Redpoll Ringing in Grampian 2023 - 25

The redpoll is an attractive small finch which breeds in Scotland, northern England and Wales. It was formerly split into three different species (lesser, common and arctic) but taxonomists have now amalgamated these three into a single species. Redpolls are on the red list of conservation concern due to a sharp population decline in the late 20th century. In summer their diet consists mainly of invertebrates, but outside the breeding season they are largely dependent on the seeds of birch, Alder, larch and spruce. In years when these seed crops are locally plentiful redpolls are largely sedentary, but when these food sources fail they will move south to southern England, with some birds even crossing the channel to winter in France and the low countries.

 

Redpolls react well to recordings of their song, and large numbers can be caught in mist nets when this is played in areas of low trees and shrubs to bring birds down from the treetops. We also had success when playing the song in a reedbed where we presume birds were coming to drinking pools to access water (Figure 1).

Figure 1. A catch of redpolls coming to drinking pools at Loch of Leys in September 2025 (Innes Sim).

Grampian Ringing Group (GRG) has ringed several thousand redpolls over the years, with especially high numbers ringed in 2016 (2,325), 2019 (3,856) and 2020 (906). Numerous recoveries resulted from these ringed birds which are summarized in previous blogs (https://grampianringing.blogspot.com/2019/12/redpoll-madness.html; https://grampianringing.blogspot.com/2020/). GRG has continued to ring redpolls in recent years and here we report on those ringed during 2023-25. 

The majority of birds ringed during 2023-25 were ringed during the main dispersal period in September, October and November (hereafter autumn; 93 – 98%; Table 1). Recovery rates outside Grampian were 1 in 330 (2023), 299 (2024) and 281 (2025); movements into Grampian were a lot higher in 2024 (1 in 187) than in 2023 (1 in 1,320) and 2025 (1 in 561; Table 1).

Table 1. Total numbers ringed, numbers ringed during autumn, number of birds recovered outside Grampian and number of birds ringed elsewhere and recovered in Grampian.

 The majority of redpolls were ringed at four sites: Brathens, Slack Wood, Loch of Leys (all Aberdeenshire) and Tiendland Forest (Moray). All of these sites are mixed birch, larch and spruce forests, except for Loch of Leys which is dominated by birch trees. 2023 and 2025 were birch mast (heavy seeding) years and 62% and 94% of redpolls were ringed at Loch of Leys in those years. In contrast, 2024 was not a birch mast year and only 0.5% of redpolls were ringed at Loch of Leys, with 92% ringed at the other three sites. Birch trees mast every 2-3 years, while spruce and larch mast every 3-5 years, and presumably redpolls adapt to feed on the most plentiful seed in each year. So, it appears that redpolls particularly targeted birch seeds in the mast years of 2023 and 2025, but then switched to feed on spruce and/or larch seeds in 2024. Presumably redpolls will remain close to their breeding areas as long as the seed food source remains abundant, and only move south in autumn in response to diminishing seed resources. 

It appears that most of the birds that pass through Grampian in autumn travel as far as southern England by late autumn/winter, with only a single Belgian control recorded during 2023-25 (Figure 2). The majority of birds appear to migrate down the eastern half of England with only two birds being controlled in Gloucestershire, close to the Welsh border. There are only two records of birds ringed during the winter of 2023/24; one from Belgium in December 2023 and one from Nottinghamshire in February 2024 (which may have already been heading north). So, we don’t have a clear idea yet of where these migrants actually winter – perhaps we might get some more information on this in the coming winter. And we also have little information on where these autumn migrant redpolls breed, although one bird ringed on South Uist in late May may indicate that at least some birds breed on the west coast of Scotland. There is clearly still a lot to discover on redpoll movements! 

Figure 2. Map of North East Scotland Redpoll Movements

 

  

 

Biometrics and movements of Redwings ringed in Grampian 1989 – 2024

(Innes Sim)

 Grampian Ringing Group ringed 701 Redwings between 1989 and 2024, and here we test if there is variation in wing length and body weight between different age classes and in different months of the year. Data on wing length and body weight were available for 6 months (Jan, Feb & Sept – Dec), but sample sizes were low in Jan, Feb and Sept (n = 14, 8 & 11 and 12, 7 & 3, respectively), so these three months were excluded from the analyses.

 Wing length

Figure 1 shows the distribution in wing length, with a mean of 119 mm and a range of 106 – 131 mm. There was no significant difference in mean wing length between the three age classes; first-year (119.1 ± 3.1 s.d., n = 438), adult (119.7 ± 3.2, n = 240), unknown age birds (119.7 ± 3.5, n = 18) (ANOVA F = 2.11, d.f. = 2, 696, P = 0.12). There was, however, a marginally significant difference in mean wing length between the 3 months: Oct (119.7 ± 3.3, n = 364), Nov (119.2 ± 3.2, n = 100), Dec (118.9 ± 3.1, n = 198) (ANOVA F = 3.40, d.f. = 2, 659, P = 0.03). Mean wing length apparently decreased during the 3 months, but the difference of 0.8 mm was small and well within the measurement accuracy and range of measurement error of different ringers (1 – 2 mm). Hence, we conclude that there was no difference in mean wing length, and thus approximate body size, in birds in different age classes or months.

