Reporting Guidelines – Blue-winged and Golden-winged Warblers
Mark Read, Ontario Parks
February 18, 2022
Although visually distinct, there is actually very little genetic difference between Blue-winged (Vermivora cyanoptera) and Golden-winged Warblers (Vermivora chrysoptera). A 2016 study by Toews et al., identifies a total of just six areas of the genome that vary between the species, four of which appear to directly control plumage details. Put another way, the genetic difference between the two species is just 0.03% (or vice versa, they are 99.97% similar). In Gustave Alexson’s excellent summary of that paper (which by the way contains some great photos), he suggests this “…could be considered akin to the differences between humans with and without freckles.” He goes on to say, “The research also shows that golden-wings and blue-wings have even less genetic differentiation than two subspecies of the Swainson’s Thrush, the Olive-backed and Russet-backed forms.”
The purpose of this article is not to go into the detailed genetics of the two species but instead to illustrate the need for caution when reporting observations of this species-pair, whether that’s to eBird, your local naturalist club, or in this case to Atlas-3. Accurately reporting the correct species is critical to better understanding distribution, habitat preferences, and zones of hybridisation. These data can then be used to address conservation needs.
Blue-winged Warbler – Tony Castro
Golden-winged Warbler – Caleb Putman
If you go no further than this, here are the key points:
If they’re visually distinct, what’s the issue?
It has been known for almost 200 years that the two species hybridise where their breeding ranges overlap. The offspring of these pairings are fertile and are able to reproduce with either species, or other hybrids. Classic hybrid forms include Brewster’s Warbler and Lawrence’s Warbler. Until recently, it was believed that Brewster’s Warbler was an F1 (first generation) hybrid between the two species and that Lawrence’s Warbler was likely a second-generation hybrid or backcross. However, Baiz et al. (2020), now suggest that the reality is more complicated and that the F1 generation hybrids are more similar to the parent (certainly in terms of underpart colouration) than previously believed.
Figure 1: The different species and hybrids of Vermivora warblers. From Baiz et al. (2020) The Auk
So, we’re still good at this stage. A genetically (or phenotypically) pure Blue-winged or Golden-winged Warbler is readily identified visually. Even the hybrids known as Brewster’s and Lawrence’s Warblers are distinctive. However, anything other than this should always be reported as Golden-winged x Blue-winged Warbler (hybrid).
During Atlas-2 (2001-2005) there wasn’t a great deal of overlap in the distribution of the two species, with Blue-winged Warbler far more common in the southwest (see Figure 2).
Figure 2: Distribution of Blue-winged and Golden-winged Warblers during Atlas 2 (2001-2005)
Figure 3: Estimated breeding range of Vermivora warblers in 2019, based upon eBird data 2005-2020
Although Figure 3 shows the distribution of both species across their entire breeding range, a couple of interesting observations can still be made. With respect to Golden-winged Warbler, it appears that the breeding range remains much as it was twenty years ago with a minor expansion (in Ontario) west and north into the Thunder Bay and Kenora regions. The story of the Blue-winged Warbler is quite different, with an apparent small increase in the south-west of the province but a very clear expansion into the south-east where the populations of both species appear to overlap in a way that they didn’t during Atlas-2.
So not only are we expecting a far greater overlap in the distribution of the two species during Atlas-3 but, in the last few years, we have also become far more aware of the complexity (or should that be simplicity) of the genetics of the two. A study by E. Rondel during the 2015-2016 seasons (personal communication, April 2021), primarily in the Frontenac Arch and Carden Alvar, leads us to the next challenge: that of the songs.
An Auditory Challenge: The real problem for observers comes from the overlap in songs. It has always been known that both species will, at times, sing the primary (or Type 1) song of the other species but that has been poorly understood or documented. Personally, I clearly remember a well-known ‘Golden-winged Warbler’ that used to hang out on Opinicon Road (north of Kingston) that would incessantly sing the primary song of a Blue-winged Warbler. It would also respond strongly to playback of that species, less so to that of the Golden-winged Warbler. Rondel, in her studies, visually confirmed the identity of 269 Vermivora individuals that she heard singing. Of those, 6 individuals were singing the song of the other species, including one individual that would switch between both Type 1 songs successively. Another 25 individuals were hybrids and these would sing the primary song of either species. Converting those figures to percentages, Rondel has discovered that 11.5% of the population she studied are not identifiable to species by song alone; and it is this that we need to be cautious of. To make matters worse, both species also perform a secondary (or Type 2) song, which is typically associated with territorial defence. Rondel’s studies indicate that there is no difference between the Type 2 songs and that the two species cannot be identified by this vocalization alone. Working in the Frontenac Arch at Murphys Point Provincial Park, I have personally observed that ‘our’ Golden-winged Warblers rarely sing their Type 1 songs, preferring their secondary songs. This could be because we have such a high population and defence is more important than ‘advertising’ for a mate but this is supposition. Whatever the reason, this is why it is important to no longer assume the species based upon song alone.
These days, there are numerous sources available to listen to various vocalisations. Below you will find examples from Xeno Canto but other sources include Dendroica, the Macaulay Library, as well as numerous apps.
Although I have picked classic examples of each species’ vocalization above, it is clear from field research that song is an unreliable indicator of species. It will certainly alert you to the presence of a Vermivora warbler but it is important to then try and gain a visual look at the bird too. Of course, the bird’s welfare comes first and you should always be careful to never put the bird under undue stress, but spending an extra couple of minutes in the area will often be all you need.
Summary of key points
- Type 1 song is an unreliable indicator of species – make sure to confirm identity visually.
- Type 2 song cannot be used to identify species at all.
- Record the species or hybrid based upon visual clues. Brewster’s and Lawrence’s Warblers are distinct.
- Other hybrids should be reported as Golden-winged x Blue-winged Warbler (hybrid)
- If you cannot visually confirm the identity of the bird then report it as Golden-winged/Blue-winged Warbler.
Axelson, G. 2016, Golden-winged and Blue-winged Warblers are 99.97 Percent Alike Genetically. Living Bird, Summer 2016. Accessed at https://www.allaboutbirds.org/news/mixed-wing-warblers-golden-wings-and-blue-wings-are-99-97-percent-alike-genetically/
Baiz, M. D., Kramer, G. R., Streby, H. M., Taylor, S. A., Lovette, I. J., & Toews, D. P. (2020). Genomic and plumage variation in Vermivora hybrids. The Auk, 137(3), ukaa027.
Bird Studies Canada, Environment Canada’s Canadian Wildlife Service, Ontario Nature, Ontario Field Ornithologists and Ontario Ministry of Natural Resources. 2006. Ontario Breeding Bird Atlas Database, 31 January 2008. http://www.birdsontario.org/atlas/aboutdata.jsp?lang=en
Maps: eBird data from 2005-2020. Estimated for 2019.
Fink, D., T. Auer, A. Johnston, M. Strimas-Mackey, O. Robinson, S. Ligocki, W. Hochachka, C. Wood, I. Davies, M. Iliff, L. Seitz. 2020. eBird Status and Trends, Data Version: 2019; Released: 2020. Cornell Lab of Ornithology, Ithaca, New York. https://doi.org/10.2173/ebirdst.2019
Toews et al., 2016, Plumage Genes and Little Else Distinguish the Genomes of Hybridizing Warblers. Current Biology 26, 2313–2318, September 12, 2016 © 2016 Elsevier Ltd. http://dx.doi.org/10.1016/j.cub.2016.06.034