RELATIONSHIPS OF THE AVIFAUNA OF SAN ESTiBAN ISLAND, SONORA RICHARD C. BANKS Bureau of Sport Fisheries and Wildlife U.S. National Museum Washington, D.C. 20560 San Esteban Island, Sonora, Mexico, lies in of the avifaunal areas of Sonora, where he the Gulf of California approximately midway states (van Rossem 1945b:24): “The San between the mainland of Mexico and #the Lucan, a term coined by Dice for the southern peninsula of Baja California, at a latitude of three-fourths of Baja California, is used with 28” 40 ’ N. It forms the center link of a chain extreme reservation. If employed here it does of islands across the Gulf, being slightly nearer not imply acceptance of the proposal to rec- Tiburon Island on the Sonoran shore than to ognize Baja California as a separate Biotic‘ South San Lorenzo Island on the Baja Cali- Province ’ but to emphasize the character of fornia side. Tiburon is a large island, very the avifauna of San Esteban Island. This is- near shore, with a large avifauna and several land which lies almost exactly in mid-Gulf is endemic forms (van1 Rossem 1932), whereas a political accident so far as the present re- South San Lorenzo is small with a meager avi- port is concerned for only one bird, a thrasher, fauna. The birds of Tiburon are all closely belongs to the Sonoran mainland avifauna. So related to those of Sonora, and the species on far as is known at this time all the other in- San Lorenzo are all identical with those of dicative resident races (except for an Am- Baja California. Although the avifauna of San phispiza) are those of Baja California or more Esteban Island is small, it includes seven spe- particularly of the San Ignacio District (of cies which vary geographically and of which, Grinnell, 1928).” The San Ignacio District as therefore, one can attempt to determine rela- outlined by Grinnell (1928:8) covered about tionships. the central two-thirds of the peninsula and was The first impression that one obtains, from characterized as “poorly developed as regards looking at maps or from visiting the area, is endemic differentiates.” of the proximity of San Estdban to Tiburbn, The subspecific assignments of the birds of which suggests that the affinities of the biota San Esteban made by van Rossem (194513) of the former should be with Sonoran forms. were accepted by the committees that pre- A. J. van Rossem, who is responsible for vir- pared the most recent A.O.U. and Mexican tually all the published information about the check-lists. My own work in Baja California birds of San Esteban Island, originally held and on the islands in the Gulf of California that viewpoint and assigned specimens from (Banks 1963; see also Lindsay 1962,1986) has that island to subspecies found on Tiburon and led me to question some of the reported sub- the Sonoran coast. For example, van Rossem specific determinations and thus to reopen the (1930b) stated that his “Tiburon Island Dis- question of the relationships and affinities of trict” of Sonora “includes also San Esteban the San Esteban avifauna. In the course of Island.” the reassessment reported here I have exam- In later writings, however, van Rossem re- ined all available material from that island, investigated the subspecific status of the geo- including most of that seen by van Rossem graphically variable San Esteban birds and and several recently taken specimens. In each came to quite different conclusions. In 1942 of the seven species accounts which follows, I he assigned specimens of three species to races present briefly the history of the nomenclature found in Baja California rather than to Sonoran of the San Esteban population prior to setting forms, and he emphasized his changed view- forth my own concept of its relationship. point on a number of subsequent occasions, stating, for example, “San Esteban Island, al- DENDROCOPOS SCALARIS though politically a part of Sonora, is Baja When he first reported the Ladder-backed Californian in its avifaunal relationships . .” Woodpecker on San Es&ban Island, van Ros- (van Rossem 1945a:243). The clearest expres- sem ( 1931) listed his single specimen under sion of this concept is set forth in a discussion the name D. s. cactophilus. Later (van Rossem The Condor, 71:88-93, 1969 1881 AVIFAUNA OF SAN ESTBBAN ISLAND, SONORA 89 1942) he assigned that specimen and one ad- TABLE 1. Mean measurements (mm) of Myiurchus ditional one to the race D. s. lucasanus of cinerascens. southern Baja California, noting that the race All Mexican on the peninsula opposite the island was not cinerascenscinerascens San Est6ban pmtinax lucasanus but eremicus. Birds of this species Wing length, 8 99.9 98.9 98.5 96.4 from nearby Tiburon Island were referred to Winn length. Q 94.2 93.3 92.2 90.9 cactophilus while those from the mainland Tailieng&, 2 91.4 