A major challenge for real-time streaming overlays is to distribute high bit-rate streams with uninterrupted playback. Hosts usually have sufficient inbound bandwidth to support streaming, but due to the prevalence of asymmetric links in broadband networks, the bottleneck is the aggregate, overlay-wide outbound bandwidth. If this bandwidth is less than what is required to forward the stream to the overlay members, then a large number of users potentially experience poor playback. We argue that for successful streaming in bandwidth constrained situations overlays need to be able to adapt to the aggregate available bandwidth. We present four bandwidth adaptation policies for tree-based streaming overlays and evaluate their efficiency using a large-scale emulation testbed with realistic broadband link characteristics.