AMERICAN ROBIN
Turdus migratorius
| Records | 38 records of 36 birds |
| Trend | No significant trend |
| Mortality rate | 11.1% |
| HL | 2.95 |
| HT | 2.81 |
| Variability Index | 1.60 |
| Irruptivity Index | 3.113 |
| CR | 0.0556 |
| CF | 0 |
| Last Record | Eastbourne, Sussex, February 2022 |
At A Glance
Hover over the circles on the map to view a breakdown of records by recording area.
Records in Detail
Click on the map pins to see the records in detail, or scroll through the table.
Analysis
Records by Year
American Robin is one of only a handful of American landbirds with records dating back to the pre-BBRC era, with three Category B records from Ireland dating back as far as 1891. The ‘boom-and-bust’ cycles typical of many Nearctic landbirds are not reflected as strongly in this species, but the occurrence pattern still shows some level of disparity as evidenced by the variability and irruption index scores for this species (see top). Nevertheless the species is a true rarity in all contexts, although with a distinct bias to the SW of England, with Cornwall, Isles of Scilly and Devon accounting for a third of all birds recorded in the UK and Ireland (12 out of 36) which follows the pattern of many Nearctic vagrants in autumn.
However, when looking more generally, a more interesting picture begins to emerge: the occurrence pattern morphs more into that of the Siberian thrushes than the classic Nearctic vagrants, as a surge of mid-winter records (14 records in the months December – February) suggests that arrivals are capable of bedding in for the winter in gardens and other urban areas. This eastward filtering may occur very early, as evidenced by a record from the middle of Surrey in mid-October 1984; however, with mid-winter records from Isles of Scilly (December 1963), north Cornwall (December 2003) and, most intriguingly of all, a bird which landed aboard a trawler off the Skellig Islands in SW Ireland (January 1965), there may be propensity for this species to move out of North America in mid-winter in a manner similar to Killdeer Charadrius vociferus, if not to the British Isles directly then maybe via trans-Atlantic shipping.
This species may also arrive with Continental thrushes in late autumn; it has reached Norway, Denmark and Sweden at least once each, and several winter and early spring records from Spain (1999, 2014, 2017), Belgium (1965) and the Czech Republic (1874) (Tarsiger, 2023) in a similar pattern to Siberian thrush species illustrate the (at least) theoretical possibility of vagrant American Robins tagging onto eastward-moving winter thrushes.
Records by Arrival Date
As Figures 2A – 2D illustrate, the arrival dates of American Robins in the British Isles have not changed significantly either generally (Figure 2A), or during the main autumn arrival period from October – November (2C). There is also no significant correlation between latitude (2B) or longitude (2D) and the arrival date of this species, although non-significant trends do occur and are illustrated by the black lines. The apparent increase in latitude of discovered American Robins over the course of the winter is notable and perhaps suggests that birds move north after arriving before settling into winter quarters.
Intriguingly, while the longitude of American Robins “arriving” in Britain has not changed significantly, the latitude is showing an apparent, sustained decrease since the early 1980s, illustrated in Figure 2E. The lack of recent records in the western part of the British Isles* compared to the early 1980s illustrated in Figure 2F also suggests a potential southward and eastward shift in the records; however, the statistical significance of these trends have not been firmly established, and indeed the amount of variation in the data suggests that any trend is unlikely to be significant on a statistical level. Many of the records at the top of Figure 2F come from western Ireland and the lack of recent records there is likely to account for the perceived drop.
*An unconfirmed record from County Cork in autumn 2023 would be notably further west than other records in recent times.
While exact trends in records of American Robin in the British Isles are hard to establish, it is, however, possible to identify clusters of arriving American Robins to establish how unusual any American Robin records are in terms of their arrival date in relation to the latitude and longitude at which they are arriving. Figure 2G (below) show the results of cluster analysis carried out on the data inputted to Figures 2B and 2D.
As shown in the map, the clustering algorithm identified 5 distinct “groups” of arrivals of American Robin records. These are:
- Records in spring (green)
- Records from the SW of England and the southern Irish Sea, any season (purple)
- Autumn records in N & W Ireland and the Outer Hebrides (red)
- Autumn and winter records in NE Scotland and the Northern Isles (blue)
- (Primarily) winter records in the SE and NE of England (orange)
Owing to the lack of any solid metric to test the ‘fit’ of these clusters, conclusions are hard to draw, but the separation of records by the algorithm does merit comment. As a first point, there are potential outliers that have lower ‘degree of fit’ within their cluster compared to other records, the two most evident being the 1988 Inverbervie record and the 1984 Surrey record with their incongruous dates of arrival.
Another interesting point is the allocation by the algorithm of the western Irish records to the Hebridean cluster rather than the southern Irish Sea/SW England cluster. While the spatial separations between the three areas of interest suggest this is counterintuitive, Figure 2H shows that the distribution of the arrival date across the three areas is much more congruent with the clustering analysis results. While there isn’t really enough data to infer whether the arrival dates of the Hebridean records are significantly separated from those of the western Irish records, we can say that there is a very significant statistical difference between the western Irish arrival dates and the ones from SW England and Wales (t10 = 5.938, p = 0.000141). This further supports the assertion that birds arriving in western Ireland do so later than records in SW England and Wales, which hints at the very outside possibility of differences in the arrival mechanisms to these areas.
Figure 2G: Map showing American Robin records grouped by cluster, 1950 – 2022.
To find out more about the methods used in the clustering analysis, please read our methods document here. [Note, December 2023: COMING SOON]
Records by Length of Stay
Figure 3A shows the distribution of the stay lengths of American Robins in the British Isles, and this distribution does conform to an exponential decay pattern at a statistically significant level (D17 = 0.765, p = 0.0002). The estimated rate of decay was roughly 0.087, meaning that, in general, the number of birds that stay n+1 days will be 91% lower than those that stay n days. The stay length has not changed over time (Figure 3B) and does not seem to vary significantly by latitude (Figure 3C).
An apparent decrease in stay length the further west the bird occurs (Figure 3D) was supported by third-degree polynomial regression analysis being statistically significant (R2 = 0.195, p = 0.016). On this basis, it can be reasonably concluded that American Robins generally tend to stay longer the further east they are found, albeit with some exceptions. The lengths of stay also conform to the expected pattern during seasonal progression, with the stay lengths of American Robins increasing as winter progresses from autumn and decreasing again into spring, as shown in Figure 3E. However, this trend is not statistically significant and the pattern can only be used as a generalisation not a hard-and-fast rule.
