There was an extremely large passage of Manx Shearwater down channel here today and I took some counts at 1/2hr intervals as shown below.
1030hrs - 35/min
1100hrs - 42/min
1130hrs - 78/min
1200hrs - 83/min
1230hrs - 59/min
1300hrs - 46/min
1330hrs - 32/min
Although the total figure maybe regarded as a best guess estimate and not a completely accurate count. I used the1030hrs total for the next 30mins giving a total of 1050 (35*30) birds between 1030 - 1059. Then repeated this for the next 1/2 hr spell and so on. Working on this premise, the total for the 3 1/2 period would make a total of 11,250 birds. Next time I will try the same method at 15min intervals for even greater accuracy. The peak count coincides with the hour/hours after high tide when birds start to leave the upper channel as the tide drops.
Other birds seen :- 2 Storm Petrel [1 d/c @ 1042hrs, 1 u/c @ 1316hrs], 2 Common Tern [d/c @ 1220hrs], 26 Fulmar, 29 Gannet, 4 Guillemot.
Other observeres present - M. Meehan, G. Roblin
Comments on how to count manx and other methods welcome.
5 comments:
Nice work Martyn. The interesting thing is even if you use rates at hourly intervals the numbers are fairly consistent.
10:30, 11:30, 12:30, 13:30(for 1/2hour) = 11,280
and
10:30(for 1/2hour), 11:00, 12:00, 13:00 = 11,310
The larger figure showing an error of 0.5% against your more accurate method.
It will be interesting to analyse at 15min intervals to see how that compares?
Great number of birds.
I've often wondered about the flight patterns of Manx in the Bristol Channel. Could it be that over a 3 of 4 hour watch the birds fly past more than once?
An interesting question Joek - so i did a bit of research - Manx GPS tracked birds travel at an average of 45km/h acoording to the latest survey. If they were to follow a circuit of Bridgend - Rhossili - Ilfracombe - Weston - Cardiff - Bridgend. It would still take the birds 5hrs to complete because of the distances travelled. So i'd say anything under 5hrs seawatch we are only counting the birds once. The birds would have to cross the channel immediately after passing Porthcawl to complete a 3hr circuit. Of course i think they follow a much bigger circuit. At Portenyon last year birds did not turn up until 0915hrs from the west after which they passed in a steady stream, which is about right if coming from Skomer/Skokholm. The Lundy population has numbers less than counted.
Nice maths, Martyn ;-) It is interesting to know what their flight patterns are, and the reasons for flying in the Bristol Channel. Looking at a paper about tracked Skomer birds, if the majority are away for up to a week at a time feeding North and West, what would they be doing flying East to Glamorgan?
I think the birds only come into the channel in great numbers when a strong storm either tires them out or they have no choice but to follow the wind patterns. It is interesting to note that none of the GPS birds were recorded in the channel at all. I presume that all the birds at this time of year are adults, as in august/september the juvs can get anywhere in a storm, we have even picked 1 up in Llynfi valley at this time. Hope these GPS surveys are long running so we can get a better picture of what is happening. Thnx Joek
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