Thursday, 8 May 2014

More on 5 May Poms

Following record numbers of Pomarine Skuas off Seaford and Birling on 5 May 2014 I have collected information from observers involved to document the passage and try and draw conclusions.

At Seaford, where coverage was from 05:00 to 20:00, a total of 133 were noted.  These are listed in Table 1.  Passage was slow to start (only two definites by 09:00) but picked up and continued throughout the day with few gaps of more than 30 minutes and none of more than 50 between sightings.  Despite a site (county?) record day total the largest flock was of just 11 birds.

A final thought is that at least 150 different Pomarine Skuas were recorded along the Sussex coast on 5 May 2014.  133 at Seaford, at least 15 different ones at Birling and at least 3 at Selsey (details below).

Table 1.  Pomarine Skua times off Splash Point, Seaford on 5 May 2014

time
number
morph
comment
06:10
[1]
pale
some uncertainty about a tail-less bird
07:53
1
pale
08:52
1
pale
09:14
1
pale
09:23
1
pale
possibly the same as 09:14?
09:29
9
all pale
I might not have seen the last one [-1]
09:40
10
all pale
09:59
10
9pale1?
10:10
5
4pale1dark
10:16
2
all pale
10:37
7
all pale
11:15
2
1pale1dark
11:29
1
pale
11:40
1
pale
12:10
4
all pale
12:16
2
both pale
one noted by me [-1]
12:30
1
pale
12:50
1
pale
12:59
3
2pale1dark
13:26
2
both pale?
following 5 Arctics, I missed them [-2]
13:35
11
10pale1dark
13:53
10
pale
14:00
1
pale
14:10
3
pale
14:42
4
all pale?
14:52
1
pale
15:16
4
all pale
15:34
5
all pale
15:43
3
all pale
16:10
1
dark?
16:45
3
all pale
17:04
2
both pale
17:53
1
pale
18:05
1
pale
not noted by me [-1]
18:15?
4
2pale2dark
} conflicting times noted and whether
18:33?
5
4pale1dark
} both groups landed on the sea
18:50
5
all pale
one joined four on sea, flew past at 19:20
19:23
3
all pale
19:25
1
pale
19:29
1
pale
total
133
me [-5] making my total 128

Thanks to Ewan Urquhart and Simon Linington for filling in some gaps and comments on the above.  Our notes were very similar, other than a very few minor timing issues and a bit of confusion towards the end (we were all pretty fatigued by then).

Table 2.  Comparison with Pomarine Skua flocks off Seaford and Birling

time
Seaford
Birling
time
Seaford
Birling
07:53
1
12:50
1
08:52
1
1
12:59
3
09:14
1
13:12
14
09:23
1
13:26
2
09:29
9
13:35
11
09:40
10
13:44
12
09:52
13
13:53
10
09:59
10
14:00
1
10:10
5
14:04
7
10:15
12
14:10
3
10:16
2
14:26
5
10:22
4
14:42
4
10:26
1
14:50
1
10:37
7
14:52
1
10:38
3
14:55
6
10:48
1
15:05
1
10:58
9
15:16
4
10:59
3
15:25
6
11:15
2
15:34
5
11:29
1
15:40
9
11:40
1
15:43
3
12:10
4
15:51
2
12:16
2
16:07
1
12:17
8
16:10
1
12:29
1
16:35
1
12:30
1
16:45
3
12:44
2
16:55
2
by 12:45
58
58
by 17:00
110
125

Data from Birling was kindly provided by John & David Cooper.  On the face of it this table shows no direct correspondence between observations at Seaford and Birling.  None of the flocks can be tracked from Seaford to Birling and only one single could be assumed to have flown directly and then only if the Seaford bird at 14:52 was that at Birling 13 minutes later.  At first I found this very surprising as the sites are only 6.5 km (~10 minutes) away as the Pom flies.  However David Cooper suggested that the Poms might be using Kittiwakes returning to the colony at Seaford Head as a refueling stop based on their observations of Poms mugging Kittiwakes, a behaviour we witnessed several times too.  This might explain the delays in birds reaching Birling from Seaford but is perhaps not the only explanation for the apparent constantly changing composition of flocks, at least not if there was any continuity in their travelling companions.  A suggestion that Pom’s might migrate already paired isn’t really born out with a lot of infidelity apparent if they do.  Such a phenomenon (paired migration) is much more evident with migrating Mediterranean Gulls.  For Poms, at least evidenced by these sightings, it seems to be a very fluid affair with birds leaving and joining groups.

