BLYTH'S REED WARBLER
The 2020 spring influx

 

Figure 1. By far most Blyth's Reed Warblers (Acrocephalus dumetorum) seen in the Netherlands in the spring of 2020 were 'one-day-wonders', this individual  at Voorschoten, Zuid-Holland however, stayed for ten days (Vincent van der Spek)

by Vincent van der Spek & Diederik Kok

Preface

Parallel to their range extension to the west, Blyth's Reed Warbler (Acrocephalus dumetorum) records have increased in the Netherlands over the past three decades, though the numbers are still (very) small. Despite this increase, the unprecedented spring influx in May and June 2020 came as a complete surprise. This article - published in Dutch on DutchBirding.nl - presents an overview of the influx within the context of pre-2020 records, and it summarizes records elsewhere in Europe. The original Dutch article also includes clues on how to find and identify a singing Blyth's Reed. These two subjects will be split into separate articles on this website. Notes on their vocalizations can be found at the ID section (coming soon!).  

Introduction
Despite the increase, Blyth's Reed Warbler is still considered a national rarity in 2020. In the year report for 2018, the Dutch rarity committee CDNA concluded:

The increase of records continues, and one may wonder how much longer the species will be considered.

(Gelling et al. 2019)

That increase indeed continued. The first record was as recent as 1990, and it has been annual since 2009. The best years prior to 2020 were 2013 (9), 2018 (11) and 2019 (8). Up to and including 2019 there were 60 accepted records, resulting in an average of exactly 2.0 records per year since 1990. The main criterion to no longer consider a species is that it has - on average - more than two records a year over the past 30 years. In January 2020, the CDNA therefore decided to consider Blyth's Reed for 'probably one more year'. And then spring 2020 came...

The 2020 influx
Between 24 May and 30 June, Blyth's Reed Warblers were found on at least 30 sites (>=31 individuals). The total number of records for the Netherlands increased from 60 to 90 in little over a month! See Table on the bottom of this article for the individual records. Up to 2019, Blyth's Reed was mainly an autumn bird (78% of the records), so despite the range extension to the west (the Baltic States and Finland were colonized fairly recently), a spring influx of this magnitude came as a complete surprise.  Prior to 2020 there were (only) 13 spring records, including a small influx in May-June 2014 with six, accounting for nearly half of all previous spring records.

Geographical distribution of the influx
There seemed to be a pattern in the distribution of the records of the spring influx (Figure 2). The influx was mainly noted along the coast, though Zeeland (in the SW) was left without. Only four birds were found over 10 km away from either the North Sea or the Wadden Sea. The Wadden Sea coast accounted for no less than (at least) 21 birds, with the island of Texel as the purple patch with 12, followed by the island of Schiermonnikoog with at least three. During 23 days (28 May-19 June) more birds were found on Texel alone than there were country wide during all of 2018, the previous record year (11 records). Note that before 2020, inland records were also rare. 

Figure 2. Distribution of Blyth's Reed Warbler records in the Netherlands in spring 2020. 

Temporal distribution

Blyth's Reed Warbler is amongst the last species to arrive on the European breeding grounds. Both prior to and during the 2020 influx, the number of June records exceeded the number of birds in May. The earliest Dutch record was on 22 May 2014. 

 

The influx seemed to show two peaks (Figure 3): 
1) a larger one between late May and early June
2) a smaller one in mid-June

Please mind that the numbers are still small and that no firm conclusions can be drawn. On 1 June no less than nine birds were present, equalling the second best year (2013) in a single day. From 4-8 June there were five days without any new birds (also see: explanation of the influx).

In general the Netherlands apparently were not the final stop for the birds. By far most birds (23; 74%) stayed for one day only. Two birds were reported on two days and only six stayed longer, ranging from 4-21 days. 

Figure 3. Temporal distribution of Blyth's Reed Warbler sightings in the Netherlands in May-June 2020

Field sightings vs. ringed birds

Before 2020, the vast majority of the spring records were found in the field (11 field records vs. 2 ringed birds), whereas ringed birds dominated the autumn records (12 vs. 35). In that sense, the 2020 influx showed a familair spring pattern: only two were ringed, against 28 field sightings (29 birds). 

