Balearic Sheawaters through west Lyme Bay in 2017 compared with previous years.

As November draws to a close and I've seen no Balearic Shearwaters on the last two seawatches from Berry Head despite being ideal conditions (although we did have a record November count of Sooty Shearwaters)! It appears the Balearic Shearwater season is pretty well over, so I thought its time to look how the passage through west Lyme Bay (WLB) compares with previous years. But first a bit about that data. From previous years observations, the key months for WLB (from Berry Head / Start Point) are June–November, with on average peak numbers between 21 August–20 September (see graph, based on 2006–2015 data). While I have data going back to 1998, only since 2006 have I been able to put in at least 100 hours seawatching into the June–November period each year (an average of 175 hours per year), so I limit any comparisons to 2006 on. For comparing years, I take the total number of birds passing June–November and divide by hours watched. This gives an effort based measure of birds / hour (usually not possible from Bird Reports such as Devon Birds as nil events aren't recorded, which are on my system).
Balearic Shearwaters (birds/hr) through west Lyme Bay 2006–2015, by 10 or 11-day period (NB. When I get a chance I'll redo this graph adding in 2016–17).
 So my 2017 final June–November totals are 952 birds in 173 hours. Which gives 5.5 birds/hr. So 2017 has been slightly above average (mean for all years 2006-2017 is 4.4 birds/hr). Of the the 12 years 2006–2017, its actually the 4th best year, only beaten by  2011, 2013 and 2015. See graph below.
Balearic Shearwaters (birds/hr) through west Lyme Bay, by year, 2006–2017.
But no two years are ever the same with Balearic passage through WLB and 2017 was certainly different.  2017 showed  a relatively slow pickup through June and early July and in fact I didn't get my first double figure count until 22/07/17, usually its a bit earlier, sometimes in June. But things changed dramatically from late July to early August, when WLB's biggest peak occurred earlier than normal.  In fact I saw my best July passage day of 185 birds on 28/07/17. The best day of the year was shortly after on 03/08/17 with 203.  This year the period 22/07/17–17/08/17, produced the majority of this years' passage, with 726 birds in 70 hour obs (10.3 birds/hr). There was another smaller peak in early September, traditionally the peak time, but not matching the earlier peak. There were four three-figure counts and the general trend was more birds moving through outer Lyme Bay - so bigger numbers at Start Point rather than Berry Head.

What will next year bring?

2 comments:

  1. Excellent comparison Mark - and some great data too.

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    1. Cheers Mike. Data is only valuable if we keep recording in a 'sort' of systematic way. Looks like Start Pt access is going to be closed for much of 2018, so back to more recording at Berry Head?!

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