1405 Buenos Aires, Argentina Corresponding author: email@example.com Received on 28 May 2014. Accepted on 17 March 2015. ABSTRACT: White-crested Elaenia (Elaenia albiceps chilensis) is a Neotropical austral migrant that is sexually monomorphic in plumage. We first determined whether the brood patch and cloacal protuberance are good indicators of an individual’s sex by comparing these characters with individuals sexed using molecular techniques. Second, at several localities encompassing a 1500 km latitudinal gradient across the breeding range, we evaluated whether morphological measurements can be used for sex determination, using discriminant analysis and molecular sexing as dependent variables. Finally, the effectiveness of the discriminant analysis was assessed by a Jackknifed validation, and by a cross-validation process through the classification of a new sample. Sexing using genetic techniques and by cloacal protuberance size and presence of the brood patch produced the same results. We did not find differences in body measurements among study sites. In all localities, males were significantly larger than females in wing and tail lengths, whereas other variables did not exhibit differences between sexes. The best classification of sex b y discriminant functions was obtained by including wing and tail length as discriminatory variables. The discriminant function correctly determined the sex of 86 % of all samples, with correct identification of 90 % of males and 75 % of females. The Jackknifed va lidation and cross-validation of a new sample resulted in similar sex classifications as those produced using the discriminant function. Discriminant analysis thus represented a simple and cost-efficient way to determine the sex of White-crested El aenias for field ornithologists. KEY-WORDS: Discriminat analysis, latitudinal gradients, Neotropical austral migrant, sexual size dimorphism. INTRODUCTION al. (2005), who separated male and female Chiffchaffs (Phylloscopus collybita) based on the bi-modal distribution Sex determination of birds is key to understanding of wing length. However, although this method is various aspects of the demography of a population as it based on sound assumptions, there is no independent verification of its reliability. Relatively unproblematic may influence survival, dispersal, recruitment and other life history parameters (Gowaty 1993, Newton 1998), and reliable is taking a blood or tissue sample for later but sex differentiation is difficult in species with sexually molecular sex determination in the laboratory (Griffiths monomorphic plumage, even for captured individuals. et al. 1998). Many recent authors have identified useful During the breeding season, the sex of most individuals morphological measurements for sex determination using molecular methods on a sub-sample of the species of many species can be determined either by the presence or extent of a brood patch in females or the cloacal under study (Hipkiss 2007, Ottvall & Gunnarsson protuberance in males (Ralph et al. 1993). In some species, 2007). The sex is treated as a dependent variable in either however, the male can develop an incubation patch, but a discriminant analysis or a logistic regression with a it does not typically develop as extensively as in females number of morphological measurements as explanatory variables. In recent years, these methods have been used (Skutch 1957, Pyle 1997). Furthermore, during the non- breeding season these characters are not useful, although to investigate morphological variables useful for sexing a sexing birds in the non-breeding season is desirable (e.g., number of non-passerine and passerine bird species (e.g., sex-specific migration phenologies, migration routes, and Campos et al. 2005, Svagelj & Quintana 2007, Cardoni wintering habitat use). et al. 2009). Although these methods can lead to reliable sex Morgan (2005) discussed the possibility of analyzing multi-modal distributions of morphological characters to determination of the population under study, their general differentiate sexes. This approach was used by Catry et application has some caveats (Ellrich et al. 2010). For Revista Brasileira de Ornitologia, 23(1), 2015 Sex determination by morphometry of adult White-crested Elaenia (Elaenia albiceps chilensis) Víctor R. Cueto, Susana