What sensor do I need to measure a change in physical weight in ounces?

I'm still a noob when it comes to Arduino and electronics. But, I've learned a few things since I purchased an Uno last February. I'm at a point where I'm developing my own projects, but my knowledge of sensors and other components is clearly still lacking at this point.

A project I want to develop down the road triggers a camera to take a photograph when a change in weight is measured.

The practical application would be used in bird photography. What I want to do is set a camera near a bird feeder and focus on a specific pre-placed perch. I want to use some type of weight-sensing component to trigger the camera to take a photograph if a bird lands on the perch. But, I don't want the sensor to appear in the photograph.

I've found a variety of sensors that might be useful for this application at Flex / Force - SparkFun Electronics?

But, my knowledge, as I said, is limited.

Could someone suggest which sensor would be best for my project. I initially thought the load sensor, but I'm measuring changes in ounces and not pounds.

Any help, advice or suggestions is appreciated.

Could you just rig the perch to a spring loaded microswitch instead of weighing the bird (unless you are logging the actual weight of the bird)?