Nitrogen fixation occurs naturally in the soil by bacteria. As the nitrogen is consumed by different species up from one trophic level to another, from plants to insects to bird, the nitrogen gets more and more concentrated.

Birds are fairly high up the food chain, and their excrements are very rich in various nutrients, including nitrogen, phosphorus and potassium.

The Haber Process is faster but requires a lot of energy, and provides no phosphorus or potassium, so you have to source it elsewhere. Also it requires no technology and can be installed pretty much anywhere.