Partial Migration, Oversummering, and Intermittent Breeding by Shorebirds
Ronald C. Ydenberg: Read the article
To breed or not to breed: explaining partial migration in shorebirds
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Life history trade-offs are hallmarks of evolutionary biology theory. Due to finite resources, all organisms must prioritize and differentially invest in many traits and behaviors. A classic example includes the trade-off between growth and reproduction: because both are expensive, many species have separate, distinct periods of life devoted to each task. A less familiar example is the concept of intermittent breeding. Organisms may choose to skip a breeding opportunity and not have offspring during that period, as foregoing the opportunity may confer fitness advantages in some other areas of life.
Long-distance migratory shorebirds spend the non-breeding season (October to March) in coastal South America, then migrate north to breed in the Arctic during the northern hemisphere summer (April to September). However, some individuals remain in or near their non-breeding grounds and do not make the journey north to breed (termed “oversummering”). Why is this happening? In his article “Partial Migration, Oversummering, and Intermittent Breeding by Shorebirds”, Ronald C. Ydenberg outlines that the majority of hypotheses surrounding these intermittent migration patterns focus on prevention (as in, the birds are constrained from migrating due to sickness, inexperience, sexual immaturity, etc.). Ydenberg argues that oversummering is an evolved life history trait, where the decision to remain in non-breeding grounds and not migrate and reproduce represents a trade-off for fitness benefits elsewhere. Ydenberg proceeds to develop a mathematical model for the oversummering behavior in shorebirds.
The basic assumption of Ydenberg’s model is that oversummering conveys a survival advantage because the mortality experienced is lower than that of migration. For oversummering to be favored, this survival gain must outweigh the cost in fitness of missing a breeding opportunity. Ydenberg expands the model by distinguishing between two age classes of birds: yearlings (first-year breeders) and adults. Since yearlings produce half as many offspring per year as do adults, they will more readily oversummer due to the reduced fitness cost of not reproducing. Individual health differences can also affect migration success, generating variation in survival within each age class. This expanded model is therefore able to predict whether all or some individuals migrate (“complete” vs. “partial” migration).
Ydenberg tests his model by comparing two species that are very similar except for their migratory behavior: western sandpipers (Calidris mauri) and semipalmated sandpipers (Calidris pusilla). In western sandpipers, 100% of yearlings oversummer, while 100% of adults migrate (representing complete migration). In semipalmated sandpipers, 28% of yearlings oversummer, and 19% of adults oversummer (representing partial migration). Using actual field data from Paracas, Peru, Ydenberg’s model correctly and accurately predicts these proportions: the key difference between the two species was their migration mortality. Semipalmated sandpipers were much more likely to die during migration, thereby increasing the survival gained by oversummering. However, higher migratory mortality also meant fewer future breeding opportunities, raising the cost of a missed breeding opportunity. Together, these patterns yielded a partial migration pattern in the semipalmated sandpiper, while the lower migratory mortality in the western sandpiper yielded a complete migration pattern. Thus, Ydenberg’s model was strongly supported.
Ydenberg’s work has important implications for behavioral ecology and conservation. Ydenberg succinctly connected broader evolutionary theory to intermittent breeding in shorebirds, demonstrating how trade-offs can lead to complex behaviors at the population level. His model provides a solid framework for future work in other migratory species. Additionally, by understanding the oversummering behavior in shorebirds, we can better characterize their distributions and abundance, thereby aiding in developing more robust population models and, ultimately, wetland conservation efforts. To breed or not to breed: that is indeed a worthy question!
Faye Romero is a PhD candidate and NSF GRFP Fellow at the University of Rochester in Dr. Nancy Chen’s lab. She is investigating the underlying genetic causes of inbreeding depression in the Federally Threatened Florida Scrub-Jay. Faye is also an avid swing dancer and birdwatcher, and is passionate about increasing accessibility to STEM careers for young people.
Image caption: Faye Romero holding a Florida Scrub-Jay.