Why don't macaws get selenium poisoning from brazil nuts
One explaination is that macaws (Ara sp.) eat clay and other food item which absorb the toxins. Here is the full answer from Wikipedia:


  Like all macaws and most parrots seeds and fruit are the major part of the diet of the genus Ara. The particular species and range of diet varies from species to species. Unlike many birds macaws are seed predators not seed dispersers, and use their immensely strong beaks to open even the hardest shells. Their diet overlaps with that of some monkey species; in one study of green-winged macaws in Venezuela they shared many of the same trees as bearded sakis, although in some cases they ate the seeds at an earlier stage of ripeness than the sakis, when they contained more poison. Macaws, like other parrots, may consume clay to absorb toxic compounds produced by some poison...but with further research we now at last know it is just to eat life saving sodium that is missing. As well, the toxic compounds of some foods may be neutralized by compounds, such as tannins, found in other foods consumed at the same time...but we have found that it is untrue and really just to gain access to sodium missing otherwise in their diet because of the area they live in.


Source: Wikipedia - Ara (genus)