Thursday, 25 April 2019

SABAP2 Sequence Index explained


Sequence Index

The new SABAP2 website (sabap2.adu.org.za) has introduced some new data formats and data terms. One of these is the Sequence Index. The sequence index is the average position that a species normally occurs on a list, and is expressed as a percentage on the SABAP2 website. Unlike the traditionally used reporting rate (how often a species occurs on a set of lists, also usually expressed as a percentage) where a high score is indicative of high abundance (and/or detection), a high sequence index indicates lower abundance (and/or detection) i.e. it took a long time to detect a species, indicated by the fact that many other species were reported before it. Simply put, a rare species should have a high sequence index and low reporting rate, while common species should have high reporting rate and low sequence index. This is probably easiest to see for a migratory species, e.g. Barn Swallow, which is absent in winter. Here the sequence index (yellow line) goes up over the winter period, while reporting rate (blue line) goes down, and then up again over summer.



We thus say that the sequence index is negatively correlated to reporting rate. This is more clearly seen if we use a set of species, for instance 58 of South Africa’s endemic bird species. Calculating mean reporting rate across a species range, we see that species with high reporting rate have a low sequence index, while species with low reporting rate have a high sequence index.



While both indices thus give indications of apparent abundance (and/or detectability), there is an important distinction in how these are calculated: reporting rate is calculated based on the number of lists where a species was present or absent; while sequence index is calculated from IF a species was detected, what position on the list did the species hold? This apparently subtle distinction does have implications though: for instance, with the BirdLasser revolution lists now have fewer species compared to the period before, for reasons we won’t explore here. This means there is a small, but significant, probability of NOT reporting a species. This means that reporting rate can appear lower but in this situation this does not reflect a change in abundance; it is a change in reporting probability. It appears that the sequence index on the other hand is robust to this change, given it is calculated independently of the number of cards submitted. The sequence index is thus an additional useful tool in the box of information that SABAP2 can deliver.

 

Tuesday, 19 March 2019

An atlas hack up Peak Formosa




Smoke and ash billowed from the mountain top, while earthquakes shook the small town below it. Rivers of lava started to flow down the erupting volcano, but that wasn’t enough to stop Pierce Brosnan driving through them to rescue the family of the town’s mayor. As you see – our ascent of Peak Formosa had started off on a slightly strange note, as Anja was unable to sleep and we had decided to abandon attempts to sleep further at 4:30, turning on the television for some entertainment, to find Dante’s Peak fittingly relegated to the early hours of the morning. 

Our preparations for the hike up Tsitsikamma’s highest peak had actually started several weeks earlier at least. The peak was targeted some time back for our annual ‘birthday’ ascent after conquering Bloupunt last year. As access to Peak Formosa is over private land, and we’d struggled to get hold of the owner (Marius Strijdom) or a response from a guesthouse we’d identified nearby, we’d driven out to the agri-industrial town of Louterwater to track the people down face to face. We felt this was a good move, as there is a veritable labyrinth of farm roads through apple orchards below the peak: it could be super easy to get lost, even with Google Maps. We also managed to track down a receptionist of the Louterwater Landgoed guest cottages to confirm a booking. 

Our mission to find Marius Strijdom was less successful: instead we crossed paths with his son and heir-apparent, Darius. His demeanor was not of the friendly type, and he requested a R100 per person ‘gate opening’ fee, so if you can get hold of Marius, so much the better: the ‘free’ access to the peak looks set to change. 

While our ascent attempt was originally scheduled for 10th March, yr.no had been warning us about poor weather conditions all the previous week, so we delayed till the following weekend. You don’t want to be tackling a new difficult mountain ascent and dealing with weather at the same time. So, we did a warm up hike up the familiar Bloukop on the 10th instead. 

As it was, the day of the ascent was perfect weather: cool temperatures to start, only a mild breeze, and mostly clear skies. We got to the massive dam, home to just a few Egyptian Geese and nervous Reed Cormorant, that marks the start of the trail at around 7:15, with sun just touching the high peaks. While white marks on burnt trees indicate the trail, this was so overgrown we decided to keep to the edge of the dam and rejoin the trail where we had learnt it crossed a stream feeding the dam. That was a bit of effort to find, and the trail at that stage is still massively overgrown with tall fynbos, which the Cape Sugarbirds think is fantastic. With the early morning dew, we were soon so wet we may as well have set off in the rain. On the plus side, at least there is a trail: there hasn’t been one on the last 2 summits we’ve done.  

Once over the second stream, the ascent begins. The lower slopes are sickeningly covered in Hakea and Pine. My dad had gifted me a hand-saw for my birthday, inspired by Donovan Kotze’s talk on the sport of alien hacking given at the Gouritz Cluster Biosphere Reserve. This proved a useful way of pacing myself with Anja: moving ahead and sawing down a sapling while waiting for Anja to catch up. I hope that other hikers will similarly do their duty to preserving the beauty of this fynbos by similarly tacking some of the invasive vegetation along the trail: neighbouring slopes have been completely converted to pine forests. That is a bad thing, because as we all know, grizzly bears live in pine forests: so, save yourselves and cut down a pine today!

