3 July 2020

Spring Migration 2020 - and ten years of data

In putting together a short synopsis of the migrant birds this spring for a Wren virtual meeting I assembled a table of the date of their first arrival. Chiffchaff and Blackcap are excluded as they over winter.


First date for spring migrants 2020
1
N Wheatear
16.03.20
2
Sand Martin
20.03.20
3
House Martin
03.04.20
4
Swallow
04.04.20
5
Cetti's Warbler
06.04.20
6
Willow Warbler
06.04.20
7
Yellow Wagtail
09.04.20
8
Whitethroat
12.04.20
9
Ring Ouzel
13.04.20
10
Lesser Whitethroat
14.04.20
11
Tree Pipit
15.04.20
12
Whimbrel
19.04.20
13
Swift
19.04.20
14
Green Sandpiper
21.04.20
15
Reed Warbler
22.04.20
16
Redstart
23.04.20
17
Sedge Warbler
24.04.20
18
Hobby
24.04.20
19
Common Sandpiper
25.04.20
20
Common Tern
26.04.20
21
Garden Warbler
28.04.20
22
Whinchat
28.04.20
23
Spotted Flycatcher
06.05.20
24
Cuckoo
30.05.20

This looks exactly as birders have come to expect, with Northern Wheatear usually the first to arrive followed by the hirundines and so on. We have excellent data recorded by many birders in the “Wanstead Birders collective” over the last ten years. Please note that only six birds, House Martin, Whitethroat, Lesser Whitethroat, Reed Warbler, Swift and Hobby breed in our area. Out of curiosity I then checked the first arrival date for these 24 species over the last ten years and was a little surprised that no species had its earliest ever arrival date this year. Thinking of global warming and the idea that migrants are getting earlier each year I decided to check the first arrival date for each of the species above and drew a simple graph of which year recorded the most first arrival dates.



Earliest ever date




1
N Wheatear
11.03.17



2
Sand Martin
14.03.15



3
House Martin
02.04.11



4
Swallow
25.03.16



5
Cetti's Warbler
17.03.19



6
Willow Warbler
29.03.17



7
Yellow Wagtail
09.04.16



8
Whitethroat
06.04.11



9
Ring Ouzel
30.03.11



10
Lesser Whitethroat
09.04.14



11
Tree Pipit
08.04.11



12
Whimbrel
(rarity)



13
Swift
18.04.15



14
Green Sandpiper
21.03.12



15
Reed Warbler
14.02.12



16
Redstart
02.04.11



17
Sedge Warbler
13.04.15



18
Hobby
20.04.10



19
Common Sandpiper
16.03.11



20
Common Tern
01.05.17



21
Garden Warbler
23.04.17



22
Whinchat
15.03.13



23
Spotted Flycatcher
26.04.15



24
Cuckoo
02.05.16




 
Year/No.
1
2
3
4
5
6
2020






2019






2018






2017






2016






2015






2014






2013






2012






2011






2010
2010















The best year turned out to be 2011 with six species having their earliest date of arrival, and the three most recent years did not give the impression that our migrants are getting to the patch earlier each year. The advantage of good data is that you can delve deeper, and rather sadly perhaps, I thought I ought.I put together a table of the first arrival date for each species that has regularly migrated through our area in spring over the last ten years. It was then easy to count each date as the number of days after Jan 1st, add one day for each leap year and plot a graph for each species.  To focus on spring for the graphs I used the number of days after 1st March, and to try to even out the variation I plotted a linear trend line in black. As an example, Northern Wheatear clearly shows a trend to be arriving earlier each year. The same is partly true for the hirundines, although there is variation, with Sand Martin clearly showing an earlier trend while House Martin shows only a shallow slope towards earlier arrival and for Swallow the slope may be better seen as no trend.

To summarise, and avoid many graphs:

1 Northern Wheatear, Sand Martin and Common Whitethroat show clear earlier arrival date trends. 


2 House Martin, Yellow Wagtail, Swift, Spotted Flycatcher and Hobby show a shallow slope towards an earlier arrival date.

3 Swallow, Whinchat, Reed Warbler, Sedge Warbler, Lesser Whitethroat, Tree Pipit and Cuckoo seem not to have altered their spring arrival time in our area over the last ten years. 

4 Redstart and perhaps Willow Warbler are two regular spring migrants that appear to have a later arrival time trend. Is this real or a data error?
Although the data set is pretty good there are many variables that make any conclusions tentative and I have not attempted a statistical analysis. The weather and a bias towards weekend recording are two factors, but I suspect the overall fall in the numbers of our spring migrants makes a difference too.

It will be interesting to plot the earliest arrival dates over the next few years and see whether the trends observed here are supported, this will confirm the observed trends are valid and we can then speculate on reasons.

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