Table 3.  Pomarine Skua totals by 15 minute periods, Selesy, Seaford and Birling

Selsey
Seaford
Birling
Selsey
Seaford
Birling
06:15-06:29
1
13:00-13:14
4
14
06:30-06:44
13:15-13:29
2
06:45-06:59
13:30-13:44
3
11
12
07:00-07:14
13:45-13:59
10
07:15-07:29
14:00-14:14
4
7
07:30-07:44
14:15-14:29
5
07:45-07:59
1
1
14:30-14:44
2
4
08:00-08:14
14:45-14:59
1
1
7
08:15-08:29
10
15:00-15:14
1
08:30-08:44
4
15:15-15:29
4
6
08:45-08:59
1
1
15:30-15:44
8
9
09:00-09:14
4
1
15:45-15:59
2
09:15-09:29
10
16:00-16:14
1
1
09:30-09:44
1
10
16:15-16:29
4
09:45-09:59
10
13
16:30-16:44
1
10:00-10:14
5
16:45-16:59
3
2
10:15-10:29
1
2
17
17:00-17:14
2
10:30-10:44
7
3
17:15-17:29
1
10:45-10:59
13
17:30-17:44
1
11:00-11:14
1
17:45-17:59
7
1
11:15-11:29
3
18:00-18:14
1
11:30-11:44
1
1
18:15-18:29
11:45-11:59
15
18:30-18:44
4
12:00-12:14
4
18:45-18:59
6
12:15-12:29
2
9
19:00-19:14
3
12:30-12:44
9
1
2
19:15-19:29
9
12:45-12:59
4
Total
74
133
125

Justin Atkinson listed the Poms seen at Selsey and sightings from Selsey, Seaford and Birling are listed in 15 minute periods in the above table.  Few clear patterns emerge although the 10 at Selsey (at 08:29, 9 pale and 1 intermediate) could have been the 10 at Seaford at 09:49 (9 pale, 1 uncertain), 1 hour 20m being a pretty good time to cover the c70 kms they would have to fly if tracking within sight of the coast (53 km/hr or 33 mph) with no Kittiwakes to distract them.  Few other links jump out of the above table as worthy of investigation although selecting 15 minute periods is entirely arbitrary and could hide some.

The tables also support the view that birds are crossing the channel all the time with sites to the east picking up birds that did not pass further west for although Seaford cumulated a higher overall total, Birling was 15 ahead when observations there were curtailed.

The last 3 recorded at Selsey could not have reached Seaford before watching stopped there at 20:00 so a county day total of at least 133 at Seaford + 15 extra at Birling + 3 extra at Selsey = 151 would be an absolute minimum.

David Cooper suggested looking at cumulative totals and speculated that the difference on observations could be explained by the 9 birds seen arriving from the south at Birling at 10:58 and 6 at 15:25.  Ignoring these cumulative totals for Seaford and Birling are shown in Table 4.

Table 4.  Cumulative totals of Pomarine Skuas considered to have been recorded at both Seaford and Birling

Seaford
Birling
Seaford
Birling
pre 08:45
1
0
12:45-12:59
62
49
08:45-08:59
2
1
13:00-13:14
62
63
09:00-09:14
3
1
13:15-13:29
64
63
09:15-09:29
13
1
13:30-13:44
75
75
09:30-09:44
23
1
13:45-13:59
85
75
09:45-09:59
33
14
14:00-14:14
89
82
10:00-10:14
38
14
14:15-14:29
89
87
10:15-10:29
40
31
14:30-14:44
93
87
10:30-10:44
47
34
14:45-14:59
94
94
10:45-10:59
47
38
15:00-15:14
94
95
11:00-11:14
47
38
15:15-15:29
98
95
11:15-11:29
50
38
15:30-15:44
106
104
11:30-11:44
51
38
15:45-15:59
106
106
11:45-11:59
51
38
16:00-16:14
107
107
12:00-12:14
55
38
16:15-16:29
107
107
12:15-12:29
57
47
16:30-16:44
107
108
12:30-12:44
58
49
16:45-16:59
110
110

Offsetting the time at each site by various amounts suggests that birds took longer to complete the 6.5km between Seaford and Birling in the morning than the afternoon.  Looking at the differences between the cumulative totals for each fifteen minute periods is interesting.  If birds were moving directly between Seaford and Birling little differences would be expected except those caused by the arbritrary nature of using 15 minute periods.  
Squaring differences give greater significance to a few bigger differences than many smaller ones, and stops big pluses and minuses cancelling each other out (less of an issue here)

