Here's why:
* Evolutionary History: Birds evolved from a lineage of theropod dinosaurs, a group of reptiles. This is supported by fossil evidence and genetic analysis.
* Shared Features: Birds and reptiles share many characteristics, including:
* Amniotic eggs: Both groups lay eggs with a protective membrane (amnion).
* Scales: Birds have scales on their legs, which are homologous to the scales of reptiles.
* Similar skeletal structures: The bones of bird wings are modified reptilian forelimbs.
While birds and mammals share some characteristics, such as being warm-blooded and having a four-chambered heart, these traits developed independently in each lineage.
Therefore, the evolutionary relationship between birds and reptiles is much closer than that between birds and mammals.