Across most of the tropics it is often difficult to tell whether extensions of known range by bird species reflect genuine change or are artefacts of increased observation. Yellow-vented Bulbul Pycnonotus goiavier is a mainly Sundaic passerine known from southern Indochina (southern Thailand, Cambodia and southernmost Vietnam) for many decades. The first Lao record came only in 1996 but since then the species has spread north through Lao PDR along the Mekong and some major tributaries. All Lao records are from the lowland plains, in non-forest habitat. Inspection of the locations of records and of birdrecording patterns in Lao PDR over time indicate that this bulbul's northern expansion, paralleled by one in adjacent Thailand, is surely not simply the belated finding of a species long-overlooked. Both climate change and habitat change (forest conversion and fragmentation) are plausible causes. That five of six non-forest passerines that have colonised (or massively expanded in) Lao PDR in the last 30 years are from the south, with one from the west but none from the north, may suggest climatic change as the cause, but their rates of expansion far outstrip northward temperature movement. While Yellow-vented Bulbul's change in national status is obvious, even though Lao PDR is one of the least known countries for birds in South-east Asia, disentangling the roles of these two simultaneous major effects in avifaunal change in South-east Asia is challenging.