# Ocean Chlorophyll-a Concentration and the Extension of the Migration of Franklin’s Gulls (Leucophaeus pipixcan) in Southern South America

## TL;DR

Franklin’s gulls adjust their migration to Chile based on food availability along their route, as indicated by ocean chlorophyll levels.

## Contribution

This study shows how ocean productivity influences the migration extension of Franklin’s gulls, linking environmental conditions to migratory patterns.

## Key findings

- Higher chlorophyll-a concentrations in Peru during winter correlate with fewer gulls reaching Chile.
- Gull populations in Chile and Peru show a negative correlation, suggesting redistribution based on food availability.
- Ocean productivity along the migration route affects how far Franklin’s gulls migrate.

## Abstract

Some migratory birds do not reach the same destination every year. Instead, they may change their itinerary depending on environmental conditions, especially food availability. Franklin’s gull (Leucophaeus pipixcan) is among these species, arriving in variable numbers on the coast of Chile during the southern summer. In this study, we used 18 years of bird-count data from three estuaries in central Chile to investigate whether ocean conditions influence the number of gulls reaching the area. We focused on chlorophyll-a (chl-a), a pigment found in phytoplankton, which is commonly used as an indicator of ocean primary productivity and food availability for marine food webs. We found that in years when chl-a concentrations were higher along the Peruvian coast during winter, fewer gulls continued their migration to Chile. Then, when productivity was lower in Peru, more birds arrived in Chile. This suggests that Franklin’s gulls adjust how far they migrate depending on where food is more abundant along their route. These findings provide evidence of how ocean productivity can contribute to bird migration, highlighting the need to consider oceanographic conditions when planning conservation strategies for migratory species in the face of environmental change.

Although many long-distance migratory birds choose stable wintering sites and staging posts, irruptive migrants may exhibit considerable interannual variability in their migratory patterns, often depending on food availability. The Franklin’s gull (Leucophaeus pipixcan) is a common long-distance migrant along Chile’s coast during the austral summer. Using census data from three estuaries in central Chile (2006–2023), we analyzed variation in summer populations in relation to chlorophyll-a (chl-a) concentration along the migration route, used as a proxy for food availability. The best model predicting the number of gulls reaching Chile included a negative effect of chl-a concentration on the Peruvian coast (0–10° S) during winter (June–July). Considering the time lag associated with the transformation of phytoplankton into seagull food, this result suggests that primary productivity along the route may influence how far south these birds migrate in search of food. We also found a negative correlation between the summer abundance of Franklin’s gulls in Chile and an eBird index for the species in Peru during the same period, suggesting redistribution of individuals between the two countries in response to resource availability. Models such as ours provide a useful tool for understanding and managing populations of migratory waterbirds.