Do you want to know when there's a bird in your bird feeder? There's a circuit for that!

This uses a PIR motion sensor that works with infrared light. And it will light up an LED when there's a bird moving inside the feeder:

You can get such a sensor for about ten bucks at most stores that sell electronic components.
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Basic Steps to Build:

1. Install the Sensor: Position the PIR motion sensor so it has a clear line of sight to the area where birds will be feeding. Make sure it is sheltered from rain and direct sunlight to avoid false triggers.
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2. Connect the Circuit:
   • Connect the battery to the PIR sensor's power input.
   • Connect the output pin of the PIR sensor to the positive leg of the LED via a resistor of around 330 ohm.
   • Complete the circuit by connecting the negative leg of the LED back to the battery's negative terminal.
3. Mount the LED: Place the LED at a location visible from inside your house, such as near a window, to give you a visual cue when a bird is detected.
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This is how it works

When the battery is connected to the PIR sensor, the sensor will store an image of the infrared light it sees. This process might take 30 to 60 seconds.

Once it has stabilized, the output should go LOW.

Now, if the sensor detects a change in the infrared image, the output will go HIGH for a period of time (that you can adjust via a potentiometer on the sensor).

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This means that when a bird comes into the field of view of the sensor, the LED will light up (since birds, like most other living beings, emit IR light).

Note that this type of sensor will only detect a change in the IR light. So it will trigger when the bird arrives. But if the bird manages to sit still for a bit, it will become invisible.

You can solve this by tweaking the sensitivity of the sensor and the time that the output pin stays high after a detection.

I like this circuit because it's a great example of the fact that you don't need to know a lot of complex theory to build practical and useful circuits for everyday life