ISRAEL JOURNAL OF ZOOLOGY, Vol. 46, 2000, pp. 149Ð154

NOTE: PREDICTING THE SEX OF BLACKSTARTS (CERCOMELA MELANURA) BY DISCRIMINANT ANALYSIS

NOAM LEADER Department of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel

Cryptically dimorphic species, in which the sexes look alike by plumage but show small differences in size and shape, are difficult to study in the field when one wants to investigate behavioral aspects and sex-roles in their breeding biology. The blackstart (Cercomela melanura; Aves; Turdidae) is a small (14Ð16 g), common resident of the SaharoÐSindian desert belt. The occurring in Israel, C. m. melanura, is widespread along the depression south to the Sinai and northwest and central Saudi Arabia (Bundy, 1986, Cramp and Simmons, 1988). Although quite abundant in desert regions of Israel, this species has been poorly investigated, and many aspects of its social pattern and behavior are not known (Paz, 1986, Cramp and Simmons, 1988). Male and female blackstarts are similar in plumage coloration, and currently can be sexed only during the breeding season, when the females have an incubation patch, or by observing copulation behavior. Thus, they are difficult to identify in the field. During a study of the breeding biology and behavior of the blackstart in the wild (Leader and Yom-Tov, 1998), a technique was required that would provide us with a simple means of sexing individually marked . Discriminant functions derived from morphometric data are commonly used to sex a variety of avian species. (See, e.g., Green and Theobald, 1989; Counsilman et al., 1994; Winker et al., 1994; Evans et al. 1995; Sweeney and Tatner, 1996; Phillips and Furness, 1997.) When applied to the problem of sexing birds, discriminant function analysis can circumvent the need for gonadal inspection or intensive observation of breeding behav- ior (Sweeney and Tatner, 1996). Adult blackstarts breeding in En Gedi (31°28′N, 35°23′E), Israel, were trapped on their breeding territories during the months MarchÐJune in both 1994 and 1995. Each breeding bird which was at least one year old was ringed with a unique combination of colored and numbered aluminum rings (Israel Nature Reserves Authority banding permit no. 93/12779). Wing length (maximum flattened chord) was measured using a stopped wing-ruler. Tail length was measured by inserting a ruler to the base of the tail. Both were measured to the nearest millimeter. Tarsus length was measured to the nearest 0.1 mm using calipers, from the middle point of the joint between tibia and tarsus behind the leg to the

E-mail: [email protected] Accepted December 1999. 150 N. LEADER Isr. J. Zool.

Table 1 Morphometric variables (mean, SD, and range) of known-sex blackstarts trapped in En Gedi, 1994Ð1995, and used in discriminant analysis. Sample size in parentheses Measurement Male Female t-test P ± ± Wing length (mm) 82.5 2.2 (17) 76.5 2.0 (21) t36 = 8.8 < 0.001 range 79.0Ð86.0 72.5Ð80.0 ± ± Tail length (mm) 64.6 2.5 (17) 60.5 1.8 (20) t35 = 5.7 < 0.001 range 61.0Ð70.0 57.0Ð65.0 ± ± BillÐskull length (mm) 17.1 0.9 (17) 16.2 1.0 (20) t35 = 2.9 < 0.01 range 14.6Ð18.15 14.1Ð17.7 ± ± BillÐnostril length (mm) 10.2 0.6 (17) 9.7 0.6 (20) t35 = 2.7 < 0.01 range 9.3Ð12.0 8.8Ð10.8 ± ± Tarsus length (mm) 23.8 0.4 (17) 22.8 0.5 (20) t35 = 6.5 < 0.001 range 23.1Ð24.9 22.0Ð24.0 middle point of the joint between tarsus and middle toe in front of the leg. BillÐskull length (from bill-tip to the skull-kinesis) and billÐnostril length (from bill-tip to the anterior edge of the nostril) were measured using calipers to the nearest 0.1 mm. Both wing and tarsus measurements were taken on the right side of the body, and three readings were taken for each measurement. In total, 39 breeding adults were sexed. Sex was determined by observing copulation behavior of all birds. I also found that a brood patch occurs only in females and that only males give territorial displays. Prior to the fieldwork, 20 blackstart skins (13 males and 7 females) were measured at the Tel Aviv University Zoological Museum, using the same measuring techniques. Sex of the museum skins was known and was based on gonadal inspection at the time of their preparation. Only 5 skins (3 males and 2 females) originated from the En Gedi and surrounding Dead Sea depression, whereas the remaining 15 skins (10 males and 5 females) were collected from more southern parts of Israel, namely the , Elat, and the Sinai Peninsula. This small sample size did not allow me to perform a preliminary discrimi- nant analysis for the purpose of sexing in the field; however, the discriminant function derived from birds caught in the field was later fitted on the museum data to examine the external validity of the discriminant function. Male blackstarts caught in the field at En Gedi were significantly larger than females in all six morphometric variables, although in each case there was substantial overlap between the sexes (Table 1). Multivariate discriminant analyses were, therefore, performed in order to improve on the degree of discrimination possible with these single variables, using the discriminant analysis module in STATISTICA (Statsoft, Inc., 1997). Discriminant analyses were performed on untransformed data using a stepwise selection for “good” predictor