P. Bravo, Natalia Trujillo-Arias and Gustavo S. Cabanne example, statistical models are selected to fit a particular mild but dry summers. Most precipitation falls as rain sample, and will therefore fit the particular sample better and snow during autumn and winter. South of 55° S, than they would fit the entire population from w hich the precipitation is more evenly distributed throughout the sample was drawn, or a sample from another population. year (Garibaldi et al. 2011). Also, there are intra-specific differences in morphology, such as wing length, due to different migration distances (e.g., Lindström et al. 1996, Fiedler 2005). White-crested Elaenia (Elaenia albiceps chilensis) is a Neotropical austral migrant and is abundant in Nothofagus forests of Patagonia between October and March (Grigera et al. 1994). This species is a small tyrant flycatcher (Tyrannidae) sexually monomorphic in plumage (Fitzpatrick 2004). There is little research a bout the White-crested Elaenia demography, which is hampered by the lack of data enabling researchers to determine sex in the field. Our aim was to evaluate sex determination of White-crested Elaenia using molecular techniques and morphometric measurements. First, we analyzed whether the presence of a brood patch or cloacal protuberance are good indicators of an individual’s sex in known-sex individuals as determined through molecular techniques. Second, at several localities encompassing a 1500 km latitudinal gradient of the breeding range of White- crested Elaenia, we evaluated whether morphological measurements can be used for sex determination, using discriminant analysis and molecular sexing as dependent FIGURE 1. Geographical locations where White-crested Elaenias variables. Thir d, we assessed the effectiveness of the (Elaenia albiceps chilensis) were sampled in Patagonia, Argentina. discriminant analysis in terms of the proportions of individuals of known sex that were classified correctly by Sampling birds a Jackknifed validation, and by cross-validation through the classification of a new sample. Scientific nomenclature and common name follow the recommendation by the South American METHODS Checklist Committee (www.museum.lsu.edu/~Remsen/ SACCBaseline.html, accessed on 20 May 2014). We only sampled breeding individuals to avoid the possibility of Study sites capturing individuals during migration. We sampled Between October and March, White-crested Elaenias at the four National Park study sites from 1 January to mainly inhabit Nothofagus forest within a narrow strip 3 February 2012, and sampled at Esquel site from 10 December 2013 to 15 January 2014. Birds exhibiting approximately 2000 km long, extending from the territorial displays were captured by placing a White- southern tip of South America to northern Patagonia. crested Elaenia model coupled with conspecific songs and Along this latitudinal gradient, we selected four study sites in National Parks of Argentina: Parque Nacional calls delivered through a portable speaker within 2 m of (PN) Tierra del Fuego (54°49' S, 68°28' W), Province of a mist net (12 x 3 m, 38-mm mesh size). At Esquel site, we passively captured birds using 10 mist nets (12 x 3 m, Tierra del Fuego; PN Los Glaciares (49°14' S, 72°54' W), 38 mm mesh size) placed 70-100 m apart during four Province of Santa Cruz; PN Perito Moreno (47°57' S, occasions for two consecutive days. Nets were opened 72°07' W), Province of Santa Cruz; and PN Los Alerces (42°36' S, 71°38' W), Province of Chubut (Figure 1). during the first four to five hours after sunrise when For cross-validation classification of a new sample, we weather conditions were not adverse (rainy or windy). At all sites, captured birds were banded with selected a site near Esquel (42°55' S, 71°21' W), Province numbered aluminum bands. For each captured individual, of Chubut (Figure 1). Vegetation corresponds to the we recorded the size of the cloacal protuberance and stage Subantartic Biogeographical Province (Cabrera & Willink 1980). In the northern part of this latitudinal gradient, of development of the brood patch. Cloacal protuberances the climate is characterized by cold and wet winters and were categorized as: none (0), small (1), medium (2), and Revista Brasileira de Ornitologia, 23(1), 2015 Sex determination by morphometry of adult