By Conservation Officer, Matu Booth.
What is juvenile dispersal?
Juvenile dispersal describes that part of the life cycle when the young of a species leaves its natal area in search of a territory of its own. It has two important functions – it reduces competition for the food resources in an already established territory, and it also reduces the likelihood of inbreeding.
Roaming and returning
On the one hand the breeding season is typified by birds settling in localized and defendable nest territories – dispersal on the other hand is an outflux of all the newly created genetic material across the landscape. For different species it will involve various scales of movement. For instance, juvenile bellbird (korimako) travel far further than tīeke (saddleback) or robins in their quest for resources and territory. Juvenile harrier hawks (kahu) have been tracked making journeys right around the South Island before crossing back to the North Island and heading back to take up residence close to where they set off from. For most bird species females settle into territories further from their natal territories than males.
Some birds just won’t stay where we put them
When, how far and to where, a species disperses determines how easy it will be to establish a population after a translocation. The willingness and ability to disperse has been a key factor undermining Zealandia’s attempts to establish a viable bellbird population – in other words one that doesn’t need to be supplemented by top-ups of females to replace birds which don’t return after dispersal.
Challenges from the ‘real’ world
But there is a common factor facing all species which are drawn beyond the sanctuary by their urge to disperse – for many of them it will be the first time that they have ventured into the “real world”. Their chances of survival are greatly diminished – for some like tīeke which often forage and roost close to the ground, it will only be a matter of time before they fall prey to cats or rats.
I find it interesting how little is known about the dispersal of even our most common species. It’s not surprising that the most detailed understanding of a bird’s biology will come from that part of the life-cycle which is focused upon the nest. But to gain insight into how to best protect or establish a population it’s vital to have some notion of how far they range and what sorts of risks are faced.
Tracking a moving target
Colour banding individuals is useful only if birds are seen again – most aren’t.
Radio-tagging individuals can deliver information that can begin to unravel the mystery.
From 2009 till 2012 we deployed transmitters on 49 bellbirds (mostly juvenile females). At the end of last season we radio-tagged 2 adult and 6 juvenile females. We tracked these using receivers which can pick up a signal up to about 500m away, given line-of-sight. Timing was crucial – there was a small window in which to capture target birds. The transmitters only deliver about 50 days of life so we delayed deploying the transmitters till as late as we could as we didn’t want to waste valuable battery life on a bird which continued to attend the valley feeders. At the same time we didn’t want to miss the last show before it disappeared from the feeders. And above all we were conscious that 50 days of radio-tracking only gives us a glimpse of the dispersal story.
Stalking a sleeping target
Bellbirds can cover a lot of ground in quick time, so even with the benefit of radio telemetry we might get an idea of where a bird is but still have to allow for the possibility that the bird might be a kilometer away ten minutes later. We tried to follow our target birds at various times through the day to establish foraging territories. This season we also put a lot of effort into trying to locate individual roost sites. We figured that a static point would be easier to pinpoint and might also give a more accurate map how a bird extends its range into new areas – in other words, disperses. The assumption that we made was that roost sites wouldn’t be chosen haphazardly at the end of the day but would be selected and be indicative of the bird’s daily activity. That’s the alibi that the tracking team used as we prowled suburbs late at night with aerials held above our heads.
So what have we learned about bellbird dispersal? Feeder usage reaches a peak in February. It’s at this time we see the greatest number of juveniles using the feeders. This melee lasts for a brief few weeks then the females drift away from the feeder sites – the birds that remain across winter are generally dominant and aspiring males.
Juveniles gradually expand their foraging range, which eventually sees many of them make daily journeys to and from their familiar sanctuary haunts. One consequence of their wider foraging is that they are increasingly likely to roost outside the sanctuary and that comes with the increased risk of being predated.
Two of the juveniles that we were following had been foraging and roosting beyond the sanctuary for only a week or two when their remains were at their roost sites – predated by rats. Another of the older females moved away from the sanctuary but was seen on several occasions foraging in a small area of neighbouring gardens in Karori – she was also regularly using a couple of roost sites in the same area. That female has now reappeared in the same nesting territory that she occupied last breeding season, and she’s been joined by her mate of last season.
In the larger picture its also encouraging that dispersal months will sometimes deliver the surprise of unbanded bellbirds visiting the valley – we know that they are exploring the landscape from some other point of origin because not only are they unbanded, but when they sing they do so using a discernibly different dialect. This supports the notion that dispersal isn’t a one way process. Of course it begs the tantalising question of where they might be dispersing from, and whether they could be progeny of Zealandia birds which have dispersed, settled and bred outside…
The first bellbird nests of the new season are under construction and about to receive their eggs. At this early stage we’re monitoring 8 females – 4 of which are of unknown provenance. In this there lies the hint that there may be enough influx over time to help establish a self sustainable population.
On the other hand, of 32 females that fledged last season only 3 have returned thus far to establish territories. What has become of the others? Have any settled in new areas and beginning to breed?
Sometimes I imagine how all of our rushing through the landscape with aerials aloft will be viewed in the future. It won’t take much of a technological leap forward for the mysteries of dispersal to be brought in to the light. There are already GPS transmitters which have deployed on birds of about 300g. Once the technology is refined and applicable to a bird of about 30g there will be a way of tracking bellbirds across the landscape for longer and continuous periods of time – and the information could be downloaded off the bird’s back from moment to moment.
The conservation implications would be profound. We could, for instance, target predators in areas that bellbird are using. Or we might gain a more accurate picture of what resources bellbirds are searching for during dispersal and enhance them. In the meantime it remains a slowly unraveling mystery.