Diverse apex predator guild impacts the foraging decisions of avian scavengers through carrion availability

Abstract

Scavengers are vital to maintaining ecosystem health by efficiently recycling and redistributing nutrients from carrion. However, increasing anthropogenic pressures have led to declines in many scavenger populations, particularly facultative scavengers such as large carnivores. There is limited understanding of how interactions between healthy populations of sympatric large carnivores structure foraging opportunities for scavengers through changes in the availability of carrion. In Chapter 1, I investigate how interference competition between two large carnivores, gray wolves (Canis lupus) and cougars (Puma concolor), alters the daily maximum counts and activity rates of avian scavengers at kill sites. Between 2021 – 2025, I gathered scavenger activity at 32 wolf and 27 cougar kills using direct observations and camera trapping. I found higher maximum counts of ravens, golden eagles, and bald eagles at wolf kills than at cougar kills, likely due to less-confrontational competition strategies used to limit the loss of biomass to scavengers. The subordinate status of magpies in the scavenger dominance hierarchy, along with their increased agility and heightened sense of smell, likely led to similar maximum counts at wolf and cougar kills. When wolves kleptoparasited cougar kills, there was an increase in the daily maximum counts of ravens, magpies, and golden eagles. This indicates that wolves assist scavengers in locating and accessing previously hidden and defended cougar kills. However, wolf presence only increased the activity rates and, likely, the amount of biomass acquired for ravens and bald eagles. The activity rates of magpies and golden eagles did not increase as individuals remained for less time. In Chapter 2, I investigate whether territorial common ravens made movement decisions during the winter that were consistent with optimal foraging theory, given the availability of carrion associated wolf foraging and recreational human hunting. I tracked 18 ravens (12 female, 6 male) using GPS data loggers between 2019 – 2024, resulting in 1702 days, to determine their daily foraging destinations. I fit two binomial generalized linear mixed models to represent daily raven decision-making. The first model estimated the probability that a raven left its territory. The second model, conditional on the raven leaving its territory, estimated the probability that a raven visited the recreational hunting regions. Both models included covariates for food availability, such as wolf-kill availability and visits, as well as recreational hunting season and biomass. The models also included other covariates that may affect raven movement decision-making by altering input costs, such as commute distance and whether it was early or late winter, or by providing information about food availability, such as daily temperature, snow depth, and the movement decisions of conspecifics. Ravens generally behaved consistently with predictions from optimal foraging theory, with wolf-kill encounters decreasing the probability of seeking alternative foraging options in other locations, despite their low availability on the landscape. However, raven decisions were not affected by wolf kills within their territory when a visit was not recorded. When traveling outside of their territories, ravens had a greater probability of visiting recreational hunting regions on days within hunting season and when their commute was shorter. I found no evidence that ravens tracked fluctuations in biomass availability in recreational hunting areas and are instead informing movement on the timing of recreational hunting seasons. My findings suggest that ravens are making movement decisions based on immediate knowledge of the resources available to them, including those in distant locations.