Arnhem AEB413

18 October 2014, Meijendel, Wassenaar, the Netherlands

© 2018 by Vincent van der Spek

HUME'S LEAF WARBLER
analyses of an increased vagrant
(2016)

 

Hume's Warbler, The Hague, The Netherlands, December 2012. The area between Noordwijk and The Hague has a remarkable number of records.

- Overview of all Dutch Hume's Warbler records till 2017
- Timing and distribution
- The best time to twitch one, is not in the peak season!
- One area in particular is remarkably good

An updated version was published in Dutch Birding in 2018 (in Dutch, with English summary).

 

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Introduction

The first Hume’s (Leaf) Warbler (Phylloscopus humei) record for The Netherlands dates back to 1958. Back in the eighties it was still an extremely rare vagrant. Much has changed since then. 2016 was even the best year so far for the species. But it wasn’t until the late nineties that Hume’s  Warbler records increased significantly. Up till 2016, 63 records (64 birds) have been accepted by the Dutch rarity committee CDNA. These records are analysed in this article.

Increasing numbers

Species with (on average) less than two records a year over the past 30 years are reviewed by the Dutch rarity committee. The average for 1987-2016 is 1,9 birds. This of course includes the ‘poor’ mid-eighties and the slightly better, but still not “booming” early to mid-nineties. The average over the past 20 years (1997-2016) is at 2,5 and the last decade (2007-2016) it has increased to 3,2 – though it must be said that the exceptionally good 2016 influences the average significantly. However, the increasing trend over the past 30 years is obvious and it is to be expected that records do not have to be submitted anymore within a few years. Figure 1 shows the annual totals between 1987 and 2016. In the pre 1987 periode there are another five accepted records from 1958, 1975, 1981 and 1982 (2). Old - formerly accepted - records of wintering Greenish Warblers probably referred to Hume's Warblers, though they have not been accepted. 1999 (5), 2003 (6), 2012 (6) and especially 2016 (9) were obviously good.

Figure 1. Annual totals of Hume’s Warbler in The Netherlands 1987-2016. Only the year of discovery added in the case of wintering birds.

However, despite increased knowledge about both the identification and timing of the species, increased observer coverage, internet (where observations of “Yellow-browed Warblers” can be checked) and better equipment to both clinch the identification and to convince the rarity committee, it must be noted that it’s still not an annual bird. This century alone, there are no accepted records for 2002 and 2010, while 2006 only had a wintering bird that was already found in 2005.

Timing: when to find a Hume’s yourself?

Hume’s Warbler irregularly winters, possibly in an increasing fashion. As wintering birds often stick around for weeks or even months on end, this is probably how most birders get to see one. However, it’s actually less rare earlier in the year: most are in fact reported during migration (October-November). Figure 2 shows when birds were first found over the course of the year. Months are divided in so called decades (ten day time frames; which might be unusual for some foreign readers).

Hume's is a late migrant, significantly later than Yellow-browed Warbler (Ph. inornatus), though the two do overlap (and numbers of wintering Yellow-browed seem to be increasing as well). The earliest date for a Hume’s Leaf Warbler is 11 October, more than a month later than the earliest proven Yellow-browed at 8 September (source: www.waarneming.nl).

Figure 2. Number of Hume's Warblers found per 10 day timeframe in The Netherlands 1956-2016 (n=64)

Mid-October till mid-November is especially good, though there’s also a slight peak at the end of December. One could speculate that the late peak is a Christmas holiday effect – I know for a fact that it’s true for one of the birds I found! Wintering birds truly seem to arrive late: only one (!) Hume’s found in either October or the first two decades of November stayed until (early) December, while all others were (long) gone by then.

 

In spring, the latest birds to leave their wintering grounds were reported on 22 April (n=2), though most birds are gone weeks earlier. Since only three ‘new’ birds were discovered in February, two in March and none in April, there’s no (obvious) spring migration.

 

Timing: when to twitch a Hume’s?

Figure 3 shows when Hume’s Leaf Warblers were actually present in Holland and thus includes all decades of long-staying birds after they were found. In other words, this figure could be interpreted as when twitchers have the highest chance to see a Hume’s Leaf Warbler. The mid-November migration peak is still visible, but obviously the best time to jump in a car or on your bike seems to be between mid-December and early January.

Distribution: where to find or twitch one?

Like many vagrants, Hume’s Leaf Warbler is mainly – but not strictly – a bird of the coastal provinces, though Zeeland only has a meagre two records. It’s been recorded in 10 out of 12 provinces, as well as on the Continental Shelf. Both Noord-Brabant and Drenthe got their second record in 2016. Only the inland provinces of Overijssel and Limburg are still waiting for their first record. The number of records per province is shown in table 1.

Figure 3. Presence of Hume’s Warblers per 10 day timeframe in The Netherlands 1958-2016 (n=64)

Table 1. Number of Hume's Warbler records per province 1956-2016

With 16 records (17 birds), the coastal area between Noordwijk and The Hague, Zuid-Holland – a 23 km stretch – has had a remarkable number of records. Hence, this small area hosts a quarter of all Dutch records! And this even excludes two birds that were seen relatively nearby (Noordwijkerhout and Delft). Apparently they really like it there, since all but two birds stayed for more than a day. Other hotspots are the Wadden Sea islands (14 records), especially Texel, Noord-Holland (5) and Terschelling, Friesland (4). The islands seem to be a migrating hotspot: only one bird wintered, while all others were short-staying birds in October and November.

 

A migration hotspot like Maasvlakte, Zuid-Holland on the other hand, has only one record. Remarkable since this is only 13 km south of The Hague as the warbler flies. It seems like the further south you get from the Golden Edge (Noordwijk-The Hague), the rarer they become! Not only in the already mentioned Zeeland, but it’s for instance still a very rare bird in Belgium, too: in 2016 only the 12th and 13th for the country were found. There are more records within biking distance from my house in The Hague!

 

Conclusions

So birders that desperately want to add Hume’s Leaf Warbler to their list of self-found birds have the highest chance if they start looking for them between mid-October and mid-November, and at end of December. And they should all move to the Golden Edge in Zuid-Holland of course! I’ll be more than happy to show you the good spots.

Acknowledgements
Max Berlijn, Johan Buckens and Jan Hein van Steenis contributed to this article in one way or the other.

References

www.dutchavifauna.nl