White-crested Elaenia (Elaenia albiceps chilensis) Víctor R. Cueto, Susana P. Bravo, Natalia Trujillo-Arias and Gustavo S. Cabanne large (3) and brood patch development was categorized in the DFA, and instead included only variables that as follows: not present (0), smooth skin (1), vascularized showed differences among sexes (see Table 1; Dechaume- (2), wrinkled (3), and molting (4) (Ralph et al. 1993). We Moncharmont et al. 2011). The performance of each also took five morphological measurements from each variable and its combinations were evaluated with the adult bird: bill length (from the anterior end of the nostril Wilk’s Lambda statistic. to the bill tip), tarsus length (from the intertarsal joint to The effectiveness of the discriminant analyses was the distal end of the last leg scale before the toes emerge), assessed, first in terms of the proportion of bir ds of known tail length (from the base of the feathers to the end of the sex that were classified correctly, second by a Jackknifed longest feather), wing chord (from the carpal joint to tip validation, and finally by a cross-validation process of the longest primary), and body mass. We used a digital through the classification of a new sample (Tabachnick caliper (± 0.01 mm) for bill and tarsus measurements, a and Fidell 1996). The Jackknifed va lidation process wing ruler (± 1 mm) for tail and wing measurements, and classifies each individual case using a function obtained a digital scale (± 0.1 g) to record weight. from the total sample, excluding the individual case to be classified (Ta bachnick and Fidell 1996). The cross- Molecular analysis validated process was used with a new set of individuals obtained during sampling at Esquel site. However, For molecular sexing, we collected blood samples by for this group, sex determination was not verified by piercing the brachial vein with a sterile needle and molecular sexing. Individuals were classified by the sampled blood using a 0.5-ml heparinized capillary tube. presence of cloacal protuberance (males) or incubation Blood samples were stored in absolute ethanol at room patch (females). temperature. DNA from approximately 1 mm of blood was purified, accor ding to the protocols presented in Ivanova et al. 2006. RESULTS For molecular sex determination, we amplified a fragment of the Chromobox-Helicase-DNA-Binding All DNA samples showed one of the typical PCR band (CHD) gene by utilizing PCR with primers P2 and patterns that differentiate males (one band) from females P8 (Griffiths et al. 1998). The PCR procedure used (two bands). The sex of all 34 White-crested Elaenias that approximately 40 ng of DNA and a primer concentration were assessed by size of the cloacal protuberance (males) or development of the brood patch (females) were also of 0.5 μM. PCR reactions used an initial denaturing step of 2 min at 94°C, followed by 35 cycles of 30 s at 94°C, genetically verified. 45 s at 49°C, and 45 s at 72°C and a final step for 10 min We did not find any differences in the five body at 72°C. PCR products were scored by electrophoresis measurements among study sites (Table 1). At all sites, in 3 % agarose gels stained with Ethidium Bromide. wing and tail lengths were significantly larger in males than in females, whereas the other variables did not show Amplification of the CHD gene in males produced a single band and two bands in females. All PCR product differences between sexes (Table 1, Figure 2). lengths were about 350 to 400 bp, which is the expected The classification of sex using DFA for wing, tail and weight (Griffiths et al. 1998). All studied samples (n=34) the combination of these two body measurements were were analyzed in two independent PCR assays that in the Table 2. The best classification was obtained w hen including wing and tail length as discriminatory variables included male, female and negative controls. (Table 2). The resulting function (D , with an associated Data analysis cut-off value of -0.385) was: D = (0.62 x wing length) + (0.07 x tail length) - 55.51 For statistical analysis, we used the subset of individuals Individuals with discriminant function scores greater than the cut-off were classified as male and t hose that were sexed by molecular techniques (23 males and 11 females). Two-way ANOVAs were used to evaluate with lower scores as