These wing lengths are very similar to a larger sample from the BTO (https://www.bto.org/understanding-birds/birdfacts/redwing). Unfortunately, it is not possible to sex Redwings in the hand but it appears that males have mean wing lengths around 7 mm longer than females (males 122 mm, females 115 mm; https://www.bto.org/understanding-birds/birdfacts/redwing).

Figure 1. Histogram of wing length distribution of 699 Redwings measured in Grampian 1989 - 2024.

 

Body weight

There was no significant difference in mean body weight between age classes (ANOVA F = 2.40, d.f. = 2, 521, P = 0.09), so data from all age classes were combined. Mean body weight increased significantly between Oct and Dec; Oct (62.7, ± 5.6 s.d.), Nov (65.5 ± 5.1), Dec (72.6 ± 6.1) (ANOVA F = 158.68, d.f. = 2, 521, P < 0.001; Figure 2).

Surprisingly, we could find only one published scientific paper on Redwing biometrics in autumn, and none on how these change between months. Mean body weight of birds killed when flying into Bardsey Lighthouse, North Wales, in October 1995 was around 62 g, so very similar to our values in October (https://nsojournals.onlinelibrary.wiley.com/doi/epdf/10.1034/j.1600-048X.2000.310211.x).

 

Figure 2. Mean (± 95% C.L.) body weight of Redwings measured during October to December 1989 – 2024.

 

Why is there an increase in body weight between October and December?

First, we can rule out the possibility that birds arriving in Grampian in November and/or December are simply larger than those in October, since there was no increase in wing length during these three months. The Icelandic race (Turdus iliacus coburni) is longer-winged and presumably heavier than the nominate race (T. i. iliacus), which breeds in northern Europe (https://britishbirds.co.uk/journal/article/status-and-identification-icelandic-redwing-britain), so it seems very unlikely that an influx of Icelandic birds can account for the increase in body weight. We do, however, catch a small but unknown proportion of Icelandic birds in Grampian (Figure 3). A more likely explanation for the increase in body weight is that birds arriving in October, having just undergone a taxing migration across the North Sea from northern Europe, or the Atlantic from Iceland, are in relatively poor body condition. They then increase their body condition as they feed on invertebrates and/or berries, such that by December they have increased their body weight by around 10 g (16%) on average. The origin of autumn migrants in the UK remains obscure but it is thought that many birds subsequently move south to winter in France and Iberia, especially during winters when weather is harsh and berry crops are poor in the UK (https://www.tandfonline.com/doi/abs/10.1080/03078698.2002.9674271). An increase in body weight would allow birds to perform this extra migration leg successfully, as well as helping to cope with harsher winters for those that choose to remain in Scotland.

Figure 3. Icelandic (top) and Nominate (bottom) Redwing races trapped in Grampian in December 2024.

    

Origins of Redwings arriving in Grampian in autumn

There have been only three recoveries of birds to/from Grampian out of 701 ringed during the last 35 years! However, around 250 of these were ringed in 2024 so the recovery rate excluding these is around 1 in 150. Birds ringed in northern Norway and Fair Isle, Scotland, in October were controlled in Grampian in November, while another was shot in Portugal in December having been ringed in Grampian in October. All three were ringed as fully-grown birds and we therefore have no idea as to where their natal areas were. It seems likely that the majority of birds arriving in Grampian in autumn originate in Fennoscandia, although smaller numbers may be from in Iceland (https://migrationatlas.org/node/1782#section1). Numbers in Grampian appear to peak around November and then fall off sharply, presumably as birds make their way further south and west in search of food (especially berry) sources. It seems likely that many of these birds continue south to winter in southern France and Iberia (https://migrationatlas.org/node/1782#section1), although we currently have only a single ringing recovery to back that up.

 Next steps

We currently have a very poor understanding as to the origins of Redwings passing through in autumn and wintering in Grampian, and should attempt to increase numbers ringed by targeting favoured foraging and roosting areas. It would also be interesting to identify the proportion of birds of Icelandic origin in Grampian in autumn and winter, following the guidelines identified in a recent paper (https://britishbirds.co.uk/journal/article/status-and-identification-icelandic-redwing-britain). Perhaps through implementing these two measures we can increase our knowledge of the origins and movements of this attractive, elusive and shy, but fascinating thrush.