Tres Marias Research Articles

Late Breeding on the Tres Marías Islands

There is evidence that some passerine bird species breed later on the Tres Marias Islands than on the mainland of Mexico at the same latitude (Grant, 1964). Observations on breeding activities were made on both mainland and islands in the years 1961-1963, and it was concluded that equivalent phases of the breeding season of eight species are reached from three to eight weeks later on the islands (modal value, 4 weeks) than on the mainland. These findings have been substantiated and enlarged by using other sources of data, namely references in the literature and the condition of the gonads of collected specimens (table 1). The results are as follows. Whereas at least 16 of the 19 passerine species breed later on the islands than on the mainland, according to available data, there is no clear indication that nonpasserines do so. Secondly, the breeding of most nonpasserines starts earlier in the year than does the breeding of passerines. Furthermore, at least four of the nonpasserine species have a longer breeding season than do the passerine species on both islands and mainland. The season of Amazilia rutila (Cinnamon Hummingbird) spans 10 of the 12 months of the year; and writing of Cyanthus latirostris (Broad-billed hummingbird), Schaldach (1963) refers to specimens being in breeding condition at all seasons of the year. The reasons for these differences are not obvious. The passerine species feed more extensively on insects than do the nonpasserines, a difference that might be correlated with the difference in time and duration of the breeding season if the pattern of available insect food is different than that of vegetable food. If this is so, the long breeding season of the predominantly nectar-feeding hummingbirds is perhaps related to a nearly year-round availability of flowers for food. Lack (1954) and several others stress the biological importance of the fact that the time at which most parent birds are feeding nestlings (maximum feeding activity) coincides approximately with the greatest available food supply during the breeding season. On the Tres Marias Islands the maximum feeding activity of the passerines follows the emergence of leaves on the deciduous trees. Leaf emergence is dependent upon rainfall. Perhaps, therefore, the difference in breeding season between mainland and island passerine birds is due to a different pattern of rainfall in the two regions. The rainy season starts in early to mid-June in both regions; by the end of June more than 150 millimeters of rain have fallen on the mainland at Puerto Vallarta in Jalisco and at San Blas and Tepic in Nayarit, whereas on the islands that quantity has not fallen until the beginning of August (see Contreras, 1942). Thus the breeding seasons may be related to the time of maximum food supply, which is governed by leaf emergence and in turn by rainfall. Ennion and Ennion (1962) have adopted a similar explanation to account for the unusual fact that on Tenerife (Canary Islands) some terrestrial bird species regularly breed two months in advance of the normal breeding season on that island. Any future assessment of the significance of late breeding on the Tres Marias Islands should take into account the unusual fat deposition in several island forms, which is not exhibited by their mainland counterparts (Grant, 1965). Fat is accumulated in March and April, two to three months before the advent of the rains and the onset of passerine breeding. It is used just prior to and in the early stages of breeding. There is scant evidence with which to interpret the significance of the fat deposition. It might be related to the lateness of breeding, and the following sugges-

