A photojournal of wildlife, wild flowers and forest life in a small section of Northern Ontario, Canada.
Two years ago I saw my first White Crowned sparrow in the springtime in amongst a big pile of sticks I was getting ready to burn. I've never seen one on the return route though. Did you check out that Sigma 150-500mm lens?
White-Crowns are common here. They appear to me to be the largest of the sparrows. The sigma lens is my main lens and I'm really happy with it. I bought it just as Nikon brought out their 80-400mm lens at much more money. It also will take a 1x and 2x teleconvertor which is available.I hardly ever use my Nikor 70-300mm, which will not take a teleconvertor. I'm happy with all my gear, except I find the D3000 a little slow shooting, but I'm not in the market for a new camera. The wetlands are big in width and you need the reach.I use an 18-55mm for scenes, and for an inexpensive lens I like it. Also you don't want to be near some of those animals and the sigma has the distance about a 150 foot range.
I have been tempted by the Sigma Bigma but my perusal of other people's opinions leads me to think that the effective range of this lens is closer to 400mm than it is to 500mm. So for about the same money I'm leaning toward the Canon 400mm f5.6L. It's interesting that you say the usable range of your 500mm is 150 feet, I guess you could call that your head start!
Let me think through my response.