Search Mouser for PIR sensors and look at the datasheets for what is available in terms of analog/digital outputs, detection ranges, and field of view.
Some ideas you can try out:
Differential FOV PIR:
- Use two rather narrow FOV sensors. Aim one at the pond surface and aim the other in the same direction but above the pond surface.
- Use a narrow FOV sensor and a wide FOV sensor. Aim the narrow FOV at the pond and aim the wide FOV sensor straight up. Since the FOV is wide and the sprinkler is low it should be able to capture tall things around it, including behind it. The result would be that as long as a human is nearby the sprinkler just never turns on.
In either case, you set things up so that the sprinkler turns on only when the sensor, and only the sensor, aimed directly at the pond detects something.
PIR + Proximity Sensor
PIR sensors have a pretty wide FOV, even the narrow FOV ones so the above methods might not pan out. But you could use a PIR sensor with a proximity sensor (such as near-IR) which is available in very narrow FOVs and aim it up to distinguish whether something tall is in the way. The sun could cause issues though for such a upward pointing sensor.
Get a sensor with analog output and a MCU or comparator circuitry and find a threshold of non-zero movement to delineate between large warm bodies and small warm bodies moving around. The problem is PIR sensors give stronger readings the warmer a body is, the larger it is, the faster it is moving, and the closer it is so these are all variables. For example, setting it so the typical warm bird body moving around and bird-like speeds and typical distance could still allow a human moving slowly and far enough away from the sensor to trigger it.