jocateme said:
At the beginning I thought it could be a juvenile Small-billed or White-crested Elaenia, but I couldn't see any photo of them. What do you think about the absence of the crest?
I think the seemingly complete absence of a crest almost certainly excludes Yellow-bellied & Plain-crested (yes, they can lower their crest to appear more round-headed, but I don't remember ever seeing one with
such a flat head). The remaining species mentioned often appear like this when it comes to crest (White-crested often appear slightly more crested, but can appear just as round-headed as Lesser, Olivaceous & Small-billed) and I don't see anything suggesting this being a juvenile. Also, the only Elaenias among the species mentioned in this thread that usually show such a distinct eye-ring are Small-billed and
chilensis White-crested - it's usually less distinct in Lesser, Olivaceous and Yellow-bellied (but as in so many other features in Elaenias, this isn't a completely certain feature, just basic statistics). The same goes for the white in the crest. Basically, Yellow-bellied (a species which, btw, also is larger than the other species discussed in this thread), White-crested, Small-billed and Lesser always have white in the crown. However, it is occasionally concealed in Yellow-bellied & White-crested and often concealed in Small-billed & Lesser (i.e. when seen normally it often doesn't seem as if they have any white in the crown). Olivaceous occasionally show a few white feathers in the crown, but most completely lack white (the last species mentioned in this thread, the Plain-crested, always lacks white completely). The short notes by José Fernando Pacheco on this page may be useful, although I've pretty much covered the various issues (for obvious reasons I presume you read Portuguese):
http://www.ib.usp.br/ceo/imag_sons/dica_elae.htm
Beware, however, that the three white-bellied group (Small-billed; E. parvirostris, White-crested; E. albiceps and Olivaceous; E. mesoleuca) often can show a slight yellow tinge to the lower flanks and vent. This is why I specifically mentioned the MID-belly in the earlier post (they never appear yellow there). Note also, that while the number of wing-bars isn't as good as suggested in some literature, it still works to some extend (i.e. when a third wing-bar is present it is most likely a Small-billed - it's just not completely certain as Olivaceous & White-crested with three wing-bars do occur occasionally). So, for above mentioned reasons I'd still put my money on Small-billed, but to be absolutely certain the voice is needed (a significant problem is, however, that White-crested only sings during the breeding season, i.e. it doesn't sing when in Brazil!). Arthur Grosset's page (
http://www.arthurgrosset.com/) has photos of several of the species mentioned in this post and there's a photo of a Small-billed in the BF database:
http://www.birdforum.net/bird_view.php?bid=7716