You have come to ask to the best place. I happen to have co-made an edition of the Summary of the General and Natural History of the Indies, by Gonzalo Fernández de Oviedo, the Pliny of the New World. The bird was absolutely astonishing to grandpa Oviedo, but then again so were most of the animals he encountered, that is what impelled him to write the "Natural History of the Indies": a New World was in need of explaining in the same fashion as Pliny had done with the old one. The little birdie was called "mosquito bird" by captain Oviedo, as the bird is tiny and noisy. I will give you the full translation, as it is quite enjoyable:

Chapter XLVII. Mosquito bird

There are some little birds so tiny that their whole body is smaller than the size of a thumb's tip, and when skinned it is smaller by half. It is a little bird that besides its smallness has so much speed and quickness in its flight, that when seeing it in the air its wings cannot be considered something other but those of the beetles or bumblebees, and there is not a single person that seeing it does not at first think it a bumblebee. The nests are in accordance to its size. I have seen one of these birds that the animal and the nest together put in gold-weighing scale did not weigh above two tomines, that is XXIIIJ grains complete with its feathers, for if it had been skinned, the weigh would have been far smaller.

In the subtlety of its legs and hands it resembled the little birds that the illuminators tend to put on the margins of the books of hours. Its plumage is of very beautiful colours, golden, and green, and more. And the beak is as long as its body, and thin as a pin. They are very daring, and when they see a man climb a tree where it nests, it attacks him in the eyes, and with such speed it goes and comes that one has to see it to believe it. Such is the smallness of this bird that I would not dare speak of it, for without me there are other eye witnesses in His Majesty's court. These birds make their nests of cotton strands, of which there is plenty and it is very convenient to them

As you can see, Oviedo uses plenty of comparisons so that people can have a reasonable mental image of what he was describing. That is more or less the only way to expain something entirely foreign to someone, using concepts that he may be familiar with.