What observations were made about the finches on the Galapagos island?

Firstly, they aren't true finches. They were collected by Darwin on his second voyage. The main observations are there beak shapes and sizes. Depending on their location in the islands, the finch's diets vary and then because of their varying diets they have varying beaks which have adapted to their food sources. The variances in the finches and their beaks despite all living in the same island chain spurred Darwin's theory of evolution.

Since Darwin's time there have been many expeditions to the Galapagos Islands and new discoveries have been made such as the Large Cactus Finch males have two different beak types. Those with the shorter beaks have one distinct song and those with longer beaks have a different distinct song. They both feed on the same type of cacti, but in different ways due to their beak types. It's thought it's so the birds can all still feed when the food is scarce.