The first sign that Thomas Gable and his colleagues were approaching a kill site was the calls of crows and eagles cutting through the winter silence in northern Minnesota.

The ecologist in the Voyageurs Wolf Project at the University of Minnesota was surveying wolf predation last winter when snowfall was particularly low. He and his colleagues had GPS collars on wolves in two packs near Voyageurs National Park. When data were relayed back to Gable and his team’s computer, they looked for the telltale cluster of location points that indicated a potential kill.

Many of these were areas where the wolves just slept, while others were kill sites. But if they didn’t get to the area quick enough—especially in the spring or fall—it was sometimes hard to detect, as the wolves often left few traces of their meals behind. “If you didn’t know a kill was there, you could miss it if you walked by,” Gable said, adding that the blood was mostly gone after a few days in some cases.

In other cases, there was still plenty of carnage—blood on the white snow, chunks of hair and leftover stomach contents of white-tailed deer (Odocoileus virginianus). Scavengers like bald eagles (Haliaeetus leucocephalus) and crows (Corvus brachyrhynchos) were still around picking at the remains.

The trouble was that the snow that made the detection of blood easier wasn’t often there. The winter of 2023-2024 saw some of the lowest snowfall in years. In fact, while temperatures are warming in this area due to climate change, the snowfall has increased in the past decade. Gable and his team already knew what wolf predation was like in the spring, summer and fall. But they wanted to get an idea of wolf (Canis lupus) predation in the winter. They assumed the weather would be typical for the season.

“We didn’t plan on last year being the mildest winter on record,” he said. “So, in some aspects, this study kind of fell into our lap.”

The data has so far revealed that a lack of winter snow may be putting wolves on leaner diets as deer become harder to capture. Gable and his colleagues detailed these findings in a study published recently in Ecology and Evolution.

“Wolves were really struggling to get a sufficient amount of food in that winter,” he said.

Starting in September 2023, the team collared one wolf in each pack with GPS devices. In the fall, they studied the predation behavior of wolves in six packs. They continued to study the predation behavior of two of those packs in the winter, and in spring they studied the predation behavior of wolves in five packs.

They also employed trail cameras to determine pack sizes.

Throughout this period, the team visited each presumed kill site based on the clusters of GPS locations. At the end of the study, they estimated how many deer each pack killed. They then divided this number by the number of wolves in the pack to determine how much food each wolf was getting.

Other research has revealed how many deer wolves kill in a typical year, as well as how much food a wolf needs each day to maintain its body weight. This is typically about 2.3 kilograms per day for the average wolf in the Voyageurs area, which are smaller than wolves in Yellowstone or in western Canada.

During this low-snow winter, the team estimated that each wolf in the packs they tracked only received about 2.1 kilograms per day, assuming no loss to scavengers like ravens (Corvus corax) and eagles. But in reality, scavengers likely consumed 10-20% of kills, meaning each wolf likely only received about 1.8-1.9 kilograms per day.

This drop in sustenance is particularly significant because the winter is usually an easier time for wolves in Minnesota. In the summer, it’s often harder for wolves to hunt deer, which are in better condition to escape predators due to abundant forage and lack of snow. They may eat a lot of fawns in June, but these animals are so small that wolves usually just squeak by in this period, Gable said. Plus, wolves also have cubs to feed at this time, which require additional food.

In the winter, heavy snowfall makes it difficult for deer to move, and the leafy vegetation that deer like to eat is absent. This makes them easier prey since there are weaker adult deer around. If heavy snow continues into spring, or there is a lot of late snow, these conditions can persist.

But last winter, low snowfall meant the deer could move around more easily and access forage through the colder months. “Mild winters are going to result in reduced predation success,” Gable said. “Most wolves were probably losing body weight throughout the majority of the winter.”

Luckily for the wolves of the Greater Voyageurs Ecosystem, last winter’s lack of snow was an anomaly. But if conditions change as the climate continues to warm over the decades, it could affect the way that wolves operate in this area. If milder winters make deer more difficult to catch, for example, the packs might be forced to expand their territory to find enough vulnerable deer.

This photo essay is part of an occasional series from The Wildlife Society featuring photos and video images of wildlife taken with camera traps and other equipment. Check out other entries in the series here. If you’re working on an interesting camera trap research project or one that has a series of good photos you’d like to share, email Josh at jlearn@wildlife.org.