Specimens (6): [Male], 40726, left testis 9 × 7 mm., right testis 4 × 2 mm., brood patch, 122.3 gm., molting--1st and 2nd primaries ensheathed (July 10), 5 km. S Champotón; [Male], 40727, testis 5 × 3 mm., brood patch, 125.6 gm., molting--1st and 2nd primaries ensheathed (July 11), 5 km. SW Champotón; [Female], 40728, 109.8 gm. (July 13), and sex? 40729, 109.5 gm. (July 15), 7-1/2 km. W Escárcega; [Female], 40730, imm., 110.3 gm. (July 21), Pisté; [Female], 40731, ovary inactive, brood patch refeathering, 121 gm., molting--2nd and 3rd primaries ensheathed (August 16), 4 km. NNE Felipe Carrillo Puerto. _Cyanocorax yncas maya_ (van Rossem): Green Jay.--There are records of breeding by this species on the peninsula for April 28 and 29, and June 4 (Paynter, 1955a:215, 216). Specimens (4): [Male], 40740, testis 5 × 3 mm., 81.2 gm. (July 16), 7-1/2 km. W Escárcega; [Female], 40739, juv., 62.3 gm. (July 21), [Male], 40737, 64.5 gm. (July 21), and [Female], 40738, 64.1 gm. (July 21), Pisté. _Psilorhinus morio_ (Wagler): Brown Jay.--Specimens (3): [Female], 40742, 203.5 gm. (July 13), [Male], 40743, juv., 170 gm. (July 14), 7-1/2 km. W Escárcega; [Male], 40741, juv., 193.7 gm. (August 14), 4 km. NNE Felipe Carrillo Puerto. _Thryothorus lucovicianus albinucha_ (Cabot): Carolina Wren.--This wren formerly was considered to be a species (_T. albinucha_). In the light of evidence presented by Lowery and Berrett (1963), it is advisable to follow Paynter (1955a:219) and consider _albinucha_ to be a subspecies of _T. ludovicianus_. The only previous evidence of breeding by this species on the peninsula was a male having slightly enlarged gonads taken by Paynter (1955a:219) on March 9. Specimen (1): [Male], 40754, juv., 16.8 gm. (August 14), 4 km. NNE Felipe Carrillo Puerto. _Thryothorus maculipectus canobrunneus_ (Ridgway): Spotted-breasted Wren.--According to Paynter (1955a:220) this species breeds from mid-March to May 23. Gonads were moderately active in our specimens as early as mid-July. Specimens (4): [Female], 40758, ovary (6 × 3 mm.) granular (July 14), and [Male], 40757, testis 5 × 3 mm., 15.3 gm. (July 14), 7-1/2 km. W Escárcega; [Female], 40755, ovary 6 × 3 mm. (ova to 1 mm.), 12.7 gm. (July 21), Pisté; [Male], 40756, ovary 4 × 3 mm., 16.8 gm. (August 3), 4 km. WSW Puerto Juarez. _Troglodytes musculus_ (Naumann): Southern House Wren.--Specimen (1): [Female], 40760, juv. (August 2), 4 km. WSW Puerto Juarez. _Troglodytes beani_ (Ridgway): Cozumel Wren.--Specimens (2): [Female], 40761, 13.3 gm. (August 9), and [Male], 40762, imm., 13.6 gm. (August 9), 3-1/2 km. N San Miguel, Isla Cozumel.