Taking thought from related well-known puzzle about burning rope, we will use

5 guavas

with none measuring device, and with out eyeballing how a lot the guava has been completed.

First, divide the parrots into 4 teams of three parrots every. Then take 3 guavas, and provides one every for the primary three teams. Then let every group of three parrots eat the guava on the similar time. In case (a) the group who completed first should comprise the particular parrot. In case (b) it is the one completed final. If all end on the similar time, it should be on the fourth group of three parrots.

Now we’ve a gaggle of three parrots which we all know should comprise the particular parrot. Similar to earlier step, take 2 guavas, let one parrot eat one every. If they end on the similar time, the particular parrot is the third one. Otherwise, we will know which parrot is determined by case (a) or case (b) based mostly on which one finishes first.

(This assumes one guava will be eaten on the similar time by a number of parrots, which, on the time of this writing, hasn’t been clarified within the query, but in addition not forbidden).

If consuming on the similar time is forbidden, then in step one we will merely rotate the guava within the group of three after it has eaten a couple of third (does not should be precise, the rotations do not even should be carried out on the similar time for all teams). The level is to let every parrot within the group have a few of the guava. If there may be any particular parrot within the group, the entire time to complete the guava shall be totally different. For instance, we will rotate the guava to the following parrot within the group each 5 seconds. As lengthy because it takes greater than 10s for any parrot to complete a guava, this can work.

Additional notice: upperbound for the reply can be 12 guavas, since we will simply give one every, and see which one finishes in several time. So any reply must be lower than 12.