SHORT COMMUNICATIONS

THE NEST, EGGS, AND RELATIONSHIPS have never been satisfactorily resolved. Most early OF THE HALF-COLLARED GNATWREN workers (e.g., Ridgway 1911, Cory and Hellmayr 1924) placed both genera in the Formicariidae, a ( CZNEREZVENTRZS) suboscine family. Miller (1924) was the first to point out that these genera possess certain external struc- tural characters, especially the presence of an after- LLOYD F. KIFF shaft, more typical of oscines. Subsequently, Wetmore (1943) found upon dissection that Ramphocaenus Although the Half-collared (or Tawny-faced) Gnat- possesses an oscine syrinx, and he placed- it in the wren (Microbates cinereiuentris) is a fairly common Sylviidae. Mayr and Amadon ( 1951) included both resident of the humid Caribbean lowlands of southern Microbates and Ramphocaenus in the tribe Poliop- Central America and northwestern South America, no tilini, the only group of New World sylviids which description of its nests and eggs has been published. they recognized. This treatment has generally been On 3 April 1971 I found a nest of this species in followed bv recent authorities. including Pavnter the understory of a mature stand of “Tropical Wet ( 1964), who commented on the probable lack of ” (Holdridee 1957) about 2 km SE of Puerto close relationship between the (Poliop- Viejo, Heredia Province, Costa Rica, at an elevation of tila) and the two genera, Microbates and Rampho- less than 50 m. The nest was located 1/2 m off the caenus. ground in a broad-leafed shrub growing in a rather A comparison of the nests and eggs of these forms, open area along a wide trail. Despite the small size insofar as they are known, further emphasizes the of the shrub ( 1 m tall), it also contained what ap- similarities between Microbates and Ramphocaenus peared to be an old sleeping nest of the White- and their shared differences from PoZio?&z. Details of breasted Wood-Wren ( Henicorkina leucosticta). the nidification of the South American Microbates At the time it was found, the gnatwren nest was collaris, the only other species in that genus, still await in the final stages of construction. While I investigated description. it, two gnatwrens, presumably a mated pair, circled Nests, and in most cases, eggs of the widespread me at close range, uttering nasal whining notes and Ramphocaenus melanurus ( including the Middle carrvinz bits of green moss in their bills. When I American rufiventris) have been described from Bra- withdrew to a v&age point 30 m from the shrub, zil (Pinto 1953, Sick 1954), Trinidad (Belcher 1932, both flew to the nest in rapid succession and Belcher and Smooker 1936), (Skutch added the bits of moss to its outer walls. 1968), Panama (Eisenmann 1953), Costa Rica I was not able to return to the site until 17 April. ( Skutch 1960), Nicaragua (Howell 1957), and Brit- When I approached to within 2 m of the nest, a ish Honduras CRussell 1964 ). I have examined an gnatwren flew off it, scolded briefly, then disappeared additional nest and set of this species collected in into the forest understory. Both the nest and the two Costa Rica by Andrew Williams and now at the eggs it contained were collected and subsequently de- WFVZ (no. 58178). posited in the collection of the Western Foundation The eggs of Ramphocaenus melanurus and Micro- of Vertebrate Zoology (WFVZ no. 58456). bates cinereioentris are virtually identical in colora- The nest was an open cup with an exterior diameter tion and shape. Those of Ramphocaenus are slightly of 10 cm and exterior depth of 15 cm, the inner cup smaller (av. of 10 = 17.8 x 13.2 mm) than the two being 5 cm in diameter and 4 cm deep. The outer known eggs of M. cinereiventris (av. = 18.65 x 14.0 walls were composed of green moss, papery bark mm), presumably owing to size differences between fragments, leaf petioles, and bits of dead brown dicot the species. The