Middle America Trench: Topography and Structure

From 1952 to 1959, during nine expeditions of the Scripps Institution of Oceanography and one of the U. S. Navy Electronics Laboratory, research vessels recorded 31,950 miles of echo-sounding traverses in and adjacent to the Middle America Trench, which extends from the Islas Tres Marias off western Mexico to the Cocos Ridge southwest of Costa Rica. The Middle America Trench is continuous at depths greater than 2400 fathoms (4400 m) for 1260 miles, except off Manzanillo and Zihuatanejo, Mexico, where submarine mountains lie in the trench. It is deeper than 3000 fathoms (5500 m) for 380 miles as the Guatemala Deep. Northwest of Acapulco it is generally U-shaped in cross section, with a steeper shoreward flank and a flat bottom suggesting sedimentary fill. From Acapulco southeast to the west side of the Gulf of Tehuantepec, the trench shoals, in a series of basins, to 2700 fathoms (5000 m). To the southeast it widens and deepens abruptly to a maximum 3500 fathoms (6400 m) off western Guatemala, then shoals gradually to merge into the sea floor off Costa Rica. The southeast segment is also asymmetrical in cross section but is V-shaped with irregular bottom. A northeast-trending band of ridge-and-trough topography, 60 miles wide, separates the 1800- to 1900-fathom sea floor outside the trench off southern Mexico from the 2100- to 2200-fathom Guatemala Basin. This zone has been traced from several hundred miles offshore to an intersection with the trench near the west side of the Gulf of Tehuantepec. Seismic-refraction studies reported in an accompanying paper (Shor and Fisher, 1961) were employed in determining the trench structure. Three refraction stations were taken along the axis of the trench west of Acapulco and two along its axis off Guatemala and El Salvador. Another station was shot on the shelf and one 60 miles seaward of the trench off Guatemala. Thick sediments were found in the Tres Marias Basin off Manzanillo and at the shelf station off Guatemala. Arrivals from rock with compressional wave velocity of 4–6 km/sec were observed at the Tres Marias Basin and Guatemala shelf stations. Off Guatemala, on a section normal to the trench, the depth below sea level to the M discontinuity is interpreted from these seismic data as about 9 km (Pacific Basin), 10 km (outer ridge), 16 km (trench), and 17 km (shelf). Below the sea floor the crust thickens from 5–7 to 10–17 km along this section. The M discontinuity is deeper and the crust below the sediments thicker under the two southern stations than under the two central trench stations. The mantle is deeper under the Tres Marias Basin, where thick (1½ km) sediments are found, than under the central stations. The Gulf of Tehuantepec marks a major change in trench configuration and possibly in age. Northwest of Tehuantepec the flat trench bottom developed in most places suggests a greater age. Southeast of the gulf the deep V-shaped trench, with thicker crustal layers but very little fill, borders a volcanically active coast. The zone of ridge-and-trough topography trending southwest from Tehuantepec may be another evidence of this boundary.

A New Clingfish of the Genus Gobiesox from the Tres Marias Islands

FOWLER, H. W. 1936. Marine fishes of West Africa. Bull. Amer. Mus. Nat. Hist., 70:1493 PP. GREY, M. 1953. Fishes of the Family Gempylidae, with records of Nesiarchus and Epinnula from the western Atlantic and descriptions of two new subspecies of Epinnula orientalis. COPEIA, 1953:135-141. . 1959. Deep sea fishes from the Gulf of Mexico, with the description of a new species, Squalogadus intermedius (Macrouroididae). Fieldiana, Zool., 39:323-346, figs. 53-57. JORDAN, D. S. 1923. A classification of fishes. Stanf. Univ. Publ., Univ. Ser., Biol. Sci., 3(2): 1-243. MATSUBARA, K. AND IWAI, T. 1952. Studies on some Japanese fishes of the family Gempylidae. Pacific Sci., 6:193-212, figs. 1-12. . 1958. Anatomy and relationships of the Japanese fishes of the family Gempylidae. Mem. Coll. Agric. Kyoto Univ., Fish. Ser., Spec. No., pp. 22-54, figs. 1-14. ROULE, L. 1922. Description de Scombrolabrax heterolepis nov. gen. nov. sp., poisson abyssal nouveau de l'Ile de Madere. Bull. Inst. Oceanogr. Monaco, 408:1-8, figs. 1-4. 1929. Consid6rations sur la nature teratologique probable de quelques formes de poissons abyssaux. 10 Cong. Int. Zool., Budapest, 1927, 1: 795-798. SMITH, J. L. B. 1937. New records of South African fishes. Ann. Natal Mus., 8:167-198, pl. 11, text figs. 1-6.

Notes on the Freshwater Fishes of Sonora with an Addition to the Known Fauna

miculatus from Cocos Island which is a cognate of T. myersi found, so far, only on the northern coast of Mexico. Thus, it can be seen that the distributional relationship between Gobiesox marijeanae and G. schultzi fits and reinforces a pattern already demonstrated by certain other pairs of species in the same family. As far as clingfishes are concerned, the Tres Marias Islands had previously been considered too close to the mainland shore for

A New Snake of the Genus Drymarchon from the Tres Marias Islands

THE revision of the Mexican forms of Drymarchon by Hobart Smith (1.941) indicates that a large Drymarchon collected by the writer in the Tres Marias in 1940 represents an undescribed form. Dr. Smith notes that a Tres Marias specimen (referred to rubidus) had a greater number of ventrals and caudals than any mainland examples available to him, and he suggests that if further specimens from the islands consistently have high counts, they should be recognizable as belonging to a different subspecies. The scale counts given by Smith for his snake, those of two Tres Marias specimens recorded by Boulenger (1894: 32), and those of the example at hand, make possible a comparison with the mainland snakes reported by Smith. It seems certain that the Tres Marias snakes form a distinctive population, which is here named.

