I was rather surprised to find this adult Tui with apparently only one functional eye at Western Springs, Auckland.
For me it raised some questions. How can a bird safely fly and defend itself from predators or competitors if it lacks 50% of its vision?
General birdwatching discussion, help with bird identification, and all other things relating to wild birds and birding in NZ that don't fit in one of the other forums.
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I can't remember which book exactly, but there was a bird book that I read which had a small passage in it about how physically disabled individual animals will adapt for survival. As an example, it mentioned an anecdote where a captive carnivorous fish (piranha?) went blind in one eye, and from then on, it would always swim in one direction so that the side of its body that its bad eye was on would always face the wall of the tank. By doing this, this fish was able to hide its weakness from the other fish of its kind, and avoid being cannibalised. In the case of this tūī, I think that lacking one eye has meant that it is more dependent on its hearing, so that it can continue to effectively compete with other tūī.
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On 16 Nov 2017 we captured and banded a male Bellbird at the Halswell Quarry in Christchurch. It was blind in its right eye. On 6 Dec 2019 the same bird was captured about 200m from the original site, except for the blind eye it appeared to be in good condition on both occasions. Other birds of various species that appear blind in an eye have been caught on several occasions.