WYOMING – A recent study led by the University of Wyoming showed a multitude of factors shaped the timing of birth in mule deer. The five-year study based in western Wyoming showed migrating deer have a lot to balance when it comes to birth timing.

Scientists long believed animals match offspring birth with the peak green-up forage at the birth site. This study concluded only deer that migrated long distances and followed the flush of spring green-up from low-elevation winter ranges to higher-elevation summer ranges were able to match birth with peak green-up. Other deer migrated short distances and gave birth earlier and out of sync with green-up.

Researchers integrated 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 during the study.

“In contrast to the existing theory, which predicts that conditions at the birth site should shape optimal birth timing, our results provide a clear example of birth timing being shaped by trade-offs arising from events occurring away from the birth site and from other parts of the annual cycle,” researchers wrote in the journal, Ecology.

In general, mule deer in western Wyoming give birth in early June, generally after migration is over but early enough for fawns to grow large enough to survive the onset of winter. The deer that were part of the study included animals that migrated long distances between winter and summer ranges, as well as those that migrated shorter distances.

Researchers also found most deer completed migration well before giving birth. Across the five-year study animals completed migration, on average, 23 days before giving birth – and does that ended migration earlier gave birth earlier.

The findings showed only animals that surfed the green wave and ended migration just before giving birth matched birth with peak green-up, whereas most gave birth after peak green-up. Although matching birth with peak green-up likely increased access to high-quality forage, doing so resulted in delayed birth and less time for offspring to grow and develop before fall migration.

Researchers also noticed 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, which allowed them to complete migration before giving birth without sacrificing the ability to surf the green wave along their migration route.

“Conceptualizing birth timing through the lens of the full annual cycle helps to illuminate additional trade-offs that migrants face when balancing reproduction with migration, foraging and accumulation of fat serves,” researchers wrote.

In addition, they said the link between movement tactics and timing of birth has important conservation and management implications. For example, human-caused disturbances to migration patterns could harm deer reproduction until animals have time to adjust. Additionally, the diversity of movement behaviors among western Wyoming mule deer is important to preserve, as animal populations with greater life-history diversity have been found to be more able to withstand environmental changes.

The University of Wyoming researchers were Ellen Aikens, Samantha Dwinnell, Tayler LaSharr, Rhiannon Jakopak, Matt Kauffman and Kevin Monteith. Other contributors were Gary Fralick and Jill Randall of the Wyoming Game and Fish Department, Rusty Kaiser of the U.S. Forest Service and Mark Thonoff of the Bureau of Land Management.