Rainfall-induced changes in food availability modify the spring departure programme of a migratory bird

Abstract

Climatic warming has intensified selection for earlier reproduction in many organisms, but potential constraints imposed by climate change outside the breeding period have received little attention. Migratory birds provide an ideal model for exploring such constraints because they face warming temperatures on temperate breeding grounds and declining rainfall on many tropical non-breeding areas. Here, we use longitudinal data on spring departure dates of American redstarts (Setophaga ruticilla) to show that annual variation in tropical rainfall and food resources are associated with marked change in the timing of spring departure of the same individuals among years. This finding challenges the idea that photoperiod alone regulates the onset of migration, providing evidence that intensifying drought in the tropical winter could hinder adaptive responses to climatic warming in the temperate zone.

Non-breeding season habitat quality mediates the strength of density-dependence for a migratory bird

“…Critical to interpreting this pattern, however, is that mean body condition of redstarts in scrub habitat was nearly always lower than that of individuals in mangrove forest. Arthropod availability in drought-deciduous scrub is typically two to three times lower than in mangrove forest [41], but can increase for short durations owing to rapid leaf flushing following periods of high rainfall [38]. Indeed, experimental irrigation of scrub habitat demonstrated that higher food availability is linked to elevated primary productivity [53].…”

“…Redstart habitat quality influences numerous measures of bird performance in both sexes. Individuals in high-quality habitats are typically in better physical condition [39,40] and depart earlier on spring migration [39,41] at the end of the non-breeding period than individuals in lower quality habitat. Annual survival probability is also related to overwinter habitat quality [36,42].…”

Climatic effects on breeding grounds are more important drivers of breeding phenology in migrant birds than carry-over effects from wintering grounds

“…Carry-over effects can occur if wintering ground climate influences the condition of birds when they come to depart on spring migration, potentially altering arrival time on breeding grounds and affecting reproductive success [6][7][8]. A recent analysis demonstrated a correlation between precipitation on West African wintering grounds and mean laying date in seven out of 11 migrant bird species [9] but did not account for breeding conditions.…”

Reconsidering the role of photoperiod in relation to effects of precipitation and food availability on spring departure of a migratory bird

“…Along with many current studies investigating the impact of climatic change on the behavioural patterns of migratory birds, the paper by Studds & Mara [1] demonstrates relationships between environmental conditions and timing of departure from an overwintering site-Font Hill Nature Preserve in southwestern Jamaica-of the American Redstarts (Setophaga ruticilla), a Neotropical-Nearctic migrant. This study adds new information of how the specific conditions of rainfall and food availability-insect abundanceexperienced on the tropical non-breeding grounds can modify the endogenous timing mechanism regulating departure at the time of spring migration.…”

“…American Redstarts in Studds & Marra [1] overwinter at sites just south of the Tropic of Cancer at a latitude of 18815 0 N and longitude of 77830 0 W where the annual change in photoperiod is 2.09 h (NOAA website). It is known that tropical birds are capable of responding to small changes in photoperiod.…”