The Identity of Forpus cyanopygius (Souancé)

OVER forty years ago, Salvin and Godman, in the 'Aves' section of their 'Biologia Centrali-Americana' (3: 581, 1897), stated very plainly that Souanc6's cotypes of Psittacula cyanopygia in the British Museum were similar to their specimens from the Tres Marias Islands. Beyond this comment they took no action for the reason that they were somewhat over-conservative and rigorously refrained from recognizing geographic variations by name. From that time onward, no one seems to have given any weight to their comment and the Tres Marias race has continued to pass under the name of insularis Ridgway. During a recent visit to the British Museum, the writer took occasion to investigate the matter and found the comment of Salvin and Godman to be justified. The male cotype, the chief basis of the description, is certainly a Tres Marias specimen. The female is, in all characters, an individual intermediate and may have come from almost anywhere on the adjacent mainland. The measurements of the male (59.11.22.38 British Museum) are: wing, 90 mm.; tail, 45; culmen from cere, 13.5; outer anterior toe, 13.5. The coloration is definitely of the darker, duller, and more bluish type which is characteristic of the Tres Marias race. Under these circumstances there is no alternative but to include Psittacula rnsularis Ridgway in the synonymy of Psittacula cyanopygia Souanc6 and to rename the mainland race which formerly passed as cyanopygia. Therefore:

Ornithological Notes from Western Mexico and the Tres Marias and Isabella Islands

Ornithological Notes from Western Mexico and the Tres Marias and Isabella Islands

The Sailing Flight of the Turkey-Buzzard

The central plain of Mexico is at a considerable elevation, ranging from 3,000 ft. at the frontier to 7,300 in the immediate neighbourhood of the City of Mexico. The air is, consequently, considerably rarefied, and one would suppose that birds would find it difficult to practise sailing and circling flight; but that does not seem to be the case, as, at any time during the day, numbers of turkey-buzzards can be seen sailing or floating and circling with only a rare flap of wing; and, as a proof that rarety of atmosphere does not seem to diminish their sailing powers, I have seen at Tres Marias, a station on the Cuemavaca line, as it crosses the hills bounding the southern limit of the plain, at an elevation of over 10,000, these birds sailing equally gracefully and without any appearance of labour. There can be little doubt that the air at this elevation has less supporting power, and, therefore, it must be compensated for by the greater speed if the upward current, and shows how wonderfully adapted the wing of the turkey-buzzard is for sailing flight, and that at a lower elevation the wing could support more than the weight of the bird.

Natural History of the Tres Marias Islands, Mexico E. W. Nelson

Natural History of the Tres Marias Islands, Mexico E. W. Nelson

Nelson on the Birds of the Tres Marias Natural History of the Tres Marias Islands, Mexico. General Account of the Islands, with Reports on Mammals and Birds E. W. Nelson

Nelson on the Birds of the Tres Marias Natural History of the Tres Marias Islands, Mexico. General Account of the Islands, with Reports on Mammals and Birds E. W. Nelson

Nelson on New Birds from Mexico Descriptions of New Birds from the Tres Marias Islands, Western Mexico E. W. Nelson Descriptions of New Birds from Mexico, with a Revision of the genus Dactylortyx E. W. Nelson

Nelson on New Birds from Mexico Descriptions of New Birds from the Tres Marias Islands, Western Mexico E. W. Nelson Descriptions of New Birds from Mexico, with a Revision of the genus Dactylortyx E. W. Nelson

The shells of the Tres Marias and other localities along the shores of Lower California and the Gulf of California

The shells of the Tres Marias and other localities along the shores of Lower California and the Gulf of California