Partial migration in a subtropical wading bird in the Southeastern U.S.

ABSTRACTThe function of migration is to allow exploitation of resources whose availability is heterogeneous in space and time. Much effort has been historically directed to studying migration as a response to seasonal, predictable fluctuations in resource availability in temperate species. A deeper understanding of how different migration patterns emerge in response to different patterns of resource variation requires focusing on species inhabiting less predictable environments, especially in tropical and subtropical areas. We provide the first individual-based, quantitative description of migratory patterns in a subtropical wading bird in the Southeastern U.S., the wood stork (Mycteria americana). Using GPS tracking data for 64 individuals tracked between 2004 and 2017, we classified migratory behavior at the individual-year level using information theory-based model selection on non-linear models of net squared displacement. We found that the wood stork population is partially migratory, with 59% of individuals seasonally commuting between winter ranges in Florida and summer ranges elsewhere in the population range (migrants), and 28% remaining in a single area in Florida year-round (residents). Additionally, 13% of storks act as facultative migrants, migrating in some years but not in others. Comparing the distribution of residents and migrants suggests that different migratory strategies might be associated with the use of different or differently distributed resources, possibly including food supplementation from human activities. The existence of facultative migrants shows the potential for plastic change of migratory patterns. Partial migration in wood storks may be an adaptation to high heterogeneity and unpredictability of food resources. We suggest that future research should focus on wading birds as model species for the study of partial migration as an adaptation to heterogeneous and unpredictable environments, by comparing populations of the same species across different wetland systems and sympatric populations of species that differ in their resource acquisition mechanisms.

Productivity and Habitat Modeling of Wood Storks Nesting in North and Central Florida

Rainfall, surface water levels, location within the state, and area and types of habitats (n = 29) surrounding wood stork Mycteria americana colonies in North and Central Florida were analyzed at 10-km, 20-km, and 30-km radii around each colony to examine their relationship with fledging rate and number of nests during 2003–2005. Seven variables within 10 km, 14 variables within 20 km, and 6 variables within 30 km of colonies were correlated with fledging rates. Fledging rate and number of nests were significantly associated with both wetland and nonwetland area and habitats. Among all the variables, fledging rate was most strongly associated with rainfall during the preceding 12–24 mo. Both larger colonies and colonies in North Florida had higher fledging rates. Although some variables had a positive association and other variables had a negative correlation with fledging rates, results were not consistent across all three radii, which suggests that the effects of hydrologic and habitat variables differs with increasing distance from a colony. The size of a wood stork colony was sensitive to a larger number of variables and varied by distances from the colony. Colonies were smaller in the northern part of Florida, and coastal colonies were larger than interior colonies. Because wood storks often use ephemeral foraging sites closer to a colony early in the season and those sites may not be available later in the season, wood storks may shift to alternate, more distant sites and habitats later in the season. A hypothesis is proposed whereby wood storks establish their colonies using proximate clues of prey availability based on the effects of past rainfall and certain preferred habitat types. These proximate cues to prey availability and foraging substrate surrounding a colony are detected by wood storks before the onset and during the initial nesting season. However, the long-term stability of a colony may ultimately be determined by yearly rainfall patterns and habitat variables at larger distances and by fledging rates that contribute to recruitment of nesting birds and an increase in number of nests.