I asked this question to an expert Birder on the NYC yahoo forum birds, Phil Jeffrey:
"Chickadees are irruptive into Central Park, and some linger after big flight years, but strangely they do not thrive there so the numbers dwindle after time.
Bird #s are probably low because of two things:
1. good food crops up north
2. mild weather
with #1 causing very low numbers of irruptives or more traditional migrants, and then in a normal winter we'd expect a few more once the weather hardens, but in this case there really hasn't been much snow to push the passerines further south. I've got very few White-throated Sparrows in my yard, for example, but in tougher years (bad crops/bad weather) I might have far more. As the ongoing Snowy Owl invasion attests to, not everything is down to current food/weather conditions, but a lot of time it is. (The Snowy Owl invasion is due to a mini population explosion this summer under very favorable breeding conditions, so it is ultimately to do with food/weather)."
Chickadee picture from a few years ago:
