2017's post-breeding migration season: acrola galore

Post date: Oct 2, 2017 10:47:38 AM

Having some available time due to the interruption of my constant-effort ringing site in Sweden, I was able to spend c. 1.5 months in Portugal during the autumn migration period of 2017. From 7 August to 19 September, I carried out 23 ringing sessions at Salreu marshlands that lasted c. 5 hours each, starting with the first light and using two sites:

(1) line paludicola (5X15m nets) and old ricefield (4 doubles + 4 singles of 12m each), which were dry this year;

(2) edge of an active ricefield (4 doubles + 4 singles) and extra 15 m net placed between a reedbed and a Typha/Scirpus stand (the ricefield had c.5 cm of water).

The chief objective was to trap as many Aquatic Warblers (Acrocephalus paludicola) as possible using two (diurnal) song playbacks at each site, and colect biometrics, blood and feather samples (for which I had the appropriate permits issued by ICNF).

The Aquatic Warbler (acrola) is a globally-threatened species that I showed to occur in Portugal, and at this particular site, in relatively high numbers during the autumn migration. Previous work on this species in Portugal was published here and here. As a member of the Aquatic Warbler Conservation Team, and as a researcher on ecology, migration and evolution especially of marshland passerines, I aimed to increase our knowledge on their abundance, phenology, distribution, fattening dynamics etc, which have fundamental as well as conservation implications.

The total number of birds caught are described in the Table below.

This represents a record number of acrolas for a single season at this site, which was surely a consequence of the greater ringing effort relative to previous years. This larger number of birds allowed to finally make some local retraps, which showed unequivocally that this species uses Salreu marshlands for refuelling and not just for a short stopover (resting) on their way to sub-Saharan Africa. It is interesting that I had five different acrolas in two sessions, and six on another (all at site 2), but did not beat my record of seven birds in a single ringing session established at line paludicola in 2011. This line was dry this year (as with all Salreu marshlands except for the active ricefields) and its vegetation is overgrown, so perhaps I should change its name! (though I did trap two acrolas there this year, in pure reedbed.)

Adult Aquatic Warblers

It is also worth mentioning that a friend and collaborator (LPS) also performed weekly ringing sessions at Salreu marshlands using a much larger number of nets placed in the active ricefields c.300 m from my site 2, and caught an additional 13 Aquatic Warblers, including one with a French ring. In fact, three of my retraps are of birds ringed by LPS. Furthermore, a friend and amateur photographer caught one Aquatic Warbler on camera at Salreu that had colour rings, and even though I could not trap it, we now know that this bird was ringed in Lithuania on its breeding grounds; whereas one of my birds ringed in 2015 was retrapped this year in Poland. Finally, another fellow ringer (PQT), caught an Aquatic Warbler at a new site, from which LPS was able to get data and samples!

Regarding other species, it is noticeable that the overall numbers are quite low, so the proportion of Aquatic Warblers is very high! Notable absentees are Wrynecks (Jynx torquilla), frequently trapped at this site during the autumn migration, and Iberian Chiffchaffs (Phylloscopus ibericus), whereas the threatened, local Reed Buntings (Emberiza schoeniclus lusitanica; my main study species), were trapped in very low numbers... It is funny that I had more foreign controls of Sedge Warblers (Acrocephalus schoenobaenus) than local retraps! (although LPS seems to have got quite a few local retraps early in the season).

Just caught one of this juvenile Woodchat Shrikes this year, but it was retrapped showing a huge increase in body mass!

I seldom trap Wheatears, but this juvenile was caught in an active ricefield!

Given all of this, I suppose I made good use of my free time and personal investment. Hope to get something published in a few months time using all data collected since 2010!

JMN