Coping with seasonal changes through alternative behavior strategies in birds
Confronted with climate change, animals can avoid extinction by either moving to more favorable regions or by plastically/evolutionarily adjusting their phenotype to the new conditions. However, the extent to which these strategies are really useful is poorly known. One way to investigate this is to ask how the decision between staying or moving has evolved in animals exposed to seasonal environments. In birds, for example, behavioral plasticity is one important part of the arsenal with which animals can cope with seasonal environments. However, this involves a high energetic cost due to the need to produce and maintain more neural tissue. The alternative is moving to less severe regions during the harsh season as a way to avoid a drop in resource availability, which is also energetically costly and makes a large brain difficult to maintain. The result is that migratory species may lack enough behavioral plasticity to cope with environmental changes, being thus forced to change their migratory behavior to cope with climate change. Here, we use a character stochastic mapping reconstruction of the evolutionary transitions of migratory behaviors and we then fit different Brownian and OU evolutionary models to explore these possibilities.
Our phylogenetic reconstructions revealed that residency is the ancestral state of birds and that migratory behavior has independently evolved 200 times from resident ancestors along their evolutionary history. Long-distance migration has never evolved directly from residency but generally through short-distance migration, and evolutionary transitions from migration to residency are extremely rare. OU models are the best supported evolutionary model for brain size, with each seasonal strategy having a different brain size optima maintained through stabilizing selection. The parameters of the models suggest selection for smaller brains (relative to body size) in migratory lineages, consistent with the existence of costs of migrating. Indeed, a narrow variation around the brain optima suggests that some constraints are preventing brain size to change in migratory birds. In addition, selection seems to have favored larger brains in resident birds exposed to high seasonal variation compared to resident birds from tropical regions, yielding support for the cognitive-buffer hypothesis. Altogether, our results provide evidence that due to the lower behavioral plasticity and difficulties to alter their migratory behavior, long-distance migrants may have trouble adapting to climate change.