To what extent and in what ways do the requirements of migration affect foraging behavior of predators? Have interactions between migratory predators and migratory prey evolved to take advantage of the cyclical and temporal nature of migration?
To answer these questions, I looked at a well-documented system of Brazilian free-tailed bats and noctuid moths in south Texas during the fall. These bats roost in caves and under bridges in huge numbers and feed on agricultural pest moths (pdf) such as corn earworm and fall armyworm, providing at least a million dollars of pest control services to farmers in the region each year (pdf). Both the moths and the bats migrate each fall down to Mexico and return in the spring.
By tracking changes in diet, bat weight, fluctuations in moth populations, and changes in bat foraging behavior at ground level and different altitudes in response to moth migration events, I studied how the passage of millions of moths through the area each fall enables the bats to prepare for their own migration. This work will enable us to better understand the value that bats provide to agriculture and the possible changes to these systems in response to future climate change.
I examined some specific questions:
Is there a relationship between bat behavior and moth migration events? In the fall, noctuid moths take advantage of favorable winds that follow cold front passages. I used pheromone and black light traps to track numbers of five species of migratory noctuid moths as they came through my study area. In addition, I caught bats as they returned to the cave after foraging, and noted the number of hours bats were actively foraging around the cave. Moth numbers fluctuated significantly in relation to various cold-front-related weather variables, and bat foraging time was also correlated with frontal passage. In addition, bats gained significantly more mass directly after cold front passage than they did over the course of the fall season. More information in this paper.
Does bat consumption of migratory moths follow a similar pattern? In addition to weight and size measurements, I also collected guano samples from bats. I used high-throughput DNA sequencing to identify insect DNA extracted from that guano, and I am currently matching those sequences to known entries in the Barcode of Life database.
Do bats change their foraging behavior in response to moth migratory events? Moths are known to migrate at altitudes of several hundred meters or higher, but it is difficult to document bat activity at these elevations. To answer this question, I deployed a hybrid helium balloon-kite (Helikite) to raise bat detectors and recorders to different altitudes. I compared relative foraging activity and acoustic parameters of calls recorded high above ground to those recorded at ground level. This study is still underway.