# Predicting the response of a long‐distance migrant to changing environmental conditions in winter

## TL;DR

This study examines how changes in eelgrass availability affect the energy needs of black brant geese during their migration.

## Contribution

The study introduces an individual-based model to predict how environmental changes impact migratory bird energy needs.

## Key findings

- Eelgrass biomass was the strongest factor influencing the geese's ability to gain enough energy for migration.
- Low eelgrass biomass correlated with lower body mass and survival rates in some years.
- Geese showed some capacity to compensate for low eelgrass biomass in certain years.

## Abstract

Access to high‐quality food is critical for long‐distance migrants to provide energy for migration and arrival at breeding grounds in good condition. We studied effects of changing abundance and availability of a marine food, common eelgrass (Zostera marina L.), on an arctic‐breeding, migratory goose, black brant (Brant bernicla nigricans Lawrence 1846), at a key non‐breeding site, Bahía San Quintín, Mexico. Eelgrass, the primary food of brant, is consumed when exposed by the tide or within reach from the water's surface. Using an individual‐based model, we predicted effects of observed changes (1991–2013) in parameters influencing food abundance and availability: eelgrass biomass (abundance), eelgrass shoot length (availability, as longer shoots more within reach), brant population size (availability, as competition greater with more birds), and sea level (availability, as less food within reach when sea level higher). The model predicted that the ability to gain enough energy to migrate was most strongly influenced by eelgrass biomass (threshold January biomass for migration = 60 g m−2 dry mass). Conversely, annual variation in population size (except for 1998), was relatively low, and variation in eelgrass shoot length and sea level were not strongly related to ability to migrate. We used observed data on brant body mass at Bahía San Quintín and annual survival to test for effects of eelgrass biomass in the real system. The lowest observed values of body mass and survival were in years when biomass was below 60 g m−2, although in some years of low biomass body mass and/or survival was higher. This suggests that the real birds may have some capacity to compensate to meet their energy demands when eelgrass biomass is low. We discuss consequences for brant population trends and conservation.

We studied effects of annual changes in abundance and availability of a marine food, common eelgrass (Zostera marina), on an arctic‐breeding, migratory goose, black brant (Brant bernicla nigricans), at a key non‐breeding site, Bahía San Quintín, Mexico. Using an individual‐based model, we predicted that the ability to gain enough energy to migrate was most strongly influenced by eelgrass biomass, rather than eelgrass shoot length, brant population size or sea level. We compare these predictions to observations from the real system and discuss consequences for brant population trends and conservation.