What phenomenon explains why northern Gambel’s subspecies tend to be slightly larger overall?
Answer
Bergmann's rule, relating to heat retention
There is a subtle geographic trend concerning the size variation among the subspecies of the White-crowned Sparrow. Birds found in the far north, such as the Gambel’s subspecies, tend to be slightly larger overall than those located further south. This pattern aligns with Bergmann's rule, which is an ecogeographical principle stating that within a broadly distributed taxonomic clade, populations and species of larger body size are generally found in colder environments. The larger body mass is advantageous for thermoregulation because it helps the bird retain heat more effectively in colder climates.
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