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Identifying Mohua
Appearance – what do mohua look like?
Affectionately nick-named custard-heads by scientists, mohua adorn a beautiful splash of bright yellow across their head and breast while the rest of the body is brown with varying tinges of yellow and olive. The male has bright yellow head and under parts with contrasting black bill, eye and legs, with under parts yellowish brown. Males weigh around 30 grams. The female is slightly less brightly coloured and smaller than the male weighing just 25 grams. The female and juveniles looks similar with the crown and nape of the neck shaded brown. The average height of mohua is 15cm from the point of their bill to the tip of their tail.
  
The male adorns a bright yellow "custard-head" (left) while the juvenile mohua is duller brown colour (middle) and has a yellow gap around its bill (right). (Photo: J. van der Wetering)
The mohua are described eloquently in the 1955 field guide: Its plumage is well described in the name [yellowhead]; the cock bird particularly is a handsome fellow, the yellows between his great black beady eyes and immediately above his dark brown bill sometimes shading almost into orange. The plumage of the female is faintlier marked. The tail shafts of the old birds are often abraded and worn, the legs are black. Gripping hard with the feet, legs spread, the strong horny bill is vigorously dug into the close fibred south and east facing moss clumps, that in the rain-forests beloved of the bush canary, glue themselves onto trunks and branches. Like the Pukeko and Kaka, the claw is utilised for holding morsels of food."(Source: Oliver, W (1955) New Zealand Birds)
A spiny tail: As mohua spend a lot of time feeding on tree trunks or on the ground the tip of their tail is often worn to spine-like shafts. The shafts are stiffened specifically for this purpose which provides a prop for the body while feeding. When you hold a bird in the hand, the projection of the spines beyond the end of the tail feathers can tell you a how long it is since the bird last moulted its feathers.
  
Tail feathers are used extensively while feeding for balance as shown by this mohua feeding on a tree trunk (left; Photo by Michael Eckstadt). The newly grown tail-feathers (middle) are compared to the worn spines from a first year male (right) after a year of feeding (Photo: J. van der Wetering) Strong legs: In proportion to their body size, mohua have large feet and powerful toes – useful tools when it comes to hanging upside down from branches when finding your next meal. In fact the whole mohua skeleton is modified for strong grasping with the pelvis bent dorsoventrally and the posterior part broadened to hold large muscles. Males generally have thicker, blacker legs than females, but this is not always the case so mohua cannot be sexed on leg-thickness alone!
A male mohua with thick black legs is held after being fitted with coloured bands (YR/BM) so that it can be individually identified by researchers. (Photo: J. van der Wetering) Some lucky mohua get to wear brightly coloured plastic bands on their legs that are fitted by scientists so that birds can be identified for monitoring programs. If birds need to be individually identified special colour combinations are fitted consisting of two plastic colour bands on one leg, and one plastic and one metal band on the other leg. The bands are read from the birds left to right and from the top to the bottom. For example the bird’s band combination in the photo to the left would be read “yellow over red on the left, and blue over metal on the right” and recorded as YR/BM.
For mohua the bands used are size "C" and aluminium metal bands (CP) are preferred as they are easier than steel bands to close correctly using banding pliers. Each metal band has a unique code identifying each bird and where it was banded. These records are forwarded to the Department of Conservation’s Banding Office who looks after the information. Plastic bands are fitted using clips and are clicked together until they close.
Banding birds requires great skill and practise and rare birds such as mohua should only be undertaken by experienced banders. Banding skills can be learnt through your local Ornithological Society or by practising banding under a Department of Conservation registered bander.
mohua moult their whole tail at one time. (Photo: J. van der Wetering)
Web: Get in touch with your local Ornithological Society to find out about learning to band birds at this link:
www.osnz.org.nz
A melodious call – what do mohua sound like?
mohua are most often heard before they are seen. High up in the forest canopy flocks or family groups can be heard uttering loud chattering calls, trills and slurs as they move through the forest.
mohua are vocal all day and all year, except during the late summer moult. Males make territorial songs that peak in late spring and have strong musical, canary-like whistles and trills – hence their reference as bush canary by early explorers.
Other calls made by mohua are loud and varied including a rapid staccato chatter, musical whistles and slurs. The female sometimes utters a buzzing call.
Did you know? mohua calls are used for conservation management purposes. When birds need to be caught to establish a new population at a new site, mohua calls are used to attract birds to mist-nets. As mohua have distinctive dialects and songs between different populations it is important to use the correct calls for the birds that you are trying to catch.
The passage below from 1873 (Birds of New Zealand, by Walter Buller) is an eloquent description of the bush canary calls and an insight into the dawn chorus that early explorers must have experienced:
"Its commonest call is a trill or rapid shivering rattle not unlike the pea whistle note of the popular cage bird. That is the ordinary means of communication in normal life. There is the sparrow-like chirp, a scolding note, a muffled, beseeching, supplicating utterance not often heard, and during incubation a whisper inaudible out of the nest hole.
"At dawn the rattling trilling chorus begins, becomes more silent towards noon and then again breaks forth, the little performers mostly unseen in the tee top greenery. When detected in full song near at hand, the very tail will be seen to vibrate with energy of the vocal outpouring, a passion of melody which is nevertheless prosaically, almost comically blocked in full jet by desire to scratch and search for vermin, a trouble peculiarly incident to the breed."
A mohua in Glenorchy captured on film making its call (Photo: Michael Eckstadt) |
