Survivorship and life history strategies in relation to migration distance in western and semipalmated sandpipers in Perú

This thesis explored the relationships between life history, migration distance, survivorship components of fitness, and molt strategies of Western and Semipalmated Sandpipers in one of the most austral non-breeding sites for both species, at Paracas, Perú. I asked how migration distance relates to pre-migratory preparation, survivorship and migratory decisions for different age classes and ecological circumstances between species and within populations. I focused particularly on how timing of first breeding relates to survivorship and thus future overall fitness. I found that adults from both species prepare for northward migration, but no juvenile Western Sandpipers did so, confirming a non-migratory over-summering ‘slow’ life history strategy for more southerly non-breeding populations. Juvenile Semipalmated Sandpipers showed bimodality in migration strategy. Most showed no migratory preparation, but ~30% fattened, molted into breeding plumage, and performed partial post-juvenal wing molt (PPW) during the pre-migratory period. The frequency of PPW is positively related to culmen length (as a proxy for eastern breeding birds with a shorter migration distance). To decompose survivorship between migrant and oversummering (resident) Semipalmated Sandpipers, I used a multi-state model with 5 years of data and found survivorship 8 percentage point higher for oversummering juveniles and 21 percentage points higher for oversummering adults compared to same aged migrant birds, as expected as compensation for the loss of a breeding opportunity. I estimated annual survivorship with an open robust multi state model using 7 years of mark-resighting data from several thousand shorebirds marked at Paracas. As predicted by some migration theories, both species had higher annual survival estimates than those obtained previously at non-breeding sites further north. Western Sandpiper juveniles also had substantially higher annual survival estimates than adults, in line with the predicted survivorship benefits needed to offset their delayed reproduction. I found that the size of the survival advantage in juvenile Semipalmated Sandpipers is migration distance dependent. Western, but not Semipalmated Sandpipers showed a negative relationship in survival with the ENSO warm phase, probably due to the former’s closer association with the Pacific migratory flyway. Finally, I corroborated that the size of the survival advantage is distance dependent. My results provide novel information on non-breeding shorebird survivorship and perspective on the interrelationships that drive avian life history strategies. I confirm that Paracas is also a site with high demographic value.