Unfortunately if it is a captive kestrel it won't be around for long. The weight is so critical on small falcons that they don't have to lose too much before it becomes fatal. Good call on the IBR as many falconers do register their birds as the fees are so reasonable and if you do get the opportunity to see it yourself look at the legs and see if it has any rings or rather large looking straps of leather that the falconer would use for holding the falcon on the glove or perch. If there are any then I would put some scraps of meat out to keep it going and use the IBR to put a call out for another falconer to capture it.