Wednesday, September 26, 2012

Perception Vs. Reality: Altitude and Nesting Birds on Vancouver Island

Humans are funny animals. And birders, naturalists, and conservationists are no exception. Even though our knowledge of the natural world is vastly greater than the average persons, we still often miss the obvious, even when it is, well, obvious. Nowhere is this more true, than in the perceptions many birders have here on Vancouver Island, as to the altitudinal preferences of nesting birds. Dark-eyed Juncos are a good example of this, as was shown recently in a discussion on one of the local internet bird chat groups. The perception of those living in more urbanized areas, seems to be that this species prefers to nest at higher elevations, because they don't appear to nest in non-forested backyards in suburbia, but they do arrive in good numbers in Autumn, so they must be migrating down the hilsides in the winter. Is this the case? Well, yes. Sort of. And it kind of misses a point that birders spend a lot of time agitating about. That of habitat protection.

 The subspecies of Dark-eyed Junco present on most of Vancouver Island is J.h. shufeldti. It prefers to nest in forests with a certain type of canopy closure. These types of forest can occur at almost all elevations found on Vancouver Island. Thus, it is influenced far more by habitat, than it is by elevation. As can be seen using the BC Breeding Bird Atlas maps, on Vancouver Island Dark-eyed Juncos nest in most squares from the coast right through the interior areas of the island.
 In winter though, they tend to prefer areas with some tree cover, but larger openings. Forest edges, hedgerows, and well landscaped back yards tend to be ideal Dark-eyed Junco habitat, and these areas will host large flocks of the birds. Are all of these birds moving into these areas from higher elevations though? No. They are not.

 In autumn, numbers at lower elevations on Vancouver Island appear to increase for at least four different reasons.

1) Dark-eyed Juncos in the nesting season exist either in pairs, or in small family groups. In Autumn, they come together into much larger feeding flocks. This makes efficient counting of the birds easier. It also gives the appearance that there are more birds in the immediate area than there were in the nesting season.

2) Dark-eyed Juncos on Vancouver Island do migrate attitudinally. As snow increases at higher elevations, more and more birds move to lower elevations.

3) Dark-eyed Juncos in most of the interior of the province, as well as birds in Alaska and the Yukon, migrate south in the winter. Some of these birds will end up on Vancouver Island. These longer distance migrants likely exists in the millions, and thus the numbers on Vancouver Island should increase substantially in the Autumn and winter.

 4) As noted previously, Dark-eyed Juncos also use somewhat different habitats in winter, than they do in the nesting season. Sometimes, even at the same sites. Here in Moorecroft Regional Park, of which about 93% is covered in forest, Dark-eyed Juncos are one of the most numerous nesting birds. I have found nests from less than a meter above sea level, to the highest point in the park, which is about 60 meters. Essentially, everywhere in the park with appropriate habitat. Right now though, it is hard to find any juncos outside of the more open areas, such as parking lot and picnic area margins. And by January, it will be surprising to find any Dark-eyed Juncos at all in the park. I have no idea where these birds go in the winter. Although it is possible that they undertake some type of longer migration, my guess is that they simply move a few kilometers to where the habitat is more to their liking. It is entirely possible that the birds that nest in Moorecroft actually move up in elevation in winter, heading up the hill to areas where there has been more clearing of forests for residential developmant. Probably one of the few sites on Vancouver Island where this is the case.

 There are quite a few nesting birds on Vancouver Island that many like to equate with an altitude higher than the main urban and residential areas. The truth though, is that we come to know these birds at higher elevations, simply because this is where humans have yet to seriously alter the habitat, and we have seriously altered vast tracts of habitat along the southeast of the island . If we built our subdivisions and industrial sites above 1000 metres, and left the lowland forests intact, we might just get the idea that Sooty Grouse, Vaux's Swifts, Olive-sided Flycatchers, Steller's Jays, Hermit Thrush and Varied Thrush, were all low elevation nesters.

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