Southern mouse-colored tyrannulet

Southern mouse-colored tyrannulet
At Piraju, São Paulo State, Brazil
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Passeriformes
Family:	Tyrannidae
Genus:	Nesotriccus
Species:	N. murinus
Binomial name
Nesotriccus murinus (Spix, 1825) [1]
Synonyms
Platyrhynchus murinus (protonym) Phaeomyias murina

The southern mouse-colored tyrannulet (Nesotriccus murinus) is a species of bird in subfamily Elaeniinae of family Tyrannidae, the tyrant flycatchers.^[2] It is found in Argentina, Bolivia, Brazil, Paraguay, Peru, and possibly French Guiana and Suriname.^[2][3]

The southern mouse-colored tyrannulet and the northern mouse-colored tyrannulet (N. incomtus) were previously treated as a single species, the mouse-colored tyrannulet. The unsplit species bore the binomial Phaeomyias murina. Genetic analysis showed that Phaeomyias was embedded within Nesotriccus and by the principle of priority, beginning in 2018 most taxonomists moved the species to Nesotriccus.^[4] Nesotriccus murinus sensu lato also included what are now the tumbesian tyrannulet (N. tumbezanus) and Maranon tyrannulet (N. maranonicus).^[5]

Beyond the reassignment to genus Nesotriccus and the split from it of the tumbesian and Maranon tyrannulets, the mouse-colored tyrannulet's taxonomy is unsettled. The International Ornithological Committee (IOC) and the Clements taxonomy treat the northern and southern taxa as separate species. Each has two subspecies; those of the southern are the nominate N. m. murinus (Spix, 1825) and N. m. wagae (Taczanowski, 1884).^[2][3] As of late 2024 the North American and South American Classification Committees of the American Ornithological Society and BirdLife International's Handbook of the Birds of the World (HBW) had not recognized the split, retaining the mouse-colored tyrannulet as a single species with four subspecies. In addition, HBW retains it as Phaeomyias murina.^[6][7][8]

This article follows the IOC/Clements model.

The southern mouse-colored tyrannulet is 10.5 to 12.5 cm (4.1 to 4.9 in) long and weighs about 7 to 12 g (0.25 to 0.42 oz). The sexes have the same plumage. Adults of the nominate subspecies have an olive-brown to gray-brown crown, a wide but ill-defined whitish to creamy supercilium, and a thin whitish eye ring on a face that is otherwise pale grayish white to white. Their upperparts are olive-brown to gray-brown. Their wings are dusky with wide whitish to dull cinnamon edges on the innermost flight feathers and tips on the coverts. Their tail is dusky. Their throat is pale grayish white to white. The center of their breast is whitish and the sides grayish, both with a pale olive wash. Their belly and undertail coverts are pale yellow. Juveniles are like adults. Subspecies N. m. wagae has less white on the throat, a darker green shade to its upperparts, and more intense yellow underparts than the nominate. Both sexes of both subspecies have a brown iris, a thick, rounded, horn-colored bill with pale pink at the base of the mandible, and gray legs and feet.^{[5][9][10][11][excessive citations]}

The southern mouse-colored tyrannulet's distribution is not clear. The IOC places the nominate subspecies in the Guianas, in central, eastern, and southern Brazil, in eastern and southern Bolivia, and in Paraguay.^[2] Clements places it similarly but adds northwestern Argentina and instead of including the Guianas says its "northern distributional limit not certain, may occur farther north in eastern South America".^[3] Cornell University's Birds of the World places it in Brazil, Bolivia, and Paraguay like the other two but not in the Guianas, and also includes northwestern and northeastern Argentina.^[5] The sources also differ on the ranges of subspecies N. m. wagae. The IOC places it in eastern Peru, western Amazonian Brazil, northwestern Bolivia, and northwestern Argentina. Clements places it in Peru, Brazil, and Bolivia like the IOC but not in Argentina. Birds of the World also places it in Peru, Brazil, and Bolivia like the other two but also in the Guianas and not in Argentina.^[2][3][5]

The southern mouse-colored tyrannulet inhabits a variety of open to semi-open landscapes, most of which are arid to only moderately humid. These include lowland and lower montane scrublands (especially those with Acacia and other loosely foliaged trees), cactus- and thorn scrub, cerrado, deciduous woodlands, gallery forest, young secondary forest, mangroves, parks, and gardens. In elevation it overall ranges from sea level up to 2,400 m (7,900 ft) but mostly occurs below 1,000 m (3,300 ft) and reaches only 500 m (1,600 ft) in Peru.^{[5][9][10][11][excessive citations]}

The southern mouse-colored tyrannulet is found year-round in the Amazon Basin. In northern Bolivia it migrates from higher to lower elevation after the breeding season. The populations in southern Brazil, southern Bolivia, Argentina, and Paraguay move north after breeding, apparently into the Amazon Basin.^[5][11]

Most of the data on the southern mouse-colored tyrannulet's diet and foraging behavior are from studies of the pre-split mouse-colored tyrannulet, and it is not certain how much of it applies to this species. As best is known, it primarily feeds on insects though fruits of mistletoe (Loranthaceae) and some other plants are a significant part of its diet. It usually forages singly or in pairs and only occasionally joins mixed-species feeding flocks. It tends to feed in dense foliage between about 2 and 8 m (7 and 26 ft) above the ground. It usually takes food by hover-gleaning and jumping up from a perch but also feeds by gleaning while perched.^[5]

The southern mouse-colored tyrannulet's breeding season varies geographically but is not fully understood. It appears to breed between October and December in Argentina and eastern Amazonian Brazil, between December and April in northeastern Brazil, and in at least January and February in eastern Bolivia. As with the data on feeding, breeding data are from the mouse-colored tyrannulet; as far as is known the data apply to both the northern and southern species. The female alone builds the nest, an open cup of plant fibers and roots, grasses, mosses, and spider web with feathers in the structure and as a lining. It is typically placed in a branch fork or tree crotch within about 4 m (13 ft) of the ground, though occasionally higher. The clutch size is two eggs; the female alone incubates. The incubation period is 14 to 17 days and fledging occurs about 17 days after hatch. Both parents provision nestlings.^[5]

The southern mouse-colored tyrannulet's dawn song is a "short husky phrase of some 3‒5 notes which is repeated for a long period rwee-chee-chew....tsee-rwee-chee-chew.....rwee-chee-chew...". Its day song is a "rising series of buzzy notes that increase in amplitude till the last few notes, which are variable in pitch and amplitude chu-chu-chu-chu-chu-chee!-chu-chew". It also makes a nasal "tjew-tjew-tju.....tjew-tjew-tee-tju...." call, "fainter short buzzy notes", and "emphatic pip notes".^[5]

The IUCN follows HBW taxonomy and so has not separately assessed the northern and southern mouse-colored tyrannulets.^[12] The southern mouse-colored tyrannulet is considered overall uncommon to common but locally rare, at least in Paraguay. The species is "[a]ble to thrive in wide variety of dry and moist habitats [and] accepts secondary and converted habitats...and is "[u]nlikely to be at any risk in the near future".^[5]

• "Mouse-colored tyrannulet" photo gallery VIREO