Haematopus ostralegus Linnaeus: Oyster-catcher.—Two individuals were seen along the beach about 6 km. W Sisal on August 10 and 11.
Charadrius alexandrinus tenuirostris (Lawrence): Snowy Plover.—This was one of the commonest shorebirds on the beaches and mudflats west of Sisal. Several immature birds were taken, along with a pair of half-grown chicks, indicating the species was breeding, or recently had bred, there. No previous record of this species breeding on the peninsula has been found.
Specimens (6): ♀, 40431, ovary inactive, 38.2 gm. (August 7), 4 mi. S Sisal; ♂, 40426 testis minute, 35.4 gm. (August 9), ♀, 40427, half-grown chick, 25.4 gm. (August 9), ♂, 40428, half-grown chick, 21.2 gm. (August 9), ♀, 40430, imm., 38.0 gm., little fat (August 10), ♀, 40429, ovary inactive, 35.7 gm., little fat, molting—1st through 6th primaries and middle pair of rectrices are new (August 10), all from 13 km. WSW Sisal. Mallophaga: Actornithophilus ochraceus, August 9; Quadraceps assimilis major, August 7.
Charadrius wilsonia wilsonia Ord: Wilson Plover.—A flock of about 11 was seen on the beach north of the village on Isla Mujeres on August 1. Two specimens were collected on the leeward side of the island about three kilometers south of the village on August 2. The only evidence of breeding was slightly enlarged testes in one male. Two individuals were sighted on the east coast of the mainland at Puerto Juarez on August 4. The species was common 3 to 13 km. west-southwest of Sisal; specimens taken there may have been migrants.
Specimens (5): ♂, 40435, testis 3 × 1 mm., 59.0 gm. (August 2), and ♀, 40436, ovary inactive, 49.3 gm. (August 2), Isla Mujeres; ♀, 40434, ovary inactive, 53.6 gm. (August 7), ♂, 40433, testis 1 mm., 54.5 gm. (August 9), and ♂, 40432, imm., 53.7 gm., little fat (August 11), 3-13 km. WSW Sisal. Mallophaga: Quadraceps assimilis major, August 2 and August 11.
Actitis macularia (Linnaeus): Spotted Sandpiper.—A total of six individuals was seen on Isla Mujeres; each was alone. The species was common at Sisal where it was often seen in small mixed flocks with Snowy Plovers and Wilson Plovers.
Specimens (3): ♂, 40437, testis 2 × 1 mm., 33.8 gm. (August 9), ♂, 40438, imm., 28.2 gm. (August 10), and sex? 40439, imm., 31.0 gm. (August 10), 13 km. WSW Sisal. Mallophaga: Quadraceps ravus; August 10.
Catoptrophorus semipalmatus inornatus (Brewster): Willet.—A flock of 15 or 20 individuals was seen almost every day of our stay on the beach 8-13 km. WSW of Sisal. Measurements of two of the specimens are large and they are clearly referable to C. s. inornatus.
Specimens (3): ♂, 40441, testis 3 mm., 242.3 gm., wing 205 mm., tail 80 mm., tarsus 66 mm., culmen (from feathers) 60 mm. (August 9), ♀, 40440, ovary inactive, 240.5 gm., wing 204 mm., tail 77 mm., tarsus 69 mm., culmen (from feathers) 59 mm. (August 11), and ♂, 40442, imm., 221.1 gm. (August 11) 8-13 km. WSW Sisal. Mallophaga: Austrominopon sachlebeni, August 9; Quadraceps carrikeri, August 11.
Arenaria interpres (Linnaeus): Turnstone.—Six individuals were sighted on Isla Mujeres on August 1 and one individual was sighted 4 km. W Sisal on August 11.