Wanstead Birding

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 1^st, 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 1^st 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.