Partial migration in a subtropical wading bird in the southeastern United States

Abstract

The 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 describing migration patterns of 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 United States, 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 nonlinear 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 in 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.

Urban specialization reduces habitat connectivity by a highly mobile wading bird

“…In this system and others, individuals captured in urban areas show isotopic signatures of anthropogenic food in their diet [41,71], suggesting that urban specialization could increase food intake rates [72] (though it can have mixed effects on health outcomes: [41,47]). In addition to these benefits, specialization could be maintained genetically if it reduces gene flow between urban and natural sub-populations; we focused on the nonbreeding season in this study, but individuals that use urban areas in the nonbreeding season can also be less likely to migrate long distances to breed (e.g., in wood storks: [73]) and ibis have recently begun nesting in urban areas in Palm Beach County (S. Hernandez, pers. obs).…”

Joint seasonality in geographic and ecological spaces, illustrated with a partially migratory bird

“…The lowest value of geographic overlap occurred between the months of January and July/August, and is still greater than most values of climatic overlap. This can be in part explained by their migratory strategy: Wood storks are partially migratory birds (Coulter et al 1999, Picardi et al 2020), which results in some individuals migrating, while others stay resident year‐round. As a consequence, the overall (monthly) distributions at the population level are not as contrasted as would be for a fully migratory species, especially between migration phases.…”

“…Here, we describe the seasonality in the distribution of wood storks, which are partially migratory birds (Coulter et al 1999, Picardi et al 2020), in both geographic and ecological (specifically climatic) spaces. We built on the approaches advocated by Fieberg and Kochanny (2005) in the geographic space only, and by Broennimann et al (2011) in the ecological space only, to propose a general framework that allows joint investigation in geographic and ecological spaces.…”

Toward a new framework for restoring lost wildlife migrations

“…Such flexibility occurs across diverse vertebrates including fish (Meager et al., 2018), birds (Fraser et al., 2019), mammals (Xu et al., 2021), and herpetofauna (Jourdan‐Pineau et al., 2012). Perhaps more surprisingly, all taxonomic groups include some individuals that go so far as to alternate between migratory and nonmigratory behavior (e.g., striped bass [ Morone saxatilis ], Secor et al., 2020; wood storks [ Mycteria americana ], Picardi et al., 2020; spotted salamanders [ Abystoma talpoideum ], Kinkead & Otis, 2007; and elk, Eggeman et al., 2016).…”