Black-crowned Night Herons (Nycticorax nycticorax nycticorax) in Taiwan were very easy to watch at close range (using the car as hide). This prompted me to try and take many images of individuals of different ages. Mu-Chi Hsiao, Cheng-Lin Li (2017) (A Field Guide to the Birds of Taiwan. Wild Bird Society of Taipei) and The IUCN-SCC Heron Specialist Group (Heron Conservation: Black-crowned Night-Heron; available here: https://www.heronconservation.org/herons-of-the-world/list-of-herons/black-crowned-night-heron) also offered some information on aging.
Older immature or juvenile.
I lacked some necessary images and have added a few from my observations, over the years, in Perak, Malaysia. I am generally reluctant to add birds in a composite from different regions but the subspecies is the same and widespread and there is no evidence of a markedly different phenotypes. I however encountered an interesting problem; many of the immature birds I encountered/imaged locally were in breeding plumage. When examining all my images from Taiwan birds I also spotted an immature bird in some breeding plumage. So I will offer what I have with come caveats about breeding plumage. In addition, note that birds are in different lighting and stance.
Non breeding adult.
I am using Keith Vinicombe’s (2014) excellent work for aging birds in general (The Helm Guide to Bird Identification. Bloomsbury Publishing PLC). I am also referring to age classification of birds by Krys Kazmierczak, OBI Editor. Keith Vinicombe generally divides birds into three groups of ages/plumage: juveniles, immature and adult but goes on to say “it is essential to clarify one particular area of confusion. Any bird to that is not mature (i.e. not an adult) is an ‘immature’. Literally ‘immature’ simply means ‘not mature’. Thus, a juvenile bird is an immature, as is a first-winter, a second-summer and so on. The term ‘juvenile’ should be confined to a bird’s very first plumage worn for a short time after leaving the nest.”
Composite image – 1 Cropped heads of six birds (above).
Composite 1 shows cropped head images of 6 birds in age progression. Composite 2 repeats the same birds, showing the whole body (below). The IUCN-SCC Heron Specialist Group suggests it takes 2-3 years to acquire adult plumage.
No. 1 is a young juvenile (Hsiao 2017 calls this ‘juvenile, 1st year’) – the brown plumage has a lot of white spots, the iris is yellow or orange-yellow (not shown well in image but observed), the lores are yellow-green, the upper bill is black (with yellow at sides) and lower mandible yellow with black at the tip.
No. 2 is an older juvenile or immature (Hsiao 2017 calls this ‘immature, 2nd year’) – the plumage has more brown with a reduction in size of white spots, the iris is orange, the lores are more green-yellow (darker), the upper bill is more black (with yellow at cutting edges) and lower mandible yellow-green with black at the tip.
No. 3 is a 3rd year immature that is becoming pale (losing the juvenile brown) and rapidly transforming into an adult. The iris have become red. The lores and bill in this bird are not indicative of the norm as this bird is unexpectedly in breeding plumage. The lores and bill should be like the non-breeding adult.
No. 4 is what I would call a Subadult (as Kazmierczak says “a bird which is almost fully adult but which still shows some, usually small, traces of immature plumage”). Keith Vinicombe does not generally favour this term and prefers using other appropriate descriptions like ‘3rd summer year’). The bird is almost like an adult but lack the full dark colours of the scalp and back. Note again that the lores and bill in this bird are not indicative of the norm as this bird is also in breeding plumage. The lores and bill should be like the non-breeding adult.
No. 5 is a non-breeding adult. The iris in red, the lores are a dirty green-yellow, the upper bill is black (with horn-yellow at cutting edges) and lower mandible horn-yellow with black more extensive at the tip, the white head plumes are short.
I seem to be differing from authorities, who say that the adult has a black bill and blue-grey or greenish-blue lores (Wells 1999, IUCN-SCC Heron Specialist Group, HBW 2019), I have tried hard to find non-breeding adults with completely black bills and that lore colour in my numerous local observations (images) and recent Taiwan images but am not able to do so.
No. 6 is a breeding adult. I have seen quite a number of breeding adults and the key changes at breeding include: a. deep blue lores (even the skin around the eye turns blue); this happens only at peak breeding/mating and they then revert to a slate-blue colour (Note that the IUCN-SCC Heron Specialist Group says that “during courtship the lores are black”). b. the beak becomes fully, glossy black c. the white head plumes become much longer and more visibly separate (especially male), the feet/legs become pink-red and the general black plumage become more glossy. I also believe the iris turns a darker, blood shot red and the birds develops a light cream front to the neck and breast.
Appreciate any differing views and observations.
Amar-Singh HSS (Dato’ Dr) – Ipoh, Perak, Malaysia
Location: Dayuan Township, Taoyuan City County, Taiwan
Habitat: Wetlands farming area near the sea
Date: 19th January 2019
Equipment: Nikon D500 SLR with Tamron SP 150-600mm f/5-6.3 Di VC USD, handheld with Rode VideoMic Pro Plus Shotgun Microphone