Why are most piciform bird families found in tropical regions?

The tropical distribution of most piciform families suggests that these birds represent specialized remnants of a once more numerous and diverse array of arboreal species. It is hypothesized that many elements of the order may have become extinct during the geologically recent expansion of the highly successful and ubiquitous order Passeriformes, which shares many similarities with the piciforms.1

While the order is distributed across all continents except Australia and Antarctica, the woodpecker family (Picidae) is the only group within the order that is widespread outside of tropical regions. Because the fossil record for most piciform families is nearly nonexistent, the evolutionary history of these groups remains largely open to speculation, though it is generally inferred that the existing tropical families are the specialized survivors of a formerly broader evolutionary radiation.12