Why can bill shape be misleading when classifying passeriforms?

Bill shape can be misleading when classifying passeriforms because it is evolutionarily plastic, meaning it adapts relatively easily to selective pressures such as changes in feeding habits. While bill shape was historically used as a primary taxonomic character to determine family relationships, it is now understood to reflect a bird's food preferences more accurately than its evolutionary lineage.1

Because of this plasticity, relying on bill shape can lead to incorrect classifications. For example, Hawaiian honeycreepers were once mistakenly split into four different families and 18 genera based largely on their diverse bill shapes, even though they are now recognized as a single family. Furthermore, similar bill shapes can arise in unrelated groups due to convergent evolution, further complicating its use as a reliable indicator of taxonomic affinity. Consequently, modern ornithologists must consider bill shape alongside other anatomical, behavioral, and genetic data to accurately classify passerine birds.12