Table 5.  Square of cumulative differences between Seaford and Birling with various offsets

offset
none
15 mins
30 mins
45 mins
60 mins
total
3310
1800
1362
1910
3318
up to 12:45
2935
1529
719
599
995
after 12:45
375
271
643
1311
2323

The numbers themselves are meaningless but the smaller totals show a better correlation.  This suggests that over the course of the day assuming a 30 minute offset, i.e. a bird passing Seaford in one 15 minute period would pass Birling two 15 minute periods later (depending on the precise times this could be a difference of between 16 and 44 minutes, e.g. 10:59 v 11:15 or 10:45 v 11:29).  The lowest total/closest match for the first half of the observations was using a 45 minute offset while a 15 minute offset was closest for the second half.  Perhaps as the day wore on the Poms became less distracted by Kittiwakes?

To me this shows that Pomarine Skua passage, at least on this occasion, is not as straight forward as I had assumed.  It was however a superb day to have devoted to seawatching (very fortunate for me being sandwiched by my 25th wedding anniversary do and work).  I’m sure it will remain long in the memories of those who were there.  Particular thanks go to Ewan Urquhart, John King, Simon Linington, Matt Eade and Liam Curson who spent longest at Seaford.  Paul & Bridget James, Tony Wilson and Julian Thomas (down from Norfolk) were also notable amongst the more usual regulars. 

Thanks too to Ewan, Simon, John & David Cooper and Justin Atkinson (via Selsey blog) for details of sightings.  I’m sure we are all hoping for repeats in the coming years to see if similar patterns emerge, and to enjoy perhaps the most eagerly anticipated spectacle of Sussex seawatching.

Pomarine Skuas flying east off Splash Point, Seaford

4 comments:

  1. many thanks for this Richard, yes I was jumping the gun on birds already being paired up, easy assumption based on the tight, almost regimented flock formations. In fact based on (quite ground-breaking) understanding of how fluid these flocks are and the movement and merging between them, I think the overall conclusion that can be regained was how little can be gleaned in terms of flocks and formation despite observations on very close quarters. However the fact that no clear answers can be gained should act not to deter but to encourage further questions!
    - In terms of flocking this article can shed some light http://www.bbc.co.uk/news/science-environment-25736049 - there quite often seemed to be interchange within the group with no bird for long keeping in the same 'position'. One could almost begin to see the birds (unintentionally) working as an organic whole - the number of birds migrating collectively being just as important as the favourable weather conditions.

    - Presumably (but not conclusively) birds joined and formed other groups by dropping back rather than resting on the sea, I certainly observed some of this flock dispersal whereas the only evidence birds pitched down was along the Beachy stretch?
    - It seemed from the increase in number at Seaford/Beachy that birds where constantly crossing the channel but this does not take into account the (lack' of) data from IOW/St Catherine's Point - I've checked IOW blogs and there seemed to be no observations - however presumably a significant number (perhaps equal to Selsey?) would have gone past SCP and bypassed Selsey, this raises the question were birds just 'cutting' in having already crossed 'en masse'.

    - do Skuas use visible landmarks (in particularly cliffs) as part of their navigation. I was present at a seawatch at Sheringham many years ago when we observed c20 LTS which were not observed at sites to our East and West (i.e. Cley) - (you can imagine how that went down). In relatively calm conditions it was suggested that the Skuas were able to 'use' the cliffs at Sheringham in order to reorientate or even 'bounce', in addition to the Kittiwake theory at Seaford/Beachy was the fact that these sites are extremely visible geographical features also a factor?

    - I'm glad you to could not identify that extremely strange bulky bird with apparent pointed tail streamers (it was the second bird that went past).

    What is wonderful is that the lack of strong conclusions that have been gained from watching this particular phenomenon have simply enhanced the enigmatic and mythical nature of this amazing creature. In tandem with this is that the combined effort of so many observers to watch individual birds for such an extended period time over such an extensive area must rank as quite unique on a global level!
    Joshua MacCallum-Stewart (Pomskua).

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    Replies
    1. Interesting (and very full) comments, many thanks. What I would like to know is whether this was a typical 'fluid' movement or something unusual brought on by the weather conditions and proximity of Kittiwake colony. I've not noted similar before but sadly it has been some years since I last had a big Pom day - hopefully I won't have to wait too long for the next. Some years, usually mid-week when I can't get down, flocks seem to pass Selsey and are seen off Seaford 80-90 minutes later. If that were the norm then paired migration might be happening as I'm sure it does with Med Gulls (although they are usually in smaller groups – often 2, 4 or 6 - and very vocal which adds to that impression).