The influx in a European context
Good numbers were noted in several other European countries. The reports elsewhere give an impression of the size and range of the influx. The Netherlands formed the western edge of the event. From the (north)east to west the following numbers were reported. The species is colonizing Norway, where it was removed from the national rarities list in 2012. It is now considered a scarce breeder (mainly in the southeast) and spring migrant. Exact numbers are hard to give, but it was estimated that 2020 was the first year that reached three figures (100+), with records concentrated (but not exclusively) in the southeast. In Poland the species is nowadays a scarce, but annual spring bird. During 2014-2019, the average per year was 34. In 2020 however, at least 110 were reported between 20 May and 30 June. In Denmark a record number of 38 was reported, and Germany clocked 24 between 22 May and 29 June. 11 of these were on Heligoland, where a trapped and ringed female seemed to be paired with a Marsh Warbler.  Prior to 2020, there were 18 records on Heligoland, the first being as recent as 2000. Northwest of the Netherlands at least 42 were found in the UK until mid-July (an overview of the first 35 can be found on BirdGuides), where the species in no longer considered since 2015. They did not seem to fly any further west or north, with no records in Ireland or Iceland. Strong passage was not noted in Central Europe, with two records in the Tsjech Republic and none in either Austria or Switzerland. Finally, large numbers did not reach countries south of the Netherlands: both Belgium (species no longer considered since 2019) and France (on 22 May, the earliest ever) saw only one. Apparently the influx took place between (north)east and northwest Europe.   

Figure 4. Blyth's Reed Warbler, Breezanddijk, Friesland, 13 June 2020 (Laurens Steijn)

Explanation of the influx

Blyth's Reed Warbler is an eastern species. They have been increasing towards the west for decades and have succesfully colonized the Baltic States and Finland. All populations winter on the Indian Subcontinent - also the most westerly breeders. With the westward spread in mind the general increase is not surprising. But that doesn't make an influx. Favourable conditions on the winter grounds, resulting in a high survival rate doesn't match the influx of Marsh Warblers (Acrocephalus palustris) that was also noted in northwest Europe, since Marsh Warblers winter in Eastern Africa. Some birders speculated that droughts might have caused them to drift, but other than in Northwest and Central Europe, no serious droughts seemed to have been reported in the east (source: European Drought Observatory). The key seems to have been in the weather conditions. This spring there were long spells of (north)easterlies during the migration period of Blyth's Reed Warblers. This theory could fit the fact that only two new birds were found in the Netherlands between 5 and 11 June: a week with westerlies and - finally, during this extremely dry spring - rain.

 

Acknowledgements 
Many people contributed to this article. We would like to thank Jochen Dierschke, Wouter van der Ham, Zbigniew Kajzer, Leander Khil, Lukasz Lawicki (Dutch Birding), Josh Jones (BirdGuides), Nicolas Martinez, Hans Matheve, Tor Olsen, Jiří Šírek, Rasmus Strack, Hugo Touzé en Peter de Vries for their help.

Table. Spring records of Blyth's Reed Warblers in 2020 used for this article¹

1 24 May, Oude Kooi, Vlieland, Friesland

2 26 May - 15 June, Kekerdom, Gelderland

3 28-29 May, De Tuintjes, Texel, Noord-Holland

4 30 May, De Krim, Texel, Noord-Holland

5 30 May, Amsterdamse Waterleidingduinen, Noord-Holland

6 31 May, Strijen, Zuid-Holland

7 31 May – 1 June, Revebos, Dronten

8 31 May – 13 June, De Branding, Schiermonnikoog (bird #1)

9 31 May, Kwade Hoek, Zuid-Holland

10 1 June, Nationaal Park Zuid-Kennemerland, Noord-Holland (ringed)

11 1 June, Robbenjager, Texel, Noord-Holland

12 1 June, De Cocksdorp, Texel, Noord-Holland

13 1 June, De Branding, Schiermonnikoog, Friesland (bird #2)

14 1 June, dorp, Schiermonnikoog, Friesland

15 2 June, Robbenjager, Texel, Noord-Holland

16 2 June, Eemshaven, Groningen

17 3 – 12 June, Voorschoten, Zuid-Holland

18 3 June, Prins Hendrikpolder, Texel

19 9 June, Meijendel, Zuid-Holland

20 10 June, Eemshaven (vangst), Groningen

21 12 June, Vuurtoren, Texel, Noord-Holland

22 12 June, Robbenjager, Texel (possibly 2), Noord-Holland

23 13 June, Den Helder, Noord-Holland

24 13 juni, Breezanddijk, Friesland

25 13-20 June, Teylingen, Zuid-Holland

26 16 – 19 June, Amsterdam, Noord-Holland

27 16 June, Jan Ayeslag, Texel, Noord-Holland

28 17 June, De Slufter, Texel, Noord-Holland

29 18 June, De Krim, Texel, Noord-Holland

30 19 June, Bollenkamer, Texel, Noord-Holland

31 28 June into July, Lauwersmeer

 

¹Choices made

The number of reports was higher than the number of records used for this article. We did not check every report ourselves on ID, but we only used birds a) of which sound-recordings and/or pictures were available on the internet, and b) that were not disputed by others. The same applies for dates and number of birds involved per site. 

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Arnhem AEB413

18 October 2014, Meijendel, Wassenaar, the Netherlands

© 2018 by Vincent van der Spek