I wonder if bird could be etymologically related to the Slovene word brdo, which means "hill, small mountain". According to Marko Snoj's Slovene Etymological Dictionary, this word comes from the Old Slavic *bъ"do, meaning also "comb" and more specifically, "weaver's reed" (a comblike tool for keeping the threads separated; this meaning has been retained in the Czech brdo and the Russian бёрдо / byordo. Snoj suggests that the original meaning was "something sharp, a sharp tool, or sharp rocks". He points out that in Latvian, birde means "loom" (the weaving machine), and birds means "weaver's reed". Strangely, he does not take the word back to its IE root, but he suggests that it is related to the Old English bord, meaning "board". I am wondering if the notion of "sharpness" could be behind the English "bird", as in the sharpness of the beak. Just a thought (and another, probably gratuitous, Slovene interpolation from me).
Actually, learning that this is Esperanto for bird has elevated that language in my estimation. A proper language ought to have kinks - even a designed one. Otherwise we might as well all just talk in algol or cobol or fortran.
Amazing. I just checked OED too, and here's what it says: "ME. byrd, bryd: OE. brid masc. (pl. briddas), in Northumbrian bird, birdas ‘offspring, young,’ but used only of the young of birds. There is no corresponding form in any other Teutonic lang., and the etymology is unknown. If native Teut., it would represent an original *bridjo-z: this cannot be derived from BROOD, BREED, and even the suggestion that it may be formed like these from the root *bru- (see BROOD) appears to be quite inadmissible."