female. This function correctly if external morphology varied with sex and study site. determined the sex of 86% of all samples, with correct We used the type III method to compute the sum of identifications of 90% of males and 75% of females squares given our unbalanced design and we checked (Table 2). Individuals with a wing length of 76 mm and a tail length of 63 mm are males, and individuals with for normality and homogeneity of variance (Shaw & Mitchell-Olds 1993). a wing and tail of 73 mm and 60 mm, respectively, are We applied a Discriminant Function Analysis (DFA) females (Figure 3). The Jackknifed c lassification correctly to morphological measurements taken from White- determined the sex of 65% of all samples, with the correct crested Elaenias of known sex. We did not use stepwise identification of 75% of males and 63% of females. The cross-validation provided similar classifications to those techniques to identify the set of variables included Revista Brasileira de Ornitologia, 23(1), 2015 Sex determination by morphometry of adult White-crested Elaenia (Elaenia albiceps chilensis) Víctor R. Cueto, Susana P. Bravo, Natalia Trujillo-Arias and Gustavo S. Cabanne produced by discriminant functions. The function derived sex of 76% of all samples, with the correct identification from the cross-validation analysis correctly determined the of 65% of males and 90% of females. TABLE 1. Male and female body measurements (mean ± SD), of adult White-crested Elaenias (Elaenia albiceps chilensis) from four National Parks in Patagonia, Argentina. All measurements are given in mm except body mass in g. Factors of a Two-way Anova were Sex (S), Origin (O) and the interaction (SxO). Only indicated the results that were statistically significant at Type I error lest that 0.05. MALES FEMALES Body Measurement n Mean SD n Mean SD Two-way Anova Wing Length PN Tierra del Fuego 6 75.8 2.6 4 72.8 2.2 F = 5.5, P = 0.027 S; 1, 26 PN Los Glaciares 4 74.5 2.4 3 73.3 2.1 F = 1.1 O; 3, 26 PN Perito Moreno 4 76.8 1.7 2 74.5 0.7 F = 0.4 SxO; 3, 26 PN Los Alerces 9 76.1 1.7 2 75.0 2.8 ALL 23 75.9 2.1 11 73.6 2.0 Tail Length PN Tierra del Fuego 5 62.6 2.1 2 60.0 1.4 F = 6.9, P = 0.015 S; 1, 23 PN Los Glaciares 4 61.0 0.8 3 60.3 2.5 F = 0.3 O; 3, 23 PN Perito Moreno 4 63.5 2.1 2 60.0 1.4 F = 0.5 SxO; 3, 23 PN Los Alerces 9 62.4 2.7 2 60.5 0.7 ALL 22 62.4 2.1 9 60.2 1.5 PN Tierra del Fuego 6 7.5 0.3 4 7.1 0.5 F = 1.3 S; 1, 26 PN Los Glaciares 4 7.4 0.6 3 7.6 0.4 F = 0.8 O; 3, 26 PN Perito Moreno 4 7.3 0.3 2 7.2 0.3 F = 0.9 SxO; 3, 26 PN Los Alerces 9 7.4 0.3 2 7.1 0.6 ALL 23 7.4 0.3 11 7.3 0.4 Tarsus Length PN Tierra del Fuego 5 20.9 1.2 2 20.3 0.3 F = 0.6 S; 1, 21 PN Los Glaciares 4 21.2 1.0 3 21.7 0.6 F = 0.2 O; 3, 21 PN Perito Moreno 4 21.8 0.7 2 21.3 0.1 F = 0.6 SxO; 3, 21 PN Los Alerces 8 21.6 0.5 2 21.7 1.8 ALL 21 21.4 0.9 9 21.2 0.7 Body Mass PN Tierra del Fuego 6 15.7 0.7 3 15.6 1.3 F = 0.1 S; 1, 25 PN Los Glaciares 4 15.4 1.3 3 16.8 1.9 F = 1.3 O; 3, 25 PN Perito Moreno 4 15.2 0.6 2 15.1 0.7 F = 1.3 SxO; 3, 25 PN Los Alerces 9 15.5 0.8 2 14.8 0.4 ALL 23 15.5 0.8 10 15.7 1.4 TABLE 2. Accuracy of sexing White-crested Elaenias (Elaenia albiceps chilensis) as percentages correctly classified using single measurements and a discriminant function D . All discriminant analyses were significant (P < 0.01). Accuracy (%) Wilk´s lambda F-value Males Females Total Tail Length 0.76 F = 8.23 55 88 63 1, 26 Wing Length 0.59 F = 18.40 90 63 82 1, 26 D 0.58 F = 8.98 90 75 86 1 2, 25 Revista Brasileira de Ornitologia, 23(1), 2015 Sex determination by morphometry of adult White-crested Elaenia (Elaenia albiceps chilensis) Víctor R. Cueto, Susana P. Bravo, Natalia Trujillo-Arias and Gustavo S. Cabanne FIGURE 2. Male and female body measurements (mean ± 95% confidence interval) of adult White-crested Elaenias (Elaenia albiceps chilensis) of Patagonia, Argentina. FIGURE 3. Relationship between wing length and tail length for males (black circles) and females (white circles) of adult White-crested Elaenias (Elaenia albiceps chilensis) of Patagonia, Argentina. Revista Brasileira de Ornitologia, 23(1), 2015 Sex determination by morphometry of adult White-crested Elaenia (Elaenia albiceps chilensis) Víctor R. Cueto, Susana P. Bravo, Natalia Trujillo-Arias and Gustavo S. Cabanne DISCUSSION Faria et al. 2007, Cardoni