No, it's not a warbler. Shape and color are good for a gnatcatcher, but it looks more like a White-lored Gnatcatcher. Are you sure that photo was taken in Canada?
As I said, shape and color makes it not a warbler. It's not a vireo either as is lacks a hooked bill and blue feet.
Chris, do you happen to have any more photos of this bird?
It doesn't look like a Tennessee Warbler's bill to me.
Absolutely taken in Canada. Have been in Costa Rica many times but never in April. Thanks Etudiant and The Bird Nuts.
You're absolutely right we left Costa Rica on March 30. I must have had a couple of pics still on the card. Then that makes this a female White-lored Gnatcatcher. Thanks The Bird Nuts.
I noticed that the photo has full green leaves in it, so I checked the EXIF data. It says it was taken on March 29th, 2018. Were you in Costa Rica then?