The Northern Beardless-Tyrannulet is one of the scarcest breeding species in Texas, being limited to parts of the Lower Rio Grande Valley and the South Texas oak forests just to the north of the Lower Rio Grande Valley. In the United States, the species nests also in southeastern Arizona and extreme southwestern New Mexico. It is fairly common in tropical and subtropical forest and scrub south through Mexico into Central America. This species lacks rictal bristles, which many flycatching species have around their mouths to aid in capturing flying insects. The tyrannulet is primarily a gleaner of slow-moving or stationary food items, such as scale insects, ants, and small caterpillars (Tenney 2000). In Texas, the Beardless-Tyrannulet’s breeding distribution and habits remained poorly known for many years, since its discovery in the Lower Rio Grande Valley in the 1870s and the first nesting studies (Oberholser 1974, Brush 1999, Brush 2005).
DISTRIBUTION. During the 1987-1992 field work seasons of the TBBA project, observers found one confirmed breeding site in latilong-quad 26098-B4 in the Lower Rio Grande Valley (Bentsen-Rio Grande Valley State Park) and two probable records in latilong-quad 26097-F6 and G7 in private ranches in the oak forest of the South Texas Sand Sheet (Kenedy County). Subsequently, researchers have also found nests in nearby areas of southern Hidalgo County, Santa Ana National Wildlife Refuge (NWR), Lower Rio Grande Valley NWR, and Anzalduas County Park (Brush 1999, Werner 2004). The species is rarely seen upriver in Starr County, where epiphyte-laden forests do not occur. A few birds have been reported in the Trans-Pecos region (western Texas), but the species’ status there is poorly known (Lockwood and Freeman 2004).
SEASONAL OCCURRENCE. Northern Beardless-Tyrannulets are permanent residents in Texas, at least in the Lower Rio Grande Valley. They are regularly seen in winter at the main breeding sites, especially in frequently-birded Bentsen-Rio Grande Valley State Park and Anzalduas County Park. Birds also winter at Santa Ana NWR but are less regularly detected in that larger tract. Tyrannulets also occur at spots where they do not apparently breed, such as Laguna Atascosa NWR and along the Arroyo Colorado in Harlingen (Cameron County). Those birds may be breeders in oak-mesquite forests. The wintering grounds of these tyrannulets in the Lower Rio Grande Valley appear to be in milder, moister habitats similar to those of the Tropical Parula [Parula pitiayumi] that show the same general distribution pattern and movements.
BREEDING HABITAT. Tyrannulets breed in two distinct habitats in Texas: riparian forest in clay soils of the Lower Rio Grande Valley and the live oak (Quercus virginiana)-dominated forest in sandy soil of Kenedy County. They forage both in canopy trees such as live oak, cedar elm (Ulmus crassifolia), sugar hackberry (Celtis laevigata) and also in understory shrubs and trees such as honey mesquite (Prosopis glandulosa and huisache (Acacia minuata). In the Lower Rio Grande Valley (and probably also in Kenedy County). Tyrannulets build nests almost exclusively in clumps of ball-moss (Tillandsia recurvata) and Spanish moss (T. usneoides). Limited information suggests clutch sizes of 2-3 eggs may be most common. Incubation and nestling periods are 14.0 and 18.5 days, respectively (Werner 2004). Nests may fail due to predation or heavy rain and cold weather (Werner 2004, Brush 2005). Northern Beardless-Tyrannulets appear to be limited to habitats containing large amounts of ball-moss or Spanish moss (Brush 1999, Werner 2004). Tyrannulets’ absence as breeders from Starr County riparian forests may be due to the lack of epiphytes.
STATUS. The lack of attention to this species and its limited Texas range make determining its conservation status difficult. It has been shown to be somewhat more numerous than expected (Lockwood and Freeman 2004). But the number of territories is small even in the largest Lower Rio Grande Valley tracts (3-5 territories at Bentsen-Rio Grande Valley State Park and 4-5 at Santa Ana NWR in 2002-2003; Werner 2004). These low numbers, plus the death of tall, epiphyte-laden trees in some parts of these parks suggest the species is vulnerable in the Lower Rio Grande Valley. As thorn forest continues to replace riparian forest in the Lower Rio Grande Valley, tyrannulets may be able to hang on, if ball-moss and Spanish moss can establish themselves on thorn-forest trees (Brush 2005). Less is known about Kenedy County populations, where the density may be lower, but there is much more potentially suitable habitat and more habitat stability.
Text by Tim Brush (2009)
Lockwood, M. W. and B. Freeman. 2004. The TOS handbook of Texas birds. Texas A&M University Press, College Station.
Oberholser, H. C. 1974. The bird life of Texas. University of Texas Press, Austin.
Tenney, C. R. 2000. Northern Beardless-Tyrannulet (Camptostoma imberbe). In The Birds of North America, No. 519 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.
Werner, S. M. 2004. Breeding biology and habitat associations of the Altamira Oriole and Northern Beardless-Tyrannulet in the Lower Rio Grande Valley, Texas. M.S. thesis, Texas A&M University, College Station.