How do you monitor a bird that is adamant it’s a blade of grass?!
As another season ‘Bitterning’ at Whangamarino wetland comes to an end, it’s time to wash the mud off the waders, empty the water out of the gumboots, and go back to the desk to work out what on earth was going on.
This was our second season of research out in Whangamarino. Our objectives were to develop a monitoring method for bittern and it’s not easy trying to monitor a bird that spends the majority of its time impersonating grass in inaccessible bogs. Yet, as only 10% of their original habitat remains today, it’s never been more important to know what’s happening with the bittern population in New Zealand.
Thankfully, male bittern make a distinct, deep ‘booming’ call during the breeding season, which we are hoping will prove useful as part of a survey method. Unfortunately, the calling-rate of bittern seems to vary greatly over time and between stations. This makes it tricky to detect trends in bittern populations using call counts alone. For example, this season at Whangamarino wetland, 234 ‘booms’ (per 1.5 hours) were recorded at one station in October. Yet, only one month later, no bittern were recorded from that same listening post. Did the bittern move or do they just have fewer reasons to call?
Hopefully we’ve got enough data this season to get some idea. In 2009, Lynette Plenderleith and I went hard out trying to see if bittern really are as crepuscular as everyone suggests. The evenings looked good, with a beautiful peak in calling-rates appearing about half an hour after sunset. Mornings were not so clear cut, with lots of sub-peaks leading up to sunrise. After several mornings of dragging ourselves out of bed at 2am, only to discover that bittern were already up and booming, we concluded that something else was going on. One morning we both came back adamant it was the moon. This has been proposed before by others, such as Philip Teal and Jono More. Teal suggested that the moon was somehow linked to bittern food sources, like the movement of whitebait. Who knows though, may be it is just easier to advertise yourself to the opposite sex when you can see the female, she can see you, and most importantly, you know where her mate is lurking.
So this year (2010) we were back out there again, trying to get more information. Unfortunately, Lynette couldn’t make it - she decided frogs were her passion, and the temptation of a full scholarship and PhD in Australia was just too much...so she passed her waders onto Elise Verschoor, and the swamp fever continued.
The objectives of this season were slightly different. We wanted to know more about the extent and cause of variability in calling-rates. We also needed to get around some of the issues that were limiting our sampling regime. Fundamentally, that we couldn’t sample more than two sites at once, and large sections of the inner wetland remained un-sampled (some places just took us too long to get to without a boat, and others you wouldn’t come back from, especially in the dark!). To solve this, Graeme Elliot and Stuart Cockburn lent us some of their bird recorders and we were able to directly compare MONO and STEREO recordings with results collected from an observer in the field.
The results so far
Its early days and there are still recordings to process, but things look promising. Recorders appear to be as reliable for detecting ‘booming’ bittern as a person physically in the wetland. In fact more reliable - at least with recordings you can ‘pause’ to write when multiple bittern boom at once.
It will take us a little longer to process the data regarding the variability in calling-rates though. What we do know already is that bittern call-rates peaked in October as predicted. Also, contrary to expectations, we detected a second peak in one small area of the wetland (the causeway). Although not common, this second peak has been observed before with the Eurasian bittern (Botaurus stellaris) and is thought to be associated with males advertising feeding areas to fledglings or females becoming available again after nest failure . So, is it a coincidence that most of the ferrets the Department of Conservation caught during their ‘mark-recapture’ trials were along the causeway this year? Who knows? The research continues, and we still have a long way to go...but we will keep you posted...