Evaluation of the importance of roe deer fawns in the spring–summer diet of red foxes in southeastern Norway

1. What they did

Red foxes kill many roe deer fawns in Scandinavia. In this study area, an earlier study by the same team found that red foxes caused 58% of fawn deaths and killed about 25% of radio-monitored fawns each year. The authors asked the opposite question: how important are fawns to the foxes?

They studied red fox diet in spring and summer, when fawns are available, in a farmland and forest mosaic in southeastern Norway.

• Collecting scats (May to August 2003 and 2004). They collected 171 red fox scats. Of these, 147 apparently fresh scats came from adults, found by walking six transects 1 to 6 km long (first cleared of old scats) or during routine radio-tracking of red foxes. Another 24 scats were collected on two visits to two breeding dens and were assigned to cubs because of their smaller size.
• Analysing the scats. Scats were broken down in water and sieved. Small mammals were identified from jaws and teeth, larger mammals from hairs under the microscope, and earthworms from their bristles (chaetae).
• Measuring diet. For each food item they recorded how often it occurred (frequency of occurrence) and its estimated volume, converted to “whole scat equivalents” so the share of each food in the total diet could be compared.
• Comparing with prey availability. They split the adult diet into spring (15 May to 15 July) and summer (16 July to 30 August), compared adults with cubs, and compared fox diet with abundance indices for small mammals (from live-trapping) and vulnerable roe deer fawns (from radio-monitored fawns). They assumed all roe deer remains in scats came from fawns.

2. What they found

A varied diet

• Foxes ate 18 species of mammals, five orders of birds, two species of amphibians and reptiles, and many invertebrates, berries, fruits and plants.
• Diet did not differ between 2003 and 2004.

Adults

• Small mammals made up the largest share of the adult diet by volume (about 32%), led by the field vole, with water voles also important.
• Medium-large mammals came second (about 23%), mostly roe deer fawns. Fawns made up about 7% of the adult diet by volume and occurred in about 9% of adult scats.
• Invertebrates (mostly beetles) occurred in 69% of adult scats but made up little volume.

Spring versus summer

• In spring, small mammals (over 40% of volume, in 61% of scats) and medium-large mammals, mostly fawns, dominated.
• In summer, berries and invertebrates took over. Berries occurred in 85% of summer scats compared with 8% in spring.
• Fawns appeared in the diet right from the start of the birth season. Their occurrence fell as summer went on, but was not linked to the index of fawn availability, which peaked on 19 June.
• Small mammals became more abundant through the summer, yet their occurrence in fox scats fell rather than rose.

Cubs

• Cubs’ diet was less varied and held far more large prey (mammals and birds) than adults’ diet.
• Roe deer remains were in 25% of cub scats compared with 9% of adult scats, and made up about 20% of cub diet by volume. Domestic pig also appeared in 17% of cub scats.
• Adults ate invertebrates and plant food more often than cubs did.

3. What they concluded

• The diet was based on small mammals but surprisingly varied for such a northern area. In summer it looked more like a Mediterranean fox diet than one expected in the far north.
• Fawns appearing early in the season supports the earlier finding that foxes actively hunt fawns as soon as they are born. Fawns may matter to vixens in spring, when berries are not yet available and vixens need to regain energy after pregnancy and while nursing.
• The large share of big prey in cub scats fits central-place foraging: a vixen carries one item back to the den at a time, so bringing large prey makes each trip worth more. Adults themselves rely on smaller, easier food such as berries and insects.
• Because small mammals here do not crash in cycles, and foxes eat a wide range of foods, the authors see fawns as an important but not irreplaceable food for red foxes.
• The relationship is therefore uneven: foxes matter much more to roe deer than fawns matter to foxes. Fawns are unlikely to trigger any feedback that would stabilise the predator–prey relationship.
• Future studies should look at how important fawns are for the growth and survival of fox cubs.

4. How tracking was or was not used

The study rests on finding and identifying red fox scats. Adult scats were found by walking transects that had first been cleared of old scats, or during radio-tracking, and only apparently fresh scats were kept. Scats at den sites were assigned to cubs by their smaller size. The paper does not say how red fox scats were told apart from those of other carnivores. The study area description also cites an earlier winter track count study, in which more red fox tracks than roe deer tracks crossed transects.

5. Tracker or observer certification

Not reported. The paper does not say who collected or identified the scats, or how fox scats were distinguished from those of other carnivores in the area. The comparison of adults and cubs depends on assigning den scats to cubs by size.

6. Other technologies, skills or innovations

• Microscopic hair analysis (cuticle, cross-section and medulla at 400 times magnification) to identify medium-large mammals in scats.
• Whole scat equivalents and average volume, combined in a single figure that shows how often each food is eaten and how much of each meal it makes up.
• Radio-monitoring of 98 roe deer fawns and live-trapping of small mammals in the same area, used as indices of prey availability.

7. Implications for conservation (from the paper)

• Red fox predation can strongly affect roe deer fawn survival, but fawns are unlikely to be important enough to fox numbers to create a balancing feedback between predator and prey.
• Where small mammals are cyclic, fawns may become more important to foxes in low vole years, as a study in southern Sweden suggested.