Firstly, which type of chip should you choose. Avoid obscure modalities which might be cheap. ASK 64 chips are out there and cheap because they are used in industry, but scanners are a problem, few can read them and hand scanners which can do it are fewer - we have a couple, but the read range of the ASK64 microchips appears very poor.
Think about who might want to check the wildlife, if its only ever going to be a University Dept then perhaps an oscure format with virtually no readers might suffice. Although you will probably spend on scanners and engineering what you save on microchips!
Use an ISO format, a trovan FDX-B. For truly unique identification you can consider a Trovan Unique microchip - but the these are not ISO compatible and will require special scanners.
The standard microchip used in pets, the trovan ISO FDX-B is simplest. Scanners are cheap and widely available. You may even get data from far afield when the animal or bird is scanned out of interest.
The photo to the left shows a trovan ISO FDX-B on the left and an ASK 64 on the right. Note the reduced winding on the ASK 64, reducing the shape of the field generated and so its read range.