Northern mouse-colored tyrannulet

Northern mouse-colored tyrannulet
In Armenia, Colombia
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Passeriformes
Family:	Tyrannidae
Genus:	Nesotriccus
Species:	N. incomtus
Binomial name
Nesotriccus incomtus (Cabanis & Heine, 1860)
Synonyms
Phaeomyias murina incomta

The northern mouse-colored tyrannulet (Nesotriccus incomtus) is a species of bird in subfamily Elaeniinae of family Tyrannidae, the tyrant flycatchers.^[1] It is found in Colombia, Costa Rica, Ecuador, Guyana, Panama, Trinidad, Venezuela, and possibly Brazil, French Guiana, and Suriname.^[1][2]

The northern mouse-colored tyrannulet and the southern mouse-colored tyrannulet (N. murinus) 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.^[3] Neotriccus murinus sensu lato also included what are now the tumbesian tyrannulet (N. tumbezanus) and Maranon tyrannulet (N. maranonicus).^[4]

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 northern are the nominate N. i. incomtus (Cabanis & Heine, 1860) and N. i. eremonomus (Wetmore, 1953).^[1][2] 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.^[5][6][7]

This article follows the IOC/Clements model.

The northern mouse-colored tyrannulet is 12 to 13 cm (4.7 to 5.1 in) long. The sexes have the same plumage. Adults of the nominate subspecies have a 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 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. i. eremonomus has dull buff edges on the wing coverts but is otherwise like 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.^{[4][8][9][10][11][12][excessive citations]}

The northern mouse-colored tyrannulet has a disjunct distribution. Subspecies N. i. eremonomus is the more northerly of the two. It is found from the Pacific lowlands of Costa Rica's San José and Puntarenas provinces south into Panama as far as Panamá Province.^{[1][2][4][8][9][excessive citations]} Sources differ on the full range of the nominate subspecies. The IOC places it in Colombia, northeastern Ecuador, Venezuela, Trinidad, and Guyana.^[1] Clements and Cornell University's Birds of the World add Suriname, French Guiana, and northern Brazil, where the IOC places the southern mouse-colored tyrannulet. In addition, Clements does not include Ecuador.^[1][2][4] Sources agree that it occurs in northern Colombia and all three ranges of the Colombian Andes and in most of Venezuela except southern Amazonas state.^[4][10][12]

The northern 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, deciduous woodlands, gallery forest, young secondary forest, parks, and gardens. In elevation it ranges up to 1,200 m (3,900 ft) in Colombia, to 600 m (2,000 ft) in Ecuador, to 1,900 m (6,200 ft) north of the Orinoco River in Venezuela and to 950 m (3,100 ft) south of it there.^{[4][8][9][10][11][12][13][excessive citations]}

The northern mouse-colored tyrannulet is a year-round resident throughout its range.^[4][13]

Most of the data on the northern 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.^[4][11][12]

The northern mouse-colored tyrannulet's breeding season varies geographically but is not fully understood. It appears to breed between March and June in Panama, between May and October in Colombia, and between May and July in Venezuela. 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.^[4][12]

The northern mouse-colored tyrannulet's dawn song is "a long, energetic, almost bubbly tu-tu-tu-tu-tu...tu-tu-Tu-Te-Teep!" that continues with no breaks for several minutes and "gradually changes to [a] more complex tu-tu-tu-Te-Tee-Tr'trp!" whose last part is louder than the rest.^[12] Its day song is a "rising series of notes that increase in amplitude till the last but one note, the latter again lower in pitch and amplitude tu-tu-tu-tu-tu-tee!-tu...repeated a few times with intervals of several seconds". It also makes a "t-trrrrt" rattle and a "tee!-tep!" call.^[4]

The IUCN follows HBW taxonomy and so has not separately assessed the northern and southern mouse-colored tyrannulets.^[14] The northern mouse-colored tyrannulet is "[a]ble to thrive in wide variety of dry and moist habitats [and] accepts secondary and converted habitats". It has expanded in Costa Rica starting in 1996 and appears to be increasing in Ecuador. It is found in many protected areas both public and private and is "[u]nlikely to be at any risk in the near future".^[4][11]