An inhabitant of várzea and flooded, swampy forest, as well as more locally in flooded forest in sandy-soil areas, Yellow-crowned Elaenia Myiopagis flavivertex is comparatively widely distributed across much of northern and western Amazonia. It is known from north-east and southern Venezuela (including Monagas south to Delta Amacuro, and Amazonas), the Guianas, north-east Ecuador (Sucumbíos, Napo), north-east Peru (in eastern Loreto, Ucayali) and Brazil across the central Amazon Basin, but also south along the Madeira River to northern Rondônia (Fitzpatrick 2004, Ridgely & Tudor 2009). There are no published records in Colombia (Salaman et al. 2010, Donegan et al. 2016), although Hilty & Brown (1986) suggested that it might be found near San Fernando de Atabapo, which is just inside neighbouring Venezuela, and McMullan & Donegan (2014) illustrated the species given that its occurrence was expected. Recently, we have recorded the species three times in the extreme east of the country, in the environs of Puerto Inírida, eastern Guainía, thereby confirming its presence, precisely in the region predicted by Hilty & Brown (1986).

On 7 October 2014, in the environs of Caño Matraca, just upstream of Pueto Inírida on the left (west) bank of the Inírida River, GMK heard and briefly observed at least one M. flavivertex inside the canopy of lower-stature flooded várzea, together with N. Bostock et al. Unfortunately, at the time, they and the other observers present, including PF, were all focused on photographing and sound-recording a pair of Orinoco Softtails Thripophaga cherriei, and GMK did not capture any vocalisations in the background of his recordings. Subsequently, the observation was more or less forgotten about for almost two years— especially as GMK was unfamiliar with the species' (non-)status in Colombia but also because its presence in the extensive várzea forest around Puerto Inírida appeared so unsurprising. In mid-September 2016, PF sent GMK photographs and a sound-recording (XC345120;  www.xeno-canto.org) from the same region, which GMK immediately recognised as being of M. flavivertex. It was only then that GMK noticed that the species was not previously known to occur in Colombia and a little later came across his notes from 2014 again.

PF's observations were made on 12 September 2016, when a single was seen with a mixed-species flock (which this species does not routinely follow), in várzea forest beside the Inirida River, In addition to the recording, his photographs show the majority of the salient features associated with a Myiopagis elaenia and specifically M. flavivertex, other than the wingbars, namely: the small bill with pinkish base to mandible, dark eyes, rather flat crown with no crest, greyish-olive breast and throat merging into more yellowish belly, lack of an obvious pale superciliary except on the supraloral region, slightly long-tailed appearance, and yellow-fringed tertials and flight feathers. However, although the underparts pattern differs from that of the otherwise very similar Forest Elaenia M. gaimardii (which has a greyish-white throat and upper breast, with a hint of olive streaking), the latter species is best identified by its voice. The only Tyrannidae that might be confused with M. flavivertex by voice is Euler's Flycatcher Lathrotriccus euleri, but the latter is very different in plumage. While M. flavivertex is a specialist of várzea forest, whereas Forest Elaenia is typically found in dry-land forest habitats, although it also occurs in inundated habitats, at least in Suriname and Venezuela (P. Boesman in litt. 2017).

Subsequently, T. M. Donegan (in litt. 2016, 2017) alerted us to the fact that other observers have also reported M. flavivertex in the same area of Colombia, among them T. Friedel, who photographed one individual on 30 January 2016 ( http://www.birdphotos.com/photos/ index.php?q=gallery&g2_itemId=228164&g2_imageViewsIndex=1). Furthermore, during another visit to the area, on 16 February 2017, PF, GMK, G. R. R. Brito and J. Lobel heard, and GMK sound-recorded, the species in the, at this season, dry várzea forest along the trail to Caño Matraca (XC365007)

A recent ornithological inventory of the Puerto Inírida region recorded only Forest Elaenia there, listing this species as occurring in both terra firme and várzea forests (Stiles & Beckers 2016), which suggests that M. flavivertex might have been to some degree overlooked by previous workers, perhaps because it is easily missed even when vocalising. The species' discovery in easternmost Colombia is plainly unsurprising, given both the extensive availability of suitable habitat in this part of Guainía and the presence of M. flavivertex at several localities in immediately adjacent Venezuela (south-west Amazonas; Hilty 2003). Nevertheless, that it should have escaped previous notice, despite considerable recent survey effort around Puerto Inirida (Stiles & Beckers 2016), surely is somewhat remarkable and hints at the possibility of still-further ornithological discoveries in this, and many other regions of the country. Indeed, while the present manuscript was in production, Ramírez et al. (submitted) reported several records of M. flavivertex from the departments of Guaviare and Meta, indicating that its range in eastern Colombia is considerably broader than believed. Given records very close to the Colombian border inside Peru (e.g. XC16599;  http://www.hbw.com/ibc/photo/yellow-crowned-elaenia-myiopagis-flavivertex/yellow-crowned-elaenia-perched), the species can also be expected to occur around Leticia, in the extreme south of dpto. Amazonas.