A “suture zone” is a zone where geographically otherwise separated faunas meet. New Guinea's Bird's Neck Isthmus potentially constitutes a double suture zone, where the avifaunas of New Guinea's northern and southern lowlands (otherwise separated by the Central Dividing Range), and also the avifaunas of the Vogelkop Peninsula to the west and New Guinea's main body to the east, are in contact. The Isthmus is a north/south corridor only 60 km long and only 160 m in maximum elevation. The watershed lies much closer to the north coast than to the south coast. We surveyed the northern 60% of the Isthmus's interior, previously unexplored ornithologically, to just south of Lake Yamur. We observed 178 species on the Isthmus, including some disproportionately in hilly terrain and others in flat terrain, and identified visually or vocally informative Isthmus populations of 30 geographically variable species. At the middle of the Isthmus, southern forms prevail over northern forms, and both western and eastern forms are represented. At least eight southern populations and one northern population extend across the Isthmus to the opposite coast. We present examples of hybridization, incipient speciation, achievement of sympatry, and ecological segregation. Three species confined to the mountains on the northern watershed descend to or near sea level on or near the Isthmus. We report the first ornithological survey of the waterbirds of Lake Yamur, Indonesian New Guinea's third largest lowland lake.