Why do scientists consider the evolutionary relationships of cuckoos uncertain?

Scientists consider the evolutionary relationships of cuckoos uncertain for several reasons, primarily due to limitations in the fossil record and ongoing debates regarding their classification.12

• Scarcity of Fossils: Cuckoos are not well represented in the fossil record. Because they prefer forest habitats, their remains are less likely to be preserved compared to birds living in open or aquatic environments, making it difficult to trace their evolutionary history.1
• Classification Challenges: While some evidence, such as osteological and myological studies, has suggested that parrots might be the closest relatives of cuckoos, this relationship remains distant and uncertain.2
• Genetic Complexity: DNA studies have identified six distinct groups within the order, including Old World parasitic cuckoos, American cuckoos, and various ground-dwelling species. The complexity of these groups has led some authorities to propose that certain subgroups, such as the coucals, should be classified as separate families.2

Additionally, while turacos were once grouped within the order Cuculiformes, most modern ornithologists now classify them as a separate order, further reflecting the shifting understanding of these birds' evolutionary placement.2