If an increase in body size is common for most evolutionary lineages, why do more recent species of birds, bats, and insects tend to be smaller?
environmental constraints. Although I question the idea that birds bats and insects are decreasing in size in general.
How are evolutionary constraints different for species of birds and bats versus terrestrial mammals?
Generally, they don't eat the same things. terrestrial mammals are more susceptible to local water pollution (since birds can fly). Bats however, tend to stay local and they are very vulnerable to Karst topographical issues. Birds tend to be more influenced by global and local magnetic fields (the current hypothesis is that this is how they know where to fly for migrations). There are other differences, just think about the differences in ecosystems, behaviours and where they fall in the tropic scale.
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