90.9 91.6 89.8 Taillength, 0 86.0 85.6 85.2 84.2 opposite San Esteban (Kino Bay) were placed Bill length, $ 14.57 14.49 14.55 14.83 first in cactophilus but later in sinaloen& Bill length, 0 14.05 14.10 14.30 14.37 (van Rossem 1932, 1945). Recent check-lists Data, except for San EstBban, from Lanyon (1961 and pen. ( A.O.U. 1957; Miller et al. 1957) followed van comm.). Wing length is of flattened wing. Rossems’ treatment of the insular populations but placed the northern limits of lucasanus at inquietus is now considered a part, from the about 29” on the peninsula, following Grinnell species cinerascens ad,ds uncertainty to any ( 1928), to the north of San Esteban. More interpretation of his early concept of the re- recently, Banks (1963) reported a specimen lationships of the San Esteban population. of D. s. sinabensis from Tiburon Island. Thus, Later, when commenting on the Baja Cali- by current treatment the San Esteban popula- fornia affinities of the San Esteban avifauna, tion is flanked by lucusanus on the peninsula van Rossem (1942) referred the same speci- and sinaloensis on the mainland, with the mens to M. c. pertinax of the southern part of southern limits of both cactophilus and eremi- the peninsula. He maintained that position cus not far away. (van Rossem 1945b) when he recognized the There are but three specimens from San distinction of nuttingi and cinerascens. This Esteban available for comparison, including final assignment of the San Esteban birds to the two mentioned’ by van Rossem and one pertinax was followed by the A.O.U. (1957) taken in 1966. All are males; two were taken and by Miller et al. (1957). Lanyon (1961) in April and one in January. The two April did not specifically mention the San Esteban birds, both of which were breeding, are quite birds; although his range map shows a symbol worn. Only the most recent specimen has a for pert&x on San Esteban Island, at the lati- complete upper mandible. tude of intergradation of pertinax and cineras- None of these specimens is like the small tens, he discussed pertinax (p. 443) only with sinaloensis, but I cannot with certainty assign reference to the peninsula of Baja California them to any subspecies. Each individual seems south of latitude 29”. to be variously intermediate, resembling one I have been able to examine all material race in some characters and another race in previously reported from the island (Town- other ways. The relatively fresh plumaged send 1923; van Rossem 194513)as well as speci- January bird is most like catophilus dorsally, mens taken in 1966, a total of eight males and but is more similar to lucasanus in the reduc- five females. All specimens were taken 13-27 tion of markings on the posterior underparts. April. Although migrants of cinerascens are Both April birds are similar tot lucasanus dor- not unexpected at this latitude at this time, I sally, but one is close to eremicus and one to consider the material examined to be members cactophiks ventrally. Thus there seems to be of the resident population. Males taken on influence from both the mainland and the 27 April 1966 had testes 13 x 5, 10 X 5, and peninsula. Certainly this species cannot be 11 x 8 mm; others taken on 17 and 18 April said to demonstrate particularly close affinities 1930 are labeled as being in “breeding condi- between the San Esteban and southern Baja tion.” California avifaunas. M. c. pertinux is a weakly defined race that averages smaller in wing and tail length but MYIARCHUS CINERASCENS slightly larger in bill length than M. c. ciner- The earliest specimens of Ash-throated Fly- ascens. Lanyon ( 1961:428) detected no sig- catchers from San Estkban Island (Townsend nificant divergence in plumage coloration be- 1923) were reported under the name M. c. tween the two races. Ridgway (1907:628) cinerascens. Van Rossem originally (1931) mentioned that the yellow of the underparts referred his material from there to the race of pertinax averaged slightly deeper, but noted cinerascens, although he considered them to .that this character was not entirely constant. be intergrades with the mainland form, in- The difference seems more useful in fresh quietus. The fact that van Rossem did not plumaged fall birds than in samples of the separate the species nuttingi, of which the race breeding populations, and I could not use 90 RICHARD C. BANKS this or any color character in trying to place Soon after the above statements appeared, the birds from San Es&ban Island. Huey ( 1930) described A. f. ignutius from the Measurements of my sample from San central part of the peninsula of Baja Cali- Es&ban are presented in table 1, with the fornia, restricting lumprocephalus to the Cape data given by Lanyon ( 1961) for the two races region proper, as van Rossem (1930a) had for comparison.