Turdus rufopalliatus graysoni (Ridgway, 1882)

Silva, Héctor Gómez de, Pérez Villafaña, Mónica G., Cruz-Nieto, Javier & Cruz-Nieto, Miguel Ángel, 2020, Are some of the birds endemic to the Tres Marías Islands (Mexico) species?, Bulletin of the British Ornithologists’ Club 140 (1), pp. 7-37 : 28-31

RUFOUS-BACKED ROBIN Turdus rufopalliatus graysoni (vs. T. r. rufopalliatus )

Coloration.—Underwing-coverts tawny-ochraceous contrasting with dull cinnamon sides and flanks vs. underwing-coverts concolorous or nearly so with the breast, sides and flanks ( Ridgway 1907). Grey breast-band vs. grey only in the streaked feathers at the lower edge of the throat; in extreme cases this causes grey, rather than rufous, to be the dominant colour of the underparts ( Grant 1965a, based on 24 male and 42 female graysoni vs. 32 male and 22 female rufopalliatus ; Howell & Webb 1995 Plate 51.3b, our Fig. 18). However, a grey breast-band sometimes shows up in mainland rufopalliatus even well away from the Tres Marías (e.g., see Fig. 19) and while some graysoni have the breast-band ‘faintly... tinged with dull salmon-color’, in mainland rufopalliatus most of the breast-band is ‘salmon color to almost cinnamon-rufous’ ( Phillips 1991). Assuming that all individuals we saw and photographed in the Tres Marías were graysoni , our photographs confirm the blurred distinction (e.g., Fig. 20).

‘Back grayish, usually more or less tinged with brownish but hardly, if at all, contrasted to nape’ vs. ‘scapulars, and usually back, strongly washed with cinnamon-rufous to russet..., in strong contrast to the grayer crown and nape (if not badly worn and faded)’ ( Phillips 1991). Howell & Webb (1995) described the back of graysoni as ‘greyish-rufous to olive-brown’. Our photographs from the Tres Marías show a range, from brown close to the upperparts of White-throated Thrush T. assimilis and showing litle contrast with the nape and crown, to more rufous contrasting with the nape and crown (e.g., Figs. 18, 20 and 22). Upperwing-coverts ‘dull cinnamon or duller’ vs. ‘salmon color to almost cinnamon-rufous’ and sides and flanks dull orange-brown vs. rufous ( Phillips 1991), which seem like subtle distinctions. All or most individuals we photographed on the Tres Marías had sides and flanks similar to some mainland rufopalliatus (e.g., Fig. 21, htp://2.bp.blogspot.com/- G0D2EpXGCkI/VHzA0MTHSjI/AAAAAAAACSQ/sMP9Yby2Yic/s1600/_ DSC 0103. JPG). Grant (1965a) mentioned that graysoni ‘show a tendency to possess paler and narrower chin and throat streaks than mainland birds, easily recognizable only when the extreme forms of the two samples are compared’ (consistent with, e.g., Figs. 18 and 20). Phillips (1991) mentioned ‘feet apparently darker’ in graysoni but did not evidence this, and it is not supported by our field observations. In conclusion, individuals of graysoni representing the extremes of plumage (e.g., with narrowly striped throats, predominantly grey underparts and / or brown backs) are easily identified, but there appears to be sufficient variance and overlap that colour differences cannot be adequately scored. The literature suggests that the main differences are duller plumage and no sexual dimorphism vs. much brighter, richer plumaged males (e.g., Grant 1965 a, Phillips 1991, Howell & Webb 1995). Individuals of unknown sex and / or in faded plumage would not be identifiable.

Morphometrics.—Larger in all external measurements, especially tarsus length and bill length (score 2). Montaño-Rendón et al. (2015), based on 268 adult specimens from throughout the range of T. rufopalliatus (sensu lato), confirmed the existence of marked morphometric differences.

Additional information.— Montaño-Rendón et al. (2015) and Ortiz-Ramírez et al. (2018) found reciprocal monophyly and deep genetic divergence between graysoni and rufopalliatus .

Reasons for uncertainty.—Apparently, graysoni occurs sympatrically with rufopalliatus in coastal Nayarit ( Grant 1965 a, Phillips 1981, Howell & Webb 1995), with specimens of graysoni from Playa Novillero (west of Acaponeta), Santiago Ixcuintla, Sauta, San Blas, Chacala and Las Varas (most from February–April, but singles from 12 May and, especially, 20 June suggest residency and opportunities for interbreeding; Phillips 1981: 306; Table 2). The absence of clear distinctions in plumage between some graysoni and other races (see above) makes it particularly difficult to identify potential hybrids based on plumage, and there is slight overlap in morphometrics, especially bill length of females ( Grant 1965a). Montaño-Rendón et al. (2015) and Ortiz-Ramírez et al. (2018) apparently did not include DNA of graysoni from the mainland, nor indeed of any rufopalliatus from the Nayarit coastal plain, where hybrids, if any, would be expected (all of their Nayarit rufopalliatus were from a single locality in the foothills). Because the extent of hybridisation, if any, is presently unknown, we recommend further studies before concluding whether graysoni represents a species.