Two weeks ago on the Po Toi thread, I predicted that this week would be the peak week for Short-tailed Shearwaters with the maximum numbers coming around 14th May. This prediction was based on my records for 2007 to 2009. I was so convinced that I changed my usual schedule in order to be on Po Toi from Tuesday to Saturday this week and record the peak numbers I expected to see on 14th and 15th.
So - the result of more than 25 hours sea-watching over the period 11th to 15th May was ….. one Short-tailed Shearwater. Just one single bird.
How could I be so wrong? Obviously the pattern of passage this year has been different to that of 2007, 2008 and 2009. In fact, the 2010 pattern has been very similar to that of 2006. I was aware that 2006 had a different pattern, but I ignored it, partly because Typhoon Chanchu passed through the South China Sea from 14th to 17th May 2006 and I thought this must have been the reason why the pattern was different in 2006. I was wrong.
What is happening can best be seen on this graph of cumulative daily totals for each year 2006 to 2010 (to date)
It’s clear from these graphs that the timing of the passage can vary from year to year. 2007 and 2008 were ‘late’ years with peak numbers occurring around 14th May. 2009 was just 3 days earlier but migration in 2006 and 2010 was 14 days earlier. It also seems that, when the passage is early, it goes on longer and has less of a peak at the end. In ‘late’ years such as 2008, the whole passage is quite brief, only about 10 days, with a large peak in the last three days.
Why should there be any pattern at all? Well, Short-tailed Shearwaters are well known for their migration pattern. Their numbers are huge, around 20 million birds making an annual loop migration, in spring up the western side of the Pacific, spending the summer in waters around Japan and further north, and then returning in autumn through the central Pacific. They tend to move in highly concentrated groups, sometimes more than a million birds passing through an area in a single day.
My guess is that what we see in Hong Kong is a reflection of the main migration, with some years being earlier and more extended than others. I will try to find out more, I’m sure someone somewhere is an expert on the annual migration of this enigmatic species.
It will take a few more years of observations to confirm the Hong Kong migration pattern, maybe something else will come out. For instance, are there intermediate years between the ‘early’ and ‘late’? It seems sure though, that the main migration is over for this year and I missed the peak which probably occurred last week around 5th and 6th May when I was not there.
Here photos of the only bird this week, seen today - a classic bird with dark head and steep forehead, only a little pale on the underwing
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