DUSKY WARBLER
in the Netherlands 1978-early 2021 (2021)

 

- Status review for the Netherlands

- Focus on the autumn 2020 influx

Figure 1. Dusky Warbler (Phylloscopus fuscatus), Meijendel, Wassenaar, Zuid-Holland, 5 November 2020 (Vincent van der Spek)

In the Netherlands, the Dusky Warbler (Phylloscopus fuscatus) is a very scarce, but annual late autumn migrant. A few individuals have also wintered. A huge influx in autumn 2020 came as a complete surprise. This article gives an overview on the occurrence of the species in the Netherlands from 1978 to early 2021. A Dutch version was published on the Dutch Birding website.

The datafile that forms the basis of this overview can be downloaded (and used freely) here (Excel, 46 KB)

Number of records 1978-early 2021
1978 to early 2020
The first Dusky Warbler for the Netherlands was found by Kees Scharringa on the island of Terschelling, Friesland in October 1978. It took another eight years for the second and third to be recorded (both trapped and ringed). Ever since, the species has been recorded more often. Over the past three decades, the records have slightly, and steadily increased. Thereby the species transformed from a rarity into a very scarce, but annual bird. It was considered by the Dutch rarity committee CDNA until 2011. Since 1999 there haven’t been any blank years.

During the first two decades of this century (2000-2019), the numbers ranged from 1-18 a year. The best years were 2003 (10), 2007 (11), 2011 (12), 2016 (18) and 2019 (10), but in most years less than ten were found (Figure 1). The huge number of birds in autumn 2020 thereby came as a bolt out of the blue.

Box 1. How did we determine the number of records?
- Until 2011 the species was considered by the Dutch rarity committee CDNA. Untill that year, only accepted  records were used (see www.dutchavifauna.nl)

- For field sightings between 2012 and January 2021, we used observation platform www.waarneming.nl. We have only included sightings supported by (recognisable) photographs and/ or sound-recordings. For ringing records we have (also) used www.trektellen.nl (also records without published photographs since misidentifications were considered slim to non-existing). For 2020 a photographed ‘Facebook’ ‘WhatsApp bird’ was added.

- In a few cases two birds were present; these were regarded as one record.

Autumn 2020 to early 2021: the influx
Between late September 2020 and mid-January 2021 no less than 91 records supported by evidence (at least 93 individuals; hereafter the number of individuals will no longer be mentioned) were found.  The total number of records for the Netherlands in was thereby increased to 224: 74 (accepted records until 2011) + 59 (2012 to first half of 2020) + 91 (the 2020/2021 influx). See Figure 1. 

 

Thereby the autumn 2020 influx (extending into early 2021) represents 41% of all Dusky Warblers ever recorded. In e.g. Sweden, Poland, Germany, Belgium and the UK larger numbers than usual were also reported. What the drivers behind the influx were, is pure guesswork.

Figure 2. Number of Dusky Warbler records per year in 1978-2020. Since 2021 is incomplete, we have not included the number in the graph.  

Distribution
Dusky Warbler is mainly, but not exclusively a coastal species. The inland provinces of Drenthe (2) and Flevoland (3) got their first records in 2020. The species has now been recorded in 11 out of 12 provinces. Noord-Brabant (NB) is the sole province without a single Dusky on the list.  During the 2020/2021 influx, records came from nine provinces. Besides NB, there were no proven reports from Overijssel and Utrecht, provinces that did have records prior to 2020. The vast majority of the migrants is found along the coast, but note that winterers are almost exclusively found more inland, in more eutrophic areas than the nutrient poor dunes.

Since 1978, Noord-Holland (NH) and Zuid-Holland (ZH) account for most records (in 2020/2021 it was the other way around), followed by Friesland (FR). This is hardly surprising, since these provinces are coastal, NH and FR have (well-watched) Wadden Sea islands and especially the west coast has a high observer coverage. The (relatively sparsely populated) coastal provinces of Zeeland and Groningen follow at some distance. The numbers in other provinces are low, though six for the land-locked Gelderland is more than decent. See Table 1.

Six municipalities have at least 10 records. The number on the island of Vlieland, FR (32) is impressive. The total number for al Wadden Sea islands combined is 63. Second is Castricum, NH (21, nearly all of them trapped), followed by Texel, NH (15), Rotterdam, ZH (12; mostly on the Maasvlakte headland), Wassenaar, ZH (10) and Katwijk, NH (10).

