You obviously agreed to participate. Do you have any documents? I think you voluntarily agreed to sign them, didn't you? Why don't you do like Kaczynski and go to the woods, running away of this shitty society if you don't agree with it? That's because society feeds you and gives you the comfort you want and, in return, you are forced to follow their rules. Contratualism itself isn't even about signing something, it's about how the society evolved to what it is today, with humans don't living in their natural state at nature anymore due to a general consensus of creating a civilization with laws made to avoid its collapse.

As I said before: this specific bird is more relevant to the society than a random deer, you liking it or not. A judicial system isn't made exclusively by reason. A judge always considers the context, the people involved and even the feelings behind a happening. If you've killed a old man just because you wanted to do so, you'll face heavy consequences. If you've killed a old man who raped your son in revenge, then this context and your feelings will be considered in the judgement and you'll still face consequences, but they're definitely going to be less intense than in the first case.

Otherwise, let's kill the President and then a random ordinary stranger and see what happens. They're just two corpses, aren't they? The consequences should be the same for these two murders, since they, objectively, are the exactly same thing in your logic: someone shot someone. As I stated before: the bird has its importance and that's why someone who kills this specific animal is going to be in jail in no time.

You can agree or disagree with that. You asked why someone who killed a bird would be in jail and I gave you the answer. You asked why a deer isn't as important as this specific bird and I gave you the answer. Now it's up to you agree or disagree with that.