My three prior trips to Costa Rica have all been mid-winter, generally between late January and mid February; our trip in April meant that some of the birds making altitudinal movements would be in different places.
Brown Violet-ear was an apparent example. In 3 trips I've never seen this species, though a couple people on one of the trips glimpsed one at Rancho Naturalista. In spring, quite to my surprise, this short-billed bird was just about the most common hummer at Arenal.
White-throated Mountain-gem, Fiery-throated Hummingbird, or even Green Violet-ear. That being said, the bright beard and ears on a plain bird are fairly unique