 

De purple patch of the 2020 influx was the coast between Noorwijk and Hook of Holland, ZH (especially the northern part), with no less than 21 records. One observer (Gijsbert Twigt) found five, other observers found four and three respectively. There’s no doubt that the high density of (good) observers influences the numbers involved. There is for instance a similar pattern for Hume’s Leaf Warbler.

Table 1. Number of Dusky Warbler records per province until early 2020 and in September 2020- January 2021.

Field sightings vs. ringed birds
In most years, both field observers and bird ringers report Dusky Warblers. Since 1978, 60 Dusky Warblers have been trapped. Ringing records represent about a quarter of all records (prior to 2020, this was about 1/3). 2009 and 2010 were the last years in which birds were ringed, while none were found in the field, and 2015 was the last year with the reverse being true. The best years for ringers were 2003 (6), 2007 (5), 2016 (6) and 2020 (12). Castricum tops the list of most birds trapped (19), followed by Vlieland (10) and my own ringing site in Meijendel, Wassenaar (6).

A bird ringed in southeast Sweden on 2 October 2004 was recovered ten days later on Vlieland. Note that there was a similar recovery in November 2020, of a bird ringed in Denmark shortly before its recovery in Belgium. A Dusky trapped on 25 November 2020 in Castricum was retrapped twice witin five days. Both its fat rate and weight increased, from 0-2 (0-5 Busse scale) and from 8,2-8,9 grams respectively (Jan Visser in lit.).   

Figure 3. Dusky Warbler Phylloscopus fuscatus, Meijendel, Wassenaar, Zuid-Holland, 30 October 2014 (Vincent van der Spek). About one in every three Dusky Warblers in the Netherlands is trapped.

Dusky Warbler Vrs Meijendel 05112020Duksy Warbler contact call
00:00 / 00:15

Recording 1. In the field, most Dusky Warblers are found by their calls

Temporal distribution
Dusky Warblers have been recorded in all months between September and April. Records in the first four months of the year are rare, however. The first winterer was only found in 2007 and prior to 2021, only five individuals had been observed in January or February, of which one remained into March and April. In January 2021, after the 2020 autumn influx, another eight were present (of which four were already found in December 2020). Only time will tell whether winter records are genuinly increasing, or not.  

But by far most birds are found within the final four months of the year. The earliest ever was a bird ringed on Vlieland on 23 September 2014, one of only four September records. Autumn passage is mainly between mid-October and mid-November, though ‘new’ birds can be found into December.  The median migration date is during the final ten days of October. Before 2020 this was also the time with the highest number of records, but during the 2020 influx there was a strong peak between 11-20 October, while there was a (weather related) dip between 21-31 October. This single event therfore caused a slight shift in the graph that perhaps does not reflect the situation for most years (Figure 4; also see Excel file for a graph excluding the influx).    

Figure 4. Records of Dusky Warblers in the Netherlands per 10 days during the autumns of 1978-2020.

Length of stay
Most Dusky Warblers are in a hurry: over 60% left within a day (N=132), and over 85% did not linger for more than three days (N=187). Only ten birds (5%) stayed for 10 days or longer (newly found birds from January 2021 are not included). There’s no clear change between the average duration of stay in the time between late September and mid-November (2 days on average; note that a single bird that stayed for 51 days increased this figure). From late November, the first birds start to settle: from then on, one day sightings form a minority for the first time in autumn (average: 3 days). Birds found between December and February (N=5 prior to 2020) usually stay long, on average 31 days. Only one was seen on one day only.  

Conclusions
Between 1990 and 2019, Dusky Warblers have increased slightly, but steadily. The number of birds observed per year is however still very small. Despite the slight increase, in which the species transformed from a vagrant to a very scarce passage migrant and occasional winterer, the 2020 influx (extending into 2021) was massive and unexpected by all means. No less than 41% of all Dusky Warblers ever observed in the Netherlands were found during the influx. Wintering birds form a new, but still rare phenomenon. The vast majority of the birds is found along the coast, but interestingly winterers are often found more inland.

 

Acknowledgements
André van Loon, Jan Visser and Arnold Wijker provided additional information on trapped birds, Ben Gaxiola and Rutger Wilschut kindly sent sound recordings of observations.

Also see ID Dusky Warbler on calls by Thijs Fijen