mean weight of 18 9 Rampho- leaves, all held together by plant fibers. The inner caenus from Central America and northern South cup was lined with a soft layer of dead leaf skeletons, America is 9.4 g, while four 9 M. cinereiuentris from plant fibers, and a few strands of fine grass. The Costa Rica average 10.4 g. nest was attached by slender plant fibers to the trunk Both species build similar nests, deep, bulky open of the shrub along one entire vertical surface, and cups placed very low off the ground (mean height another side was built around a small limb which reported for nine Ramphocaenus nests was only 4.5 grew off the trunk at a sharp angle. The overall ap- cm, ranging from 2.4 to 7.0 cm) and usually sus- pearance of the nest was that of a semi-suspended, pended between upright plant stems. The only substan- bulky cup of green moss, partially concealed by the tive difference between the nests of the two species leaves of the shrub. is that nearly all of the reported nests of Ram- The eggs measured 18.8 x 13.8 and 18.5 x 14.2 phocaenus had the outer walls composed chiefly of mm with empty dry shell weights of 0.101 and 0.103 dry grass blades or leaves (often bamboo), plus bits g, respectively. Both contained embryos approximately of moss and other plant matter, while green moss 4-5 days old. The eggs were white with a liberal formed the virtual entirety of the only known Mi- sprinkling of fine reddish-brown and dark brown crobates nest. spots over their entire surface, but slightly denser on These differences in the choice of nest materials the larger ends. They were ovate in shape and blunt may simply reflect the different habitat preferences at the small ends. of the two species. In Central America, I have in- The taxonomic relationships of Microbates and the variably encountered Ramphocaenus melanurus in similar neotropical monotypic genus, Ramphocaenus, second growth or forest edge situations, while Micro- [=I1 The Condor 79:261-281, 1977 262 SHORT COMMUNICATIONS

bates cinereiuentris seems always to be confined to in the well-defined Polioptila, the other in the Ram- the understory of mature humid forest, at least in phocaenus-Microbates group. Further study, espe- Costa Rica. Mosses are ubiquitous in the latter cially the discovery of the nest and eggs of some mem- habitat, and a mossy nest there presumably enjoys ber of Macrosphenus, all apparently undescribed, may cryptic advantages. Similarly, broad-bladed grasses shed more light on the respective origins of the New and bamboos are pervasive constituents of most neo- World sylviids. tropical second growth, and a nest constructed of I am grateful to F. G. Stiles for field assistance these readily available materials would surely be less and unpublished weight data, to E. Eisenmann and obvious to visual predators than one made mainly of M. D. Bruce for helpful comments on the manu- green moss. script, and to E. N. Harrison and the Western The Ramphocaenus nest found by Howell, now at Foundation of Vertebrate Zoology for financial sup- the WFVZ (no. 35221), appeared to be sewn on one port. side to a large green leaf in the manner of the Old World tailorbirds ( Orthotomus) . While other observ- LITERATURE CITED ers have remarked on the tendency for Ramphocaenus to surround its nest with large broad leaves, there are BELCHER, C. 1932. Notes from Trinidad and the no other reports of tailorbird-like “stitching.” I did Main. Oologists ’ Record 12:36-39. not note this characteristic in the Microbates cinerei- BELCHEH, C., AND G. D. SMOOKER. 1936. Birds of ventyis nest, although the nest was partially sur- the Colony of Trinidad and Tobago. Pt. IV. Ibis rounded by the large leaves of the shrub which held it. 6:792-813.‘ Eggs of Ramphocaenus and Microbates are super- CORY, C. B., AND C. E. HELLMAYR. 