Does your trail camera capture video or image sequence? You probably only need to analyse 1 frame every few seconds or less if you just want to get deer alerts.

I would go with a DIY solution, since Rekognition will get expensive quick ($1/1000 images). If you did 1fps running continuously, you'd pay $87/day.

Nearly any ML framework with an ImageNet pre-trained model will work. Accuracy will be better if augmented with more labeled deer images (ImageNet probably only has frontal poses) but should be alright straight out of the box.

You could run the whole thing locally on a cheap android phone for free instead of AWS cloud pricing, 1fps will be fine with TensorFlow or MXNet on Android. Then just setup email alerts.

There was a recent post on HN about getting a tensorflow model running on a raspberry pi to detect trains. The blog post describing the process was quite well done. I bet it would apply quite well to detecting deer.

https://news.ycombinator.com/item?id=13603225

http://svds.com/tensorflow-image-recognition-raspberry-pi/