      It would be interesting to know where the birds crossed the channel and how closely they follow the coast. I’m sure most pass Shoreham too far out to see (at least that is my excuse) but few seem to get past the marina un-noticed although that may say more about the observers. I’d assumed that Selsey missed out more for being too far west than from the IoW’s shadow and eleive that Justin Atkinson feels the same way.

      Jacob Everett is sending me times of the Poms he saw off Newhaven that morning which will be interesting and yours would be good to have too if convenient. Bob Self had c30 from Hope Gap many of which were on the sea or milling around which confirms the loitering conclusion between Seaford and Birling.

      Thanks again for your thoughtful comments. I'm envious of your Long-taileds at Sherringham, double my life total. I agree fully about the enigmatic nature of Poms. Wonderful birds and it was a real privilege to witness Monday's movement.

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    2. Many thanks Richard, yes feel even if its speculative comments they are useful in terms of building an overall rough picture and pin-pointing stronger or weaker arguments.
      The three things that seem have emerged from this are that Poms migrate in the right conditions, where they are able to pass an available food source AND because its beneficial for them to migrate together. Based on the Ibis info in the BBC article Poms flocks would adopt a rough 'v' formation to benefit from upwash and minimize downwash, this maybe accounts for the frequent 'chopping' and 'changing' and 'dropping back' and due to the sporadic and regularly spaced flocking the mass of birds moving as a whole benefits the individual.
      In regard to the question can you apply what happened last Monday to migration as a whole maybe we can tentatively assign 'energy retention', 'food source' and 'social coherence' as things to look out for under a very broad spectrum. However from quite a few seasons on the North coast something I learnt is almost every Skua flight is quite often radically different, confounds expectations and breaks many rules. So to sketch an answer to your question was it 'unusual'. I think its more that many features will probably be quite different but we are building up a clearer picture from what we discovered about flocking and flight timing following our observations.
      In regard to where birds cross and whether they are following the coast i.e. the line they were taking, I just found it fascinating how they very much seemed to following the same one, roughly equi-distant from the shore (based on how closely they were passing buoys). I'm sure this can be quite deceptive but certainly it seems birds going round the corner of IOW being seen from Selsey are getting a bit lost - not following the 'sensible', but its really hard to know if its more energy efficient to constantly follow the coast or cut the corner by taking a direct route from IOW to Seaford/Beachy or Dunge.

      Great discussion,

      Josh

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  2. Hi Richard
    Great write up
    Some observations from Worthing Beach noted below – I was fortunate enough to be back in Sussex for my parents joint 70th birthday celebrations so managed some of the movement but had to depart all too soon (with all Brownie points well and truly cashed in). Needless to say it was great to connect with the movement some 26 years since I got hooked on seawatching from the Marine Gardens shelter!
    My coverage was from 05:15 but there were no early Poms – indeed it was very quiet for the first couple of hours.
    Following the comments in the discussion it was interesting to note the bulky tail-less bird which matches one I saw at 05:59 so could match with the Seaford sighting if this was the second bird.
    We then logged:
    9 @ 0842 – all pale, not noted as Selsey as Owen rang through to Dave Smith almost immediately after – presumably Seaford 0929 flock.
    10 @ 0910 – 9 pale, 1 inter – the same as Selsey 0829 and Seaford at 0959 but no sign of another 10 in the interim (and we had 3-4 observers present at that time so unlikely to have missed another group of 10).
    4 @ 0917 – 1 dark, presumably same as Selsey 0836 and Seaford 1010 (having picked up another single at the latter site).
    John and Dave had another 4 around 1020 (same Brookiands 1031 as per SOS website, Selsey 0905)

    All birds were following a very similar line, fairly close in (certainly for Worthing!) but with a dropping tide and heat haze kicking in viewing was getting more difficult as the morning went on. The impression was that the birds were following the coast and not cutting across to the cliffs to the east (which were very visible), whilst movement seems very focused and with a sense of purpose. Also worth noting that the Worthing total of Common Scoter 0515-0935 was high relative to sites further east – 823 counted compared to 903? at Seaford for the full day) which was also unusual given the clear conditions. Not sure if we can draw any conclusion from that though.
    From my casual reading of observations from sites further west, good numbers off Hurst presumably saw a reasonable passage through the Solent – many more noted at Hurst than Portland suggesting birds were hitting the coast along this stretch. From memory the weather maps showed a fresher wind from northern Biscay to the south coast in the area so perhaps the birds made the most of this in crossing the channel and then hugging the English coast?
    All very interesting and, as said by others, adding to the intrigue of this species! It is hard to beat a flock of spring Poms and I shall live on those views for many a year!
    Regards
    Paul

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