1924. Catalogue ficially similar to those of Polio&a snecies in colora- of the birds of the Americas. Field Mus. Nat. tion, i.e., white with fine brown dots or small spots, Hist. Zool. Ser. 13:1-369. although at least two Polioptila species, P. caerulea EISENMANN, E. 1953. The nest of the Long-billed and P. plumbea, lay eggs with a greenish-blue ground Gnatwren ( Ramphocaenus rufiventris), Auk 70: color (which fades in museum specimens to pale 368369. bluish-white o r pure white). Polioptila eggs are HOLDRIDGE, L. 1957. The vegetation of mainland relatively shorter than those of Microbates and Ram- Middle America. Proc. 8th Pacific Sci. Coner. phocaenus. The length/breadth ratios of the eggs of 4:148-161. four species of gnatcatchers in the WFVZ collec- HOWELL, T. R. 1957. Birds of a second-growth rain tion are as follows: P. albiloris, 1.24 (N = 8 ), P. forest area of Nicaragua. Condor 59:73-111. nigriceps, 1.28 (N = 4), P. caerulea, ‘ 1.28 (i? = MAYR, E., AND D. AMA&N. 1951. A classification 20 )., and P. vlumbea. 1.28 IN = 20 ). The same of recent birds. Am. Mus. Novit. No. 1496. ratios for eggs of Microbates cinereiventris and Ram- MILLER, W. D. 1924. Variations in the structure phocaenus melanurus are 1.33 (N = 2) and 1.35 of the aftershaft and their taxonomic value. Am. ( N = lo), respectively. Mus. Novit. No. 140. Major differences exist between the Ramphocaenus- PAYNTER, R. A., JR. 1964. Subfamily Polioptilinae, Microbates group and Polioptila in the form, materials, pp. 443-455. In E. Mayr and R. A. Paynter, Jr. and placement of their nests. The nests of the above- [eds.], Check-list of the birds of the world. Vol. mentioned species in the WFVZ collection 10. Mus. Comp. Zool., Cambridge, Mass. are all neat compact cups, resembling oversized hum- PINTO, 0. 1953. SBbre a colecgo Carolos EstevLo de mingbird nests. All are composed of soft, fine vege- peles, ninhos e ovos das aves de Be&m (Pars). table fibers and are decorated on their exterior sur- Pap. Avulsos DeDartam. Zooloeia SBo Paulo XI faces with lichens, the whole mass being held together (13):201. - by cobwebs. In contrast, neither Ramphocaenus nor RAND, A. L., AND M. A. TRAYLOR. 1953. The sys- Microbates appear to decorate the exterior of their tematic position of the genera Ramphocaenus and bulky nests with lichens, and there is only one report Microbates. Auk 70:334-337. of Ramphocaenus utilizing cobwebs as g cementing RIDGWAY, R. 1911. The birds of North and Middle material ( Skutch 1960). The available data indicate America. Pt. V. Bull. U.S. Natl. Mus. No. 50. that Ramphocaenus and Microbates nests are typically RUSSELL, S. M. 1964. A distributional studv of the placed very near the ground and suspended between birds of British Honduras. Ornithol. Monogr. 1. upright plant stems. An inspection of over 200 data SICK, H. 1954. Zur Kenntnis von Ramphocaenus slips pertaining to Polioptilu egg sets in the WFVZ ( Sylviidae) and Psiloramphus ( Formicariidae) . collection indicated that gnatcatcher nests are typi- Bonner Zool. Beitrsge 5:179-190. cally placed much higher off the ground and situated SKUTCH, A. 1960. Life histories of Central American in the forks of woody branches, or placed on their birds. Pt. II. Pacific Coast Avifauna. No. 34. flat surfaces. SKUTCH, A. 1968. The nesting of some Venezuelan Rand and Traylor (1953) analyzed the external birds. Condor 70366-82. characters of Microbates, Ramphocaenus, Polioptila, WETMORE, A. 1943. The birds of southern Veracruz, and a genus of African sylviids, Macrosphenus. Con- Mexico. Proc. U.S. Natl. Mus. 93:215-340. cluding that Ramphocaenus and Microbates are far more similar to Macrosphenus than to Polioptilu, they Western Foundation of Vertebrate Zoology, 1100 suggested that there may have been two intrusions of Glendon Avenue, Los Angeles, California 90024. Ac- Old World warblers into the neotropics, one resulting cepted for publication 21 April 1977.