et al. 2009, Botero-Delgadillo 2010, Sandoval & Mennill 2013). Nevertheless, it would be important to assess other features that have been We found that White-crested Elaenias can be accurately sexed by the degree of development of the cloacal used to differentiate sexes and t hat can be measured protuberance and by the presence of the incubation during bird handling in banding stations, such as keel patch. The agreement between results of genetic sex length. Murphy (2007) found that such a morphological measurement is very useful to differentiate the sex of determination and those from measurements taken from individuals of Eastern Kingbirds (Tyrannus tyrannus). birds in the field c learly demonstrates that males do not develop a partial incubation patch. This is an important Other measurements may also be useful to differentiate result because males in some species of flycatchers exhibit the sexes in monomorphic species, for example the UV such character (e.g., in the genus Myiarchus, Pyle 1997). reflectance of plumage (Tubaro et al. 2005), although this is more difficult to obtain during fieldwork. However, these features are only useful to identify the sex The results of our research are important for the during the breeding season, and sexing birds in the non- breeding season is important for a proper assessment of analysis of geographical variation in this and other species the population dynamics of migrant birds throughout of birds, because possible morphological differences their annual cycle (Newton 2008). among localities could be due to variations in the sex ratio of captured individuals in those sites. Also, these Our results also show that at least two body results are useful to assess whether males and females measurements of White-crested Elaenias, wing and tail length, differ between sexes, with those of males being differ in their demographic characteristics and habitat larger than females. Furthermore, using the discriminant selection during the non-breeding season, a period of function generated by the combination of these two the annual cycle poorly studied in Neotropical austral migrant birds. measurements, we can differentiate the sexes with at least 90% and 75% confidence in males and females, respectively. These morphological measurements are commonly taken at banding stations and therefore the ACKNOWLEDGMENTS methods are highly standardized. In addition, obtaining these measurements does not require intensive handling We thank Alex Jahn for his comments and for improving of birds, thereby significantly reducing the stress on English syntax and grammar, which greatly enhanced this captured individuals. paper. Jason Weckstein, Erik Johnson and one anonymous A major criticism of the use of discriminant functions referee provided numerous constructive comments for derived from body measurements to differentiate between improving our paper. Research was partially financed sexes is that they are useful only for the populations by Secretaría de Ciencia y Técnica de la Universidad from which the samples were obtained (Ellrich et al. de Buenos Aires (UBACyT 645-11) and National 2010). In our study, we did not observe any geographic Geographic Society (GN 9289-13). CONICET provided differences for the five morphological variables, even institutional support. All work related to the handling of across a latitudinal gradient from 54° S (PN Tierra del birds and blood sampling was done in compliance with Fuego) to 42° S (PN Los Alerces), i.e., separated by over Argentina law (permit APN 942). 1300 km. We sampled during the active breeding period of White-crested Elaenia (corroborated by the capture of individuals in breeding condition at all sampling sites), REFERENCES so it is improbable that individuals captured at lower Botero-Delgadillo, E. 2010. 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Condor 107: 648-656. migration of Willow Warblers Phylloscopus trochilus in Sweden: results from a nation-wide co-operative project. Ornis Svecica 6: 145-172. Associate Editor: Jason Weckstein Revista Brasileira de Ornitologia, 23(1), 2015
Ornithology Research – Springer Journals
Published: Mar 1, 2015
Keywords: Discriminat analysis; latitudinal gradients; Neotropical austral migrant; sexual size dimorphism
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