Three day old North Island brown kiwi chick on Ponui Island. Photo by Ruby Mackenzie
A load shrill call interrupts the night, breaking the silence of the island. It’s not unwelcome, after all its why i’m here. This call belongs to none other than that New Zealand icon, the kiwi. The male repeats his call up to 20 times, allowing other kiwi to know his location. Calls from the kiwi can be heard throughout the night, the guttural call of the female occasionally complimenting those of a male.
Ponui Island, located in the Hauraki gulf is home to approximately 2000 kiwis. It covers an area of approximately 1800 ha. While it is highly unlikely you’ll run into one on the mainland, here, it is rare to not see a kiwi. They can be seen regularly running down a fence line after nightfall, or hiding amongst undergrowth at the edge of a swamp.
Hearing call after call throughout the night has one wondering just how many kiwi are really calling. Is it just one or two, or perhaps 5 or 10? There is such a high density of kiwi here and the hills are capable of warping how sound travels. The pros may be able to tell, but not me.
This is the idea that Jennifer Dent and Laura Molles explored in their paper “Call-based identification as a potential tool for monitoring Great Spotted Kiwi”. The aim was to provide the first measurable descriptions of male and female great spotted kiwi vocalisations.
Unlike on Ponui, finding, tracking and recording kiwi in an area with a lower abundance presents a host a problems. Kiwi are cryptic, illusive and, of course, nocturnal. Using methods that will not disturb their natural behaviour are our best bet.
Call counts have been used in the past to gather information in changes of population size over time. Reliability is questionable as kiwi calling behaviour may change due to a multitude of reasons. Individuals may have different call rates or there may be a difference between habitats. Interpreting these calls has its own host of problems, such as weather, observer bias, and phases of the moon (yes really, cue the were-wolfs).
Past studies have shown us contradictory results with call based identification of individuals. North Island Brown kiwi calls are highly distinguishable, whereas Little Spotted Kiwis are the opposite, with only a few calls being able to be distinguished from each other. For the method to be useful, Dent and Molles needed the Great Spotted Kiwi to have a high variation of calls between individuals but a low variation within each individual’s calls.
Prior to this research, 14 birds had the privilege of gaining some new ankle jewellery in the form of a transmitter. This enabled Department of Conservation staff, who were assisting Dent and Molles, to be able to track their movements and enabled them to tell when an individual is incubating an egg. Using this information, acoustic recorders were placed near the nests to be able to record the calls of the great spotted kiwi. All of the recorders were programmed to record within the hours kiwi are active, so 10 pm to 6:15 am each day, from November to March.
Since male and females sounds so different and each nest site is far from any others, it was assumed that the calls only belonged to the paired male and female who attended to the nest. Audio programs were using to filter out background noise and improve the sound. Finally, each call was analysed by looking at the syllables per call, the rate of each syllable and the duration of each syllable.
From this study of Great Spotted Kiwi calls, Dent and Molles found male and female calls to be very unique to the individual. They found that while there was an insignificant difference between male and female call length, males showed a longer and higher frequency to the syllables in their vocalisations. In contrast, female vocalisations had lower frequency elements, with shortened syllables. As a result, the technology and processes they used were highly accurate in determining each individual kiwi.
This is great news for the kiwi and their researchers. Populations, like that on Ponui Island, will finally have an accurate, non invasive way of not only monitoring the population but also observing kiwi activity. We can only make an educated guess what these guys get up to under the cover of darkness, without the watchful eyes of humans nearby. As technology advances, this creates opportunities that can improve how we manage populations. It definitely helps how we can help our endangered species, such as the kiwi, and advances our conservation efforts in New Zealand.
This article was prepared by postgraduate student Ruby Mackenzie as part of the ECOL 608 Research Methods in Ecology course.