Pretty Solid game!
This game needs music and sound effects, otherwise its fun! Increasing difficulty is a must, however. Give me a reason to try and keep destroying them :) another thing I found is the game was actually easier the more seaguls that got destroyed, since they would have to go further across the screen before going down a level. If you can get them to speed up as more are shot down, it will be more challenging and more fun in my opinion. Overall though good job!