Recent contributions to this journal have discussed the impact, actual and potential, of the first significant treaty in the field of avian conservation, the 1902 Paris Convention for the Protection of Birds Useful to Agriculture. This article seeks to expand upon that discussion by providing further information on the nature and development of the international legal system, highlighting in particular the various factors that affect the practical impact that such instruments are likely to exert. It suggests in particular that, in addition to the rigour and suitability of the substantive provisions, it is necessary to consider the effectiveness of the mechanisms established for the treaty's implementation, and the number and identity of the states that can be persuaded to participate, since the importance accorded to state comsovereignty within the international legal order is such that relatively little can be achieved without their individual consent. It has taken many decades of experimentation for treaty negotiators to devise legal instruments that are capable of bringing about significant progress in the conservation field, and the pioneering nature of the 1902 Convention was such that its practical impact was always likely to be limited. Its true significance is therefore primarily historical, serving as a form of stepping stone to the more advanced instruments of the modern era.