Why might a geographic barrier such as a large river cause th formation of a new species of small rodents but not a new species of birds?

A geographic barrier like a large river can lead to the formation of a new species of small rodents due to their limited mobility and tendency to remain close to their habitat, which can cause them to become isolated from other populations. In contrast, many bird species have the ability to fly across such barriers, allowing for gene flow between populations and reducing the likelihood of speciation. Additionally, rodents may experience different selective pressures on either side of the river, promoting divergence, while birds may not experience the same level of isolation. This difference in mobility and ecological adaptation can lead to varied speciation outcomes between the two groups.