I think you're taking that bit a little out of context.
We had a really small team and simply set out to make the best game we could. We had the opportunity to make one more game, as Rovio didn’t have any more money left. I pitched lots of ideas when we thought about what kind of game we wanted to make. I had dozens and dozens of game ideas with various characters. The birds were originally part of a different concept. I had created a piece of concept art featuring the birds – which everybody loved at the studio – so we settled on using them.
The way that I design games is by drawing pictures. I have to see how the game looks in order to understand it. I try to visualise the gameplay for myself, and drawing it helps. It gives me a feel for what the game’s about. We spent a lot of time thinking about what kind of game we could make that could be a hit. We looked at a lot of games on the web, such as Flash-based games, and we studied what kind of games people liked to play. We wanted to minimise the risks, so to speak, and go with what seemed to work for people. Two-dimensional physics-based games were really popular at the time, especially the kind of games where you launched something in the air. So we decided on that genre and then put the birds that everyone liked into the game. We got the publishing deal for Angry Birds based on that single piece of concept art.
http://www.edge-online.com/features/meet-man-behind-angry-birds/#nullThe concept of the birds was a "hit" with the team because of their experience. They had strong intuitions about what would work.
edit:There is some combination of personal taste and comprehension of the market that combines for success. Both things are important. I did not mean to say the latter was more important than the former.