“The structure of the iris of the Cinnamon Bittern (Ixobrychus cinnamomeus) has always fascinated me (above). It has a pigmentation posterior to the pupil and I have wondered about its function if any. Wells (1999) states ‘iris yellow with dark mark fore and aft of the pupil’.
“I have only observed clearly a mark posteriorly (consistent) and occasionally a faint mark anteriorly (not consistent). This is also present in the Black Bittern (Ixobrychus flavicollis) and Von Schrenck’s Bittern (Ixobrychus eurhythmus). Wells (1999), when describing the Von Schrenck’s Bittern, says ‘dark mark fore and aft of the pupil as in other Ixobrychus species’. But I have not been able to appreciate it in the commonly seen Yellow Bittern (Ixobrychus sinensis).
“In addition the pupil is more elliptical than circular (some describe is as ‘barrel-shaped’ or ‘bar-shaped’) – better seen when in bright light.
The image above shows that when the bird has the neck stretched out (‘elongated position’), dark feather between the neck and body can be seen. This is not seen well when in crouched (‘squat-position’) (below).
“Handbook of the Birds of the World (2019) says ‘tuft of loose black and dark brown feathers at base of neck’. The IUCN-SCC Heron Specialist Group states ‘a tuft of loose black and dark brown feathers on the side of and at the base of the neck is expansible for display’.
“The above shows the bittern in flight.
“All images of a male.”
Dato’ Dr Amar-Singh HSS
Ipoh, Perak, Malaysia
3rd December 2019
Location: Perak, Malaysia
Habitat: Wetlands, Padi Fields