Tuesday, November 15, 2016

Early Winter Insectivores in the Western GTA (Part 1)

At the end of October I found this Canada Warbler in Mississauga. This is record late for the area, and really crazy as, per eBird, most Canadas are already at or near their tropical wintering grounds. However, this sighting is perhaps not that surprising given the records of warblers and other insectivores in this area into early winter every year.

In the last six Decembers, at least 16 species of warblers have been seen along Lake Ontario from Toronto to Hamilton*. What are insectivorous birds doing in Canada in the winter? The answer isn't totally clear, but there are a few possibilities. Some are definitely sick or injured birds incapable of accomplishing a long migration. Others may be utterly lost. A third and most intriguing possibility is that global warming and human development are allowing certain species to winter much farther north than they used to. We could be seeing a vanguard of a change in wintering behavior for some populations, much like has famously occurred among European Blackcaps, with large portions of the population now wintering at British bird feeders.

Now, this certainly isn't occurring in quite the same way with our warblers - the vast majority of individuals disappear (and presumably perish) long before spring returns. Only a few species like Yellow-rumped and Pine can survive our winters with any regularity. Still, it wouldn't surprise me if we're seeing a northward shift in the wintering ranges of many of these species into the southern United States, especially with habitat loss in the tropics. Mostly baseless speculation, but fun to think about.

This Yellow-rumped Warbler survived the winter of 2012-2013 in Oakville.

Regardless, seeing warblers in November and December is always exciting, and looking for them and other insectivorous birds is one of my favourite things to do, especially as the chance of a major rarity is pretty good. I've thought about this quite a bit and wanted to write some quick guidelines for what I think helps in finding these birds.

First of all, when should you look? I think this is very weather-dependent. As I write this, it is still very warm, with double-digit temperatures every day and lots of insects still around. As a result, I think there are probably a lot of lingering insectivores around right now. However, they are likely so spread out as to be impossible to find. Once things get cold and food gets scarce, these birds will be forced to move to limited areas where they can still make a living. I think this is usually early to mid-November, but can be earlier or later (as in this year). And keep in mind that this is all speculation with a little bid of anecdotal evidence, and I could be totally wrong!

So where are the birds going to move? There are a few things they are probably looking for:
  • Food. I think this is the main issue. Our birds generally don't seem to have much issue with fairly cold temperatures when food is abundant, but insectivorous birds will of course have a hard time feeding once the insects die off. There are a few places where insect prey can still be found in winter:
    • Sewage treatment plants. There are a few of these scattered around, and the large numbers of midges coming out support a variety of birds each winter
    • Any other open water, whether it's Lake Ontario or a small stream
    • Anywhere warm, especially south or east facing slopes and heavily urbanised areas. 
These tracks belong to Dark-eyed Juncos and Yellow-rumped Warblers feeding on midges around a sewage treatment plant.
  • Heat. I think that the main attraction of these warmer areas is the insects present, but there are surely some benefits in thermoregulation as well. South-facing wooded or brushy slopes and areas surrounded by open water (e.g. the various artificial peninsulas in Lake Ontario) are great, as are any patches of natural habitat in heavily urbanised areas.
  • Other birds. Most songbirds hang out in flocks in winter, and anywhere with lots of other birds is worth checking. Feeders are good at bringing in a wide variety of birds (last winter I had a Hermit Thrush, which doesn't generally eat seeds, momentarily visit my bird feeder with some House Sparrows). Some of the more common species that often seem to be associated with these rarer birds include Golden-crowned Kinglet, White-throated Sparrow, wrens, and American Robin.
  • Corridors. The Lake Ontario shoreline naturally concentrates birds that are on the move, so any appropriate site on or near the shore is likely to be discovered by a desperate warbler. Rivers and other habitat corridors through urban areas would have a lesser effect.
In summary, the best areas are probably those with some combination of a sewage treatment plant, open water, proximity to the lake (and in particular peninsulas), warm sunny slopes and lots of other birds.

So what species can be expected to show up at these sites? There are too many to list, but I'll give a quick overview.

  • Yellow-bellied Sapsucker shows up fairly regularly in warm and sheltered areas.
  • Flycatchers: Eastern Phoebe is occasionally found. Other flycatchers are very rare, and have a high chance of being a major rarity. 
  • Vireos: Blue-headed and White-eyed show up sometimes, everything else is a major rarity. 
  • Swallows can sometimes show up over water treatment plants and other open water, and Cave Swallow is always a possibility.
  • Thrushes and mimids: these groups eat a lot of fruit in winter but still seem to often show up in the kinds of places listed above. In my experience, if you see robins that are foraging on the ground instead of on fruit, this is a good indication that you're in a good spot.
  • Wrens: Winter, Carolina and sometimes House are generally found in warmer, sheltered, brushy areas
  • Ruby-crowned Kinglet is very regular, and often shows up at the same places as rarer warblers and other species.
  • Blue-gray Gnatcatcher is similar to Ruby-crowned Kinglet but less common.
  • Warblers: Probably any species is possible, but they range from expected (Yellow-rumped, Common Yellowthroat) to very uncommon (Nashville, Cape May) to very rare with few or no winter records (Bay-breasted, Canada). The real hope is to find one of the rare western species (Townsend's, Black-throated Gray, Hermit, MacGillivray's) which show up in Eastern North America every winter.
  • Orioles, buntings, grosbeaks, tanagers: These groups generally show up at feeders but could appear in more natural areas too.
Next post I'll show some of the spots in my area that I think are particularly worthwhile.

Part 2

*These are the species I know of (I may have missed one or two): Black-and-white, Orange-crowned, Nashville, Tennessee, Common Yellowthroat, American Redstart, Cape May, Northern Parula, Bay-breasted, Yellow, Palm, Pine, Yellow-rumped, Black-throated Gray, Black-throated Green and Wilson's.

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