Youtube/Vimeo API limitations

I want to heavily use the Vimeo API (and possibly YouTube too), to get thumbnails for videos. So my question is, are there 开发者_Python百科any limitations to how many calls you can make to both of the services?

I've read on a YouTube FAQ that

If your application does send API traffic in excess of the quota limits, the API will return a too_many_recent_calls error

My problem with this is that I'm not sure that I actually use the API for YouTube. I get my thumbnail from an URL, which I'm not sure it's qualifying as an API call. The url is something like: http://img.youtube.com/vi/ORdY5xWzKzY/0.jpg

But on Vimeo I'm sure I use the API.

Any ideas to what the limitations are?