Sunday, April 10, 2016

A closer look at breeding White-eyed Vireos in Ontario

As a biological consultant I work and follow Species at Risk (SAR) in Ontario very closely, as they essentially dictate my workload with respect to development applications. I consider myself someone who feels strongly towards the protection of our biodiversity and I've often wondered how and why certain species are classified as at risk and others aren't.

In recent years there seems to be an overall shift in the type of species being classified as at risk. In the late 80's/90's species that were designated as at risk were generally species whose ranges barely extended into Ontario (e.g., Prothonotary Warbler, Barn Owl, King Rail, etc.). In contrast to this earlier 'phase' in species at risk protection, the last 10 years has seen many common species, albeit declining at alarming rates, classified as at risk (e.g., Barn Swallow, Eastern meadowlark, Bobolink, etc.).

I think this recent phase in SAR protection has allowed some great work to be done for protecting species like Eastern Meadowlark and Bobolink. A bi-product of these 'umbrella' species being protected is that many other species (and not just birds either) that are declining and found in the same habitats (as BOBO and EAME as an example) are also being protected (e.g., EAKI, VESP, SAVS, FISP, etc.). However, in 'analyzing' these 'phases' of SAR protection there are a few species that were 'missed' during the 1st phase in the late '80's/90's, with White-eyed Vireo being one of them (I think).

White-eyed Vireos are an increasingly rare breeding species, found exclusively in the extreme SW part of the province. For fun I asked a few people I hold in the high regard with respect to songbird populations in Ontario: 2 out of the 3 people said <12 pairs/year; the 3rd person didn't specify a breeding population.
There is a general lack of information on the species likely in part because the species isn't classified as at risk and therefore doesn't receive funding allotments that species like Loggerhead Shrikes or Acadian Flycatchers may.

During the 1st Ontario Breeding Bird Atlas (OBBA) it was postulated that because the species was (and is still, relatively) common during spring migration, due to birds overshooting from the main US range, that the species was going to expand into southern Ontario as a breeder and become more established. The population estimate in reading the 1st OBBA was roughly estimated to be ~50 pairs, with 2 squares having a population estimate of between 2 and 10 pairs. WEVIs were recorded in 19 atlas squares (2 confirmed, 8 probable, and 9 possible):

Compare that with the 2nd OBBA (2001-2005) where WEVIs were recorded in 15 atlas squares (4 confirmed, 4 probable, and 7 possible) and it is relatively clear that the expansion predicted during OBBA 1 has not materialized, in fact WEVI has become scarcer, both in terms of spring migrants (personal observation) and breeding birds.

It's hard to say exactly how much of a population reduction the change from the 2 atlases represents; the total decrease of atlas squares is 21%, though in reality this doesn't mean too much. Taking a further look, eBird provides a good tool to document the species, particularly in the past 10 years, when eBird usership has dramatically increased. It is, however, clear that there are very few records during the months of June and July (over the past 10 years), as shown below:

No records were noted from Pelee Island.

Currently White-eyed Vireos are classified by the MNRF as S2/Imperiled ("very few populations; 20 or fewer locations, making it vulnerable to extirpation").

Looking into this a little more by comparing the data presented above with data we know regarding a similar species in Ontario -- the Acadian Flycatcher, is designated as an 'critically endangered' species. Acadians are also listed as S2, with a national population of 30-50 pairs (likely closer to 35 pairs). Both WEVI and ACFL have very similar geographic ranges.

Both species according to the Breeding Bird Survey have exhibited relatively flat population trends since 1966; however, to be fair WEVI has undergone +0.4% vs. ACFL at -0.4%, per Sauer et al. 2013/BBS.

ACFL screenshot for June/July for the past 10 years.
While both species occupy largely the same geographic ranges, they occupy different habitats: ACFLs are in older growth forests, while WEVI are found in old field/thicket style habitats. The last 30 years has seen some dramatic changes in populations of both guilds that inhabit these habitats. Species like the Yellow-breasted Chat have nosedived in Ontario (and the northeast), while the forests that existed in the 1970s/80s have matured and some species have increased that prefer these habitats. Having said this, I would expect WEVI to occupy pretty similar habitats to Chats and would expect that WEVI has also experienced a fairly dramatic decline, though few would notice this decline.

Anyways this is becoming a long blog post; I'd be interested to know what others think about WEVIs in Ontario! Should it be listed as a SAR? Are there more birds than I think? Are they increasing? Declining? Is WEVI declining in Ontario similarly as YBCH?


  1. Great post Ken. An interesting observation and well backed up with data. As an observer who lives in Middlesex but doesn't get much chance to bird Middlesex during the breeding season(!) my impression of the number of records per summer is that they are around and being found in 'average' quality habitat (ie/ patch size is not large nor particularly refined) in 'unexpected' places (ie/non-traditional) but only 1-2/year. My understanding of their requirements south of the border is that they will take to small patches of sometimes poor habitat as well, and we seem to have a decent amount of it here in Middlesex/Elgin/Lambton (even Oxford). When I am out in the field and find Orchard Oriole (which has exploded- kind of the opposite of WEVI if you think about it) I am thinking WEVI in some situations, but I don't find them. When I was working on Long Point in the early 90's we had a nesting pair of WEVI that we regularly visited and the male hardly sang. This could be an aspect of their biology in Ontario to ponder. In some parts of their wintering range that I've been to, WEVI are pretty common still. I don't know why we haven't experienced the predicted increase of the first Atlas and I agree that they are more scarce as a migrant at the big lookout locations on Lake Erie now. Definitely worth a closer look. _ Peter Burke

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