A five-year study of mule deer does and newborn fawns in western Wyoming shows that migrating deer have a lot to balance when it comes to birth timing.

The study led by University of Wyoming scientists challenges the long-held assumption that animals match offspring birth with the peak green-up of forage at the birth site.

The researchers’ work appears in the journal Ecology.

To examine the factors shaping birth timing, researchers integrated highly detailed data on female deer, including movement data from GPS collars, body condition and gestational development from ultrasonography of adult female deer, and intensive fieldwork to locate newborn fawns.

Among the study’s findings:

  • Most deer completed migration well before giving birth: Across the five-year period of the study, animals completed migration, on average, 23 days before giving birth.
  • Does that ended migration earlier gave birth earlier.
  • Does appear to trade off early birth and increased time of offspring growth with matching birth to peak green-up.
  • Animals that migrated long distances had less developed fetuses in March, thereby allowing them to complete migration before giving birth -- without sacrificing the ability to surf the green wave along their migratory route.

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