The rare out of place scarlet ibis drew the family outside. They could hear the call of this unusual bird that was so far away from its indigenous home. As far as how it got there, it may have been lost while migrating, the story doesn't really specify. They went outside and looked up into the tree only to see the bird call out and fall to its death. They observed the tiny bird with its twisted neck. The bird become significant later in the story when Doodle is crushed and looks, to his brother, like the poor dead ibis so out of place, twisted, and dead.
I believe that this bird, meant for the tropics from south Florida to South America, was probably driven off its migratory course by the hurricane mentioned as having driven through Old Woman Swamp that July, 1918. At least, scientifically, this is an explanation. However, that this bird, with its slender neck and legs and its "bleeding" breast, should find that moment to light in and then die beneath the narrator's bleeding tree, only days before Doodle himself begins to decline and then ultimately die, speaks more to that element of fantastic and symbolic imagery that makes Hurst's "Scarlet Ibis" so memorable. Doodle, in death, is a human likeness of the dead ibis, slender neck thrown back, slender legs crooked beneath him, his breast red with blood.