Biological and Environmental Factors Associated with the Autumn Migration
Timing of North American Raptors
Authors: Honkomp Nora R. 
Published: January 2025

Migration is an energetically expensive and risky event in the life cycles of
many organisms, and migration impacts survival and reproductive success through
direct and carry-over effects. The timing of migration departure from the summer
breeding grounds is important for determining the probability of survival and
physical condition upon arrival at wintering sites. Many internal and external
factors contribute to departure timing. It is important to study these factors
across species, populations, and individuals to determine which traits and
conditions are most important for shaping departure timing and its variability.
Improving our understanding of the factors that affect organisms’ phenological
cycles, particularly the understudied events in those cycles, is necessary to
predict how populations, and the communities they are a part of, may be affected
by environmental change. I studied the life history correlates and weather
conditions associated with the timing of departure for North American raptor
species.

In the first chapter, I used data from eBird to calculate spatially-explicit
autumn departure timing for 15 migratory raptor species across the United States
and Canada from 2011 to 2021. Using these departure dates as a response variable
and accounting for the uncertainty involved in these estimates, I then examined
the relative importance of five life history traits and latitude in explaining
departure timing between species and the effect of three life history traits on
variation in departure timing within species across space and over time. I found
that an interaction between migration distance and latitude affected departure
timing. The negative effect of latitude (northern birds departing before
southern birds) was stronger in short-distance migrants compared to
long-distance migrants. Further, socially migrating species tended to depart
earlier than species that migrate alone, and larger-bodied species tended to
depart later than smaller species. Finally, long-distance migrants,
diet-specialists, and social migrants had more consistent departure timing
compared to short-distance migrants, diet generalists, and solitary migrants who
were more variable in timing.

In the second chapter, I selected the four raptor species (Broad-winged Hawk,
Osprey, Swainson’s Hawk, and Turkey Vulture) with the greatest eBird data
availability and assessed their departure timing within a latitudinal band
across the northern US from 2002 to 2021. Using weather data from Daymet and a
sliding window analysis, I determined the most likely periods of sensitivity to
four weather variables for each species' departure timing. I then combined these
season-specific weather variables to build a full weather model for each
species. Maximum temperature was important for determining the departure timing
of all species, but the direction of effect varied (some advancing or delaying
departure) depending on the time of year and species. Minimum temperature and
precipitation was important for determining the departure time of several
species, and snow-water-equivalent only explained the departure of Broad-winged
Hawks. Finally, I used the full weather models for each species and data from
NA-CORDEX to forecast departure timing of all four species to 2075. The forecast
analysis showed that over the next 50 years, Swainson’s Hawks will advance their
departure timing (depart earlier in the year) and Turkey Vultures will delay
departure (depart later in the year). There were no significant temporal trends
for the timing of Broad-winged Hawk autumn migration, and Osprey departure
timing is expected to advance very slightly.

Combined, these analyses provide insight into the biological and environmental
factors affecting raptor autumn migration timing in North America. This
information furthers our understanding of the relationships between life history
traits and departure timing and provides new insight into factors associated
with annual variation in departure timing, which may underlie vulnerability to
changing environmental conditions. Additionally, it identifies unique
environmental factors related to the timing of autumn departure for four common
species and provides estimates of future changes in an important part of their
full annual cycles.