Why aren't there as many birds in Central Park the year?

I have been so concerned that there are not nearly as many birds in Central Park this winter as last year.  And no chickadees at all.  (And it's been frustrating for my photography too!)

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: