Foraging patterns of predators can change in response to novel sources of prey. However, the timing of that transition and the various factors that influence when predators change their foraging behavior is understudied. The emerald ash borer (EAB) (Agrilus planipennis) is a phloem-feeding beetle native to Asia that can kill ash (Fraxinus spp.) trees. They were discovered in the Detroit, MI area in 2002 and have since spread to other hardwood forests throughout eastern and central North America. Woodpeckers are the most important native source of biotic control of EAB populations and they have shown to preferentially foraging in ash trees in areas heavily infested by EAB. We studied the EAB outbreak and the foraging behaviors of five bark foraging birds (Picoides villosus, Picoides pubescens, Dryocopus pileatus, Melanerpes carolinus and Sitta carolinensis) on the campus of the University of Northwestern - Saint Paul in Minnesota between 2014 and 2017. We hoped to determine which birds will change their foraging patterns in response to this novel food source and when that transition will take place.
We monitored the EAB outbreak by marking and surveying 225 trees in 2014 and resurveying them in the fall of 2016 for evidence of EAB infestation. The campus had four infested trees in 2014-15 and 83 infested trees in 2016-17. We collected 209 independent behavioral observations of five bark foraging birds between 2014 and 2017. Hairy and Downy Woodpeckers both showed a preference for foraging on oak (Quercus sp.) trees and White-breasted Nuthatches preferentially foraged on eastern cottonwoods (Populus deltoids). Pileated Woodpeckers preferred to forage in snags. All of the birds foraged on ash trees in proportion to their abundance on campus. The data show no evidence of any birds that are preferentially foraging on ash trees or changing their behaviors at this stage of the EAB infestation. This suggests that the adaptive change in foraging behavior in response to a novel food source documented in other studies does not take place until after that food source becomes more abundant.