. Bulletin of the British Ornithologists' Club. [Bull. 1977: 97(1)] 30 In nelsoni, 9 females averaged 104 g in weight (92-112 g), whereas 5 males averaged 121 g (117-123 g). In sanblasiana, 14 females, distinguished on the basis of breeding roles or brood patches, averaged 107 g (94-118 g, only one bird exceeding 114 g); 12 males distinguished on the basis of breeding role or cloacal protuberance, averaged 116 g (112-119 g). Thus in adults of both races, females are significantly lighter than males. The social-alarm calls of the two races of San Bias Jay are distinguishable by ear and b
. Bulletin of the British Ornithologists' Club. [Bull. 1977: 97(1)] 30 In nelsoni, 9 females averaged 104 g in weight (92-112 g), whereas 5 males averaged 121 g (117-123 g). In sanblasiana, 14 females, distinguished on the basis of breeding roles or brood patches, averaged 107 g (94-118 g, only one bird exceeding 114 g); 12 males distinguished on the basis of breeding role or cloacal protuberance, averaged 116 g (112-119 g). Thus in adults of both races, females are significantly lighter than males. The social-alarm calls of the two races of San Bias Jay are distinguishable by ear and by sonographic analysis (Fig. 2). In nelsoni this call is a rather uniform nasal-sounding chatter; in sanblasiana, to the chatter is added con- sistently another component, which is a more sustained, downwardly inflected cawing sound resembling the typical component of the social- alarm call of C. melanocyanea of Central America and the cawing sound of fr;;J. TIME IN SECONDS Figure 2. A comparison of social-alarm call structure in (left) C. s. nelsoni and (right) C. s. sanblasiana. The specimens selected show the extreme differences noted, but sometimes C. s. sanblasiana is as staccato as nelsoni. No sustained components, as shown for nelsoni, have been heard from sanblasiana. C. beecheiL When individuals of C. beecheii, , C. melanocyanea and C. sanblasiana are confronted with a tape recorder playback of their own social-alarm calls, they will respond by approaching and sometimes by uttering the same call, though individuals quickly habituate to this experi- mental procedure. When the same jays hear the social-alarm calls of the other species, they do not approach or call. On the other hand, when we played social-alarm calls of nelsoni to sanblasiana, they responded by approaching and calling. It would therefore seem that apparent difference in voice is not as yet an isolating mechanism in the process of divergence of nelsoni and sanblasiana from each other. The mino
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