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Po Toi Seawatch Autumn 2022

Po Toi Seawatch Autumn 2022

Here we are with the survey of the autumn, i.e. September, October and November. Of course, it will be interesting to compare results with last year, but not sure there will be as much surveys done this year. Anyway, with September being a pretty hot month and the forecast for easterly winds with cloudy skies, we headed out for our first autumn seawatch.

6 to 10 September

Arriving with the ferry, I started on 6 September at 11h45 and seawatched until 17h00. No clouds yet, as it was hot and sunny. Winds did come from the east, with force 2 to 3 (that breeze was just enough to cope with high temperatures). It turned out the be a very calm start of the survey.

Terns:
104 terns spec.
2 Little Terns
9 Common Terns
13 Greater Crested Terns

Passerines:
2 Eastern Yellow Wagtails

Checklist on eBird: https://ebird.org/checklist/S118544471
Count on Trektellen: https://www.trektellen.nl/count/view/3323/20220906

The clouds arrived on 7 September, with showers and thunderstorms in the morning, but mostly dry afterwards. Winds came from the northeast and turned east, with force 5. I watched from 06h00 until 17h00.

Egrets and herons:
39 egrets spec.
2 Eastern Cattle Egrets

Waders:
2 waders spec.

Terns:
433 terns spec.; many probably Common, some probably Aleutian
66 Greater Crested Terns
24 Common Terns
11 Bridled Terns
5 Aleutian Terns
5 Little Terns
3 Gull-billed Terns

Doves and pigeons:
1 Rock Dove

Swift(lets), swallows and martins:
4 Barn Swallows

Checklist on eBird: https://ebird.org/checklist/S118544605
Count on Trektellen: https://www.trektellen.nl/count/view/3323/20220907

The earlier forecast of cloudy weather already turned out to be a different reality on Po Toi on 8 September. Apart from some clouds in the morning, it soon became hot and sunny. Winds continued to come from the east, changing force during the day, from 3 to 5. Seawatching time was from 06h00 to 17h30.

Seabirds:
1 Parasitic Jaeger
26 Red-necked Phalaropes

Ducks:
11 Garganeys

Waders:
4 Eurasian Whimbrels

Terns:
673 terns spec.; mostly probably Common, few probably Aleutian
179 Greater Crested Terns
109 Common Terns
6 Little Terns
3 Gull-billed Terns
1 Bridled Tern

Swift(lets), swallows and martins:
1 Barn Swallow

Passerines:
3 Eastern Yellow Wagtails

Checklist on eBird: https://ebird.org/checklist/S118544776
Count on Trektellen: https://www.trektellen.nl/count/view/3323/20220908

And the same weather for 9 September: after a few early morning clouds, bright, hot and sunny. Winds continued to blow from the east, with only force 2 to 3. A full day seawatching again, from 06h00 to 17h30.

Seabirds:
1 jaeger spec.
1 Parasitic Jaeger
13 Red-necked Phalaropes

Ducks:
21 Garganeys

Egrets and herons:
1 Grey Heron
1 Eastern Cattle Egret 

Waders:
1 Black-winged Stilt
1 Eurasian Whimbrel

Gulls:
3 Black-tailed Gulls

Terns:
802 terns spec.; many probably Common, but most unidentifiable
155 Greater Crested Terns
112 Common Terns
4 Aleutian Terns
1 Bridled Tern

Swift(lets), swallows and martins:
2 Himalayan Swiftlets
4 Barn Swallows

Checklist on eBird: https://ebird.org/checklist/S118544985
Count on Trektellen: https://www.trektellen.nl/count/view/3323/20220909

And our last day, 10 September, it turned cloudy/overcast, with mist and limited visibility (less than 5 to 6 km.). Winds stayed the same: easter, force 2 to 3.

Seabirds:
25 Red-necked Phalaropes

Ducks:
7 Garganeys

Egrets and herons:
111 egrets spec.

Gulls:
1 Black-tailed Gull

Terns:
380 terns spec.; almost all probably Common
111 Greater Crested Terns
52 Common Terns
1 Bridled Tern

Passerines:
1 Eastern Yellow Wagtail
1 White Wagtail

Checklist on eBird: https://ebird.org/checklist/S118545355
Count on Trektellen: https://www.trektellen.nl/count/view/3323/20220910

Remarks:

Let’s turn to the most numerous birds by far, terns, and compare this year’s results with last year’s. Our survey in 2021 was a bit later, but the differences were noticeable, as is clear from this table.



Indeed, the number of terns this year was much lower than last year, and we also did not see any White-winged Terns. For example, we counted 999 terns on an afternoon last year, while only 1,074 on a full day this year. And 3,226 on a full day last year, but only 544 on a morning this year. If numbers would be similar, we would have counted much higher numbers during that morning (with unchanged weather for the whole day), as mornings are the traditionally peak (see further).

But we did count a new record for Greater Crested Terns this year, with 179 individuals (all flying west) on 8 September, while the previous highest count was 165 last year on 10 September.

Yet, our counts now lead to more and new questions. Were the numbers of terns exceptionally high last year? Or were they exceptionally low this year? And what caused the difference? Is the lower number caused by the very low counts of Aleutian Terns this year? Many of last year’s unidentified terns might have been this species, possibly explaining the different counts. These are question to which I do not have the answer. We still need to do more surveys in the future.

Another point is that we didn’t see any Swinhoe’s Storm Petrels so far this August/September, while there were certain ones last year on 29 August (from a boat) and 11 September (from Nam Kok Tsui), plus a possible one on 26 August (from Nam Kok Tsui). Maybe easterly winds were not strong enough or occurring over a large enough area? And maybe there is a link between so few Aleutian Terns and the absence of storm petrels this year? Again, questions without answers…

More on terns and weather. We’ve seen on many occasions in the past – and mentioned here a few times – that calm, dry weather seems to be their preferred circumstances to migrate. Interestingly, on 7 September, the day started rainy and turned mostly dry afterwards. While the traditional peak of tern movements in a day is from 06h00 to between 08h00 and 10h00, 80% of the terns on 7th appeared after 10h00, exactly when most rain was gone. Moreover, the peak on that day appeared between 10h00 and 14h00, which is usually the quietest part of the day. I think this largely confirms the relation between weather and tern movements, although there might always be exceptions.

The rarest bird on this survey was the Black-winged Stilt. Last year, also only one was seen, on 26 August.

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Seawatch in Autumn

Thanks for an interesting summary, Bart !  

The weather conditions must really make a huge difference.  The Swinhoe’s Stormy on 29th August 2021 may have been driven closer to HK by heavy rainfall not far offshore....

Keep up the good work !
http://johnjemi.hk

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Quote:
Original posted by John Holmes at 17/09/2022 17:06
Thanks for an interesting summary, Bart !  

The weather conditions must really make a huge difference.  The Swinhoe’s Stormy on 29th August 2021 may have been driven closer to HK by heavy rainfall n ...
Thank you for your continuous support and encouragements, John! Very much appreciated.

Yes, weather is likely to have an influence on seabirds, but it remains a complex phenomenon which I still mostly don't understand. Sometimes it makes sense and sometimes it seems completely unexplainable (at least to us, humans). So many factors that might be in play, as well.

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