Observations on the distribution, behavior, and comparative breeding biology of Neoxolmis rufiventris (Aves, Tyrannidae). American Museum novitates ; no. 3220

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New York, NY : American Museum of Natural History
"As many as 93-94 individuals of the relatively uncommon and highly terrestrial Chocolate-vented Tyrant Neoxolmis rufiventris (Vieillot, 1823) (Aves: Tyrannidae: Fluvicolinae) were encountered at 70 lowland (60-920 m) sites in Santa Cruz, Argentina, and Magallanes (Region XII), Chile, during the austral spring of 1993. Sites were typically very open and included steppes with scattered cushion plants, grass steppes, and grass-cushion plant and grass-shrub associations. Most sites (70%) held solitary birds; relatively few contained 2 (27%), 3 (1%), or 4 (1%) individuals, some of which may have been paired. No nests were found. Most sightings were in the late morning and in the afternoon and early evening, suggesting a bimodal activity pattern. Foraging on or near the ground for small prey, faint vocalizations including alarm calls emitted by an apparent pair, intraspecific aerial pursuits of 5-20 m height and up to 10 sec duration, and presumptive aerial displays, were observed. The swift, direct flight of N. rufiventris resembles sympatric, predatory Falco sparverius (Falconidae) and Muscisaxicola species (Tyrannidae), a fact that may account for the mobbing reactions that the former was seen to elicit in Geositta cunicularia (Furnariidae) and M. capistrata. Evidence of nesting in Neoxolmis rufiventris previously consisted of reports of three nests from Río Negro (1) and Santa Cruz (2), Argentina. Six additional nests and egg sets from Río Negro (5) and Santa Cruz (1) are described here, based on collections made by J. R. Pemberton in 1911 and by A. T. Waldron in 1933. The nine known nests were situated either in Patagonian steppe (8) or in steppe habitat within Monte (1). At ca. 775-1600 m, the Río Negro sites are geographically and altitudinally disjunct from the Santa Cruz localities, which lie at or below 100 m. Breeding takes place from late October-late December and is partly synchronous in the two provinces, though relatively late at higher elevations in Río Negro. Partial coincidence of breeding phenology, presence of steppe habitat in intervening parts of Chubut, and probable commingling of migrants on wintering grounds in northern Argentina, Uruguay, and southern Brazil, suggest potential gene flow between the Río Negro and Santa Cruz populations. All of the known nests of the Chocolate-vented Tyrant were placed in relatively sheltered locations, either on flat or sloping ground next to a grass tussock, shrub, or rock or, in one instance, 2.4 m above ground inside a rock crevice. The nest is an open cup, usually of dry grass, some sticks, and a lining of feathers. The clutch comprises 1-3 (modally 2) ovoidal eggs with pink or cream ground color and reddish and gray spots near the larger pole. A comparative analysis of nest site and clutch size variation in Neoxolmis rufiventris and 27 related species of the Muscisaxicola group (sensu Lanyon, 1986) suggests that open-cup tree or shrub nests are primitive for this assemblage, and that ground-nesting habits and relatively small, two-egg clutches arose independently in Muscisaxicola and Neoxolmis via separate invasions of semiarid resource-poor biotopes, respectively in the Andes and in the cis-Andean rain shadow"--P. [1]-2.
32 p. : col. ill., 1 map ; 26 cm.
Includes bibliographical references (p. 23-27).