Google research has indicated macaws love or eat a lot of Brazil nut. These are highly toxic to humans in somewhat small quantities due to their selenium content.
Given the mass difference between a human and macaw, lets say 80kg and 2kg, if toxicity for humans 'starts' around 900micrograms or ~9 nuts, maths would indicate toxicity for a macaw might be around 20% of a nut?
These figures are all based on google and the margin of error I expect is quite large, nonetheless it seems quite disparate if a macaw could consume even a single nut a day on average, without consequence?