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20th February 2019
Bookings open this evening (Wednesday 20th February)! Guided by the 2018 findings, we've set up an all-gold array of survey squares for everyone to choose...
Last year for the first time, teams of NatureTrackers took part in Where? Where? Wedgie! in May around the state, to see if we could estimate population numbers of Tasmanian wedge-tailed eagles and perhaps other species too.
With the level of interest, it looks like the method could work! 280 teams surveyed 224 4 km x 4 km squares across the three survey days. This included 44 ‘priority’ survey squares (in 2018 there were 95 of these - every 10th 4 km x 4 km square across Tasmania) – prioritised in order to get information on the birds from all common habitat types in the proportion that they exist across the state, and to reduce the risk of counting the same birds in different squares.
Wildlife statistician Joanne Potts has applied an approach called ‘occupancy modelling’ to the results of our repeated 10 minute surveys, so far focussing her analyses on the results from the priority squares. With this she's been able to determine the probability that eagles use each square, and also the probability of actually seeing an eagle during a survey in squares that eagles are using.
If we don’t see an eagle, Jo's approach can tell us how confident we can be that one isn’t there (rather than one being there but us not being able to spot it). Between us during the 2018 survey, NatureTrackers surveyed priority squares on average 8.5 times per square, with eagles being detected 33 times in 22 priority squares.
Jo's analyses indicated that there’s around an 85% chance that a wedge-tailed eagle was using the survey square during the time of the survey, and that probability of detection was very low (10%). In other words, if we visit a square that’s occupied by an eagle 6 times, we have a 53% chance of not seeing it, even though it’s there!
Knowing how likely we are to spot an eagle that's actually there is really important in guiding this year’s work, to ensure we get enough repeat 10-minute surveys to get good information. We’ve constrained the surveys to priority squares only, but greatly increased the number of these (every third square), and expanded the survey to six days. There are a few other little tweaks which you can read about under Prepare.
We’ve also used everyone's feedback towards lots of other improvements – especially around the website and app. For example, we’re now using the List Map in our booking map which includes much more detail.
So what other information do we need to estimate the population size? There are various approaches to doing this, but we are particularly interested in applying good estimates of the distances the birds move, and of the extent to which their movements overlap. We know that a territory typically contains a pair of adult birds and often one young one, but we also know that older sub-adult birds leave their parents’ territory and fly widely across Tasmania. NatureTrackers' very own James Pay is learning a lot more about this with his GPS tagged birds, which should allow us to better understand our survey data in the coming years.
While we wait for this information, our big focus for 2019 is to get a more holistic picture of where eagles are and aren’t in the landscape. We’re aiming for an overall ‘baseline’ across Tasmania, for comparison over the years. It’s tantalising, for example, that we missed out on the southwest prior to this year’s fires! Even if we can’t immediately get population estimates, we can use this baseline to assess whether numbers are increasing, declining or remaining stable. We’ll ultimately be able to apply work like James’ to any years where we have achieved a statewide survey, to get quite accurate annual population estimates – and thus to much better understand how our eagles are doing in the face of a complex combination of threats and regulatory efforts.
This year, with all the new yellow priority squares, we're aiming to go for gold! You can book your survey square(s) any time from this evening (Wednesday 20th February) until a few days before the surveys, but you might want to jump in fast to secure your favourite spot. We only need three days of survey in any one square, after which it’ll be marked as ’taken’.
We're currently preparing a detailed formal report of the 2018 analyses - watch this space.