Just after the pine trees thin out one hits the eastern ridge. From here, while the trail is steep, but the going is easier as the vegetation lower with the soundtrack provided by tireless Victorin’s Warblers. There are lots of excuses to stop to soak in the views, scan for birds, and admire the Erica’s and other fynbos. It took us 3 hours of admiring the views, hacking pines, recording birds to make it to the ‘Knife Edge’. The slopes at this section are really steep, but it would be better described as a ‘Saw Edge’ since the ridgeline is up, down and around rocks and boulders. Faced with ‘The Gulley’ on the other side, this was all too much for Anja’s vertigo: time to get out her book and wait for me to hit the summit. The Gulley is certainly a technical scramble, but after that its relatively straightforward to the summit, which I reached about an hour after the Knife Edge. 

The logbook at the top was entertaining reading. Probably the most memorable quote was ‘Please cut the grass’ from some visitors from Germany. The cross on the top is also home to a Bible, but its in Afrikaans just in case you were wondering if you should leave yours behind. While I scouted around for rumoured Cape Rockjumpers for a while, I had no luck.  

The descent was fairly straight-forward, with the exception of the growing heat. While we’d taken 2 litres of liquid each up, we were parched by the time we got to the stream at the base of the peak. Still, the excited White-necked Ravens that had been circling would have to look for dinner elsewhere that day. The swim in the dam to end it all was magnificent. 

Certainly, this hike requires a good degree of freedom, and shouldn’t be tackled by anyone, and its hard to recommend from a birding point of view: 7 hours producing just 25 species. But the views are magnificent, and there is plenty of sport to be had if you’re into alien vegetation hacking.    

We used this blog as part of preparation, which has a gps track file: https://muisvoel.com/2017/01/formosa-peak/ 






Knife Edge and the Gulley ascent up the false summit beyond

The summit

Reading material available on the mountain top

    

Monday, 11 March 2019

Sunbird refugee crisis

It was an honour to have an article published in the latest African BirdLife magazine commenting on the crazy influx of Orange-breased Sunbirds to Blue Hill over December. However, the article did not publish several charts, which I think paint a good picture of what happened, plus the consequences of a fire event on these birds. I still don't know why most birds were juveniles for instance.

This is the text:




The fires in the Outeniqua Mountains around George (Western Cape province, South Africa) that raged from the end of October to beginning of November 2018 were the largest in the region in recorded history, burning a region of some 80 000 hectares, leaving 8 people dead and many people homeless. These fires follow close on the devastating Knysna fires of 2017. Fighting fires in these remote mountain areas is dangerous: one helicopter pilot died during the most recent incident, and fires are generally only engaged with they threaten people or property. For weeks we were on fire alert, fire fighting equipment at the ready, as we woke to smoke-filled valleys from the fires from the adjacent mountain range.

Still, the vegetation of these mountains, Fynbos, is fire adapted, with many species requiring a fire event as part of their life history strategies, either to clear away dominant vegetation in the case of rapidly resprouting geophytes and grasses, or to release seed stocks stored on the plant over years, as in the case of most protea species. Similarly, bird communities change in response to fire history, with terrestrial foraging insectivores and granivores dominating recently burned veld, whereas nectarivores dominate older vegetation. So, while fire might be great news if you’re a Cape Rockjumper or Long-billed Pipit, it’s bad news if you are an Orange-breasted Sunbird or Cape Sugarbird. These nectarivores require flowering Erica and Protea species for most of their energy requirements, and these plants normally only start to flower a few years after a fire event. What happens then, to these birds after a fire?

I have been ringing on Blue Hill Nature Reserve, Western Cape, about 2 hours drive from George, since 2011. Back then, there had not been a fire on most of the property for some 15 to 25 years: tall proteas crackling with Cape Sugarbirds dominated the landscape. But at the beginning of 2012 a massive fire swept across most of the property. Predictably, the sugarbirds and sunbirds disappeared, but had they died or departed? I would retrap only two birds that had moved to an adjacent unburnt valley, so what had become of the rest? I could only speculate. 

However, just after the Outeniqua fires I was bird ringing again near a pond where I was expecting to mostly catch granivorous species like Cape Siskin and Cape Canary that come to drink. Instead, my nets were literally sagging with Orange-breasted Sunbirds. Closer inspection would reveal that the majority of these were also young birds: only a few months old and displaying residual gape, or with some youngsters with an orange feather or 2 giving away that they were aspiring to be beautiful males. The sheer quantity was overwhelming: on 14 November 2018, in 3 hours I caught 47 Orange-breasted Sunbirds from a total of 67 birds using a single 12-meter-long net. The huge number of birds could not have originated on the reserve, they must have come in from the burnt areas to the south. Certainly, for a bird traditionally associated with Erica species, there were none to be had at this ringing location: they may instead have been settling for muddy water and blue Psoralea flowers (members of the Fabaceae). 

On average, these birds were also the lightest compared to the previous 3 years, indicating some birds may have been starving. One was soon recovered dead, and by the end of December, capture rates had declined dramatically – the birds likely dispersing further or dying. Certainly, there were no refugee camps for these youngsters. Overall though, a cruel reminder of the impacts of increasing fire events in the fynbos and the consequences for its wildlife.   

By comparison Cape Weaver capture rates are 'the same'


The proportion of 'juveniles' is alarmingly high


 Female capture mass of the young birds especially was very low, and male capture rate lower than generally recorded. Note, females weight less than the males.


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