Icterus pustulatus graysonii, Cassin, 1867

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 : 25-26

STREAK-BACKED ORIOLE Icterus pustulatus graysonii (vs. I. p. microstictus and I. p.

yaegeri total score 7).

Coloration.—Very few short and narrow streaks on back, usually in scapular region vs. more prominent and numerous streaks throughout back ( Fig. 15; Grant 1965a, based on 39 adult male and 13 adult female graysonii vs. 69 adult male and 29 adult female microstictus / yaegeri [score 2]). Median coverts ‘pale yellow to yellowish-white’ vs. ‘usually (orangey) yellow’ in yaegeri, the geographically closest mainland taxon, and white in microstictus, which occurs further inland than yaegeri ( Phillips 1995 [score 1]). Grant (1965a) mentioned but did not quantify a tendency to differ in yellow vs. orange plumage. Generally, graysonii is less orange overall (consistent with the median coverts character, above, and with Ridgway 1902, Jaramillo & Burke 1999) and we support this based on our field work, although one adult graysonii among the dozens seen was intensely orange ( Fig. 16).

Morphometrics.—Larger in external measurements (score 2), but with a significantly shorter coracoid both absolutely and relatively (score 2) and a relatively shorter femur (not significantly different whereas all external measurements are significantly different [not scored]). The bill is also differently shaped (‘longer in relation to width’, Grant 1965a [not scored]).

Evidence of hybridisation.— Phillips (1995) collected a mainland specimen that he presumed to be a rare variant of yaegeri and had ‘plain yellowish interscapulars, with hardly perceptible black streaking (on the back). But the middle wing-coverts are richer, and bill shorter, than graysonii ’. This specimen is now in the IBUNAM collection (P022269). Its precise locality, according to the label, is ‘1¾ km N of Singaita’, whereon it is also mentioned ‘Apparently alone in brush, near normal-backed pair’. The bird not only resembles graysonii in its back patern but also in size ( Fig. 15). Therefore it combines phenotypic characters and is almost surely a hybrid. We found a second mainland specimen (P015891 from Santiago Ixcuintla) that resembles graysonii in its fewer and narrower back streaks, but streaks are present even on the central back ( Fig. 15). In overall length this specimen is comparable to other mainland specimens, and we believe it is also a hybrid.

Additional information.— Cortés-Rodríguez et al. (2008) and Ortiz-Ramírez et al. (2018)

found reciprocal monophyly but shallow genetic divergence between graysonii and

mainland specimens. Shallow genetic divergence is also seen in other sister species of

orioles even when they possess distinctly different plumage features (e.g. Kondo et al. 2004).