Long-term scatterhoarding by Eurasian red squirrels (Sciurus vulgaris)

1. What they did

Many tree squirrels bury food to eat later. Eurasian red squirrels are “scatterhoarders”: they bury one or a few seeds in each spot, spread across hundreds or thousands of spots in their home range. Until this study, no one had measured how much food Sciurus squirrels store this way, or how much of a squirrel’s yearly energy needs it supplies.

The authors studied red squirrels in two woodlands in northern Belgium: one mainly pine, the other mainly oak and beech. They dug up 100 caches to see what was in them, and counted recovered caches on the forest floor through the year. They also followed squirrels carrying radio transmitters and watched them burying food.

They combined these observations with their earlier measurements of how long squirrels are active, how fast they eat each seed type and how much energy they need each day. From this they worked out how many seeds a squirrel buries in a year, how many it recovers, and how many days of food that represents.

2. What they found

What is in a cache

• The 100 caches held 155 food items: 65 had a single item, and the most in one cache was 5.
• Of caches with more than one item, 43% mixed seeds from different tree species.
• Overall, 63% of the items were pine cones, 21% acorns and 16% beechnuts.
• Seeds were buried about 2–6 cm deep in the soil or leaf litter.

How much is hoarded

• Recovered caches showed as small round holes about 5 cm deep. About 5,100 to 6,700 cached seeds were dug up per hectare each year in the pine wood. With about 2.3 squirrels per hectare, that is about 2,200 to 2,900 seeds recovered per squirrel per year.
• Squirrels hoarded mostly in September and October, tailing off through to January.
• Each seed took about 44–50 seconds to hide. Acorns and beechnuts were hidden faster than pine cones, because they were often picked up from the ground.
• Almost all food was buried in the ground. Only twice did a squirrel hide food in a tree.
• A squirrel spent about 32 hours a year hoarding in the pine wood and 34 hours in the oak–beech wood, burying about 2,300 and 2,800 seeds a year respectively.
• What they hoarded depended on what was abundant: mostly pine cones in the pine wood, mostly acorns and beechnuts in the oak–beech wood.

How much is recovered

• Squirrels recovered about 83% of their caches in the pine wood and 79% in the oak–beech wood.
• They recovered nearly all buried pine cones (99–100%) and most beechnuts (83–92%), but only about half of the acorns (54–62%).
• The recovered food was enough for about 43 days of a squirrel’s energy needs in the pine wood and 68 days in the oak–beech wood.

When caches are used

• Oak–beech wood: caches were used mostly from January to April, with acorns eaten first.
• Pine wood: caches were used mostly from March to May, with pine cones eaten from April to June, when no more seeds were left on the trees.

3. What they concluded

• Red squirrels hoard less than American red squirrels in similar Canadian forests (about one-quarter in pine woods). Even so, their caches matter during the hardest part of the year, January to May, when energy-rich food is scarce.
• Woodland type shapes the behaviour. In oak–beech woods, seeds are available in the canopy for only a short time and fallen seeds are soon eaten by other animals. So squirrels hoard more there and use their caches earlier.
• Red squirrels digest acorns poorly and never eat more than 4–6 a day, which may explain why so many cached acorns are left. Unrecovered beechnuts and acorns may grow into seedlings, so squirrels may help disperse these trees.
• Hoarding may be more important in regions with colder winters or less varied food.
• Introduced grey squirrels forage more on the ground, and they may raid red squirrels’ caches in early spring, when reds depend on them most. This competition could help explain why grey squirrels have replaced red squirrels in much of Britain.

4. How tracking was or was not used

No trailing was reported. The study relied on reading signs on the forest floor:

• Recovered caches: recognised as small round holes about 5 cm deep, and counted on fixed plots through the year to estimate how many caches were dug up per hectare.
• Buried caches: found by digging through the litter and soil to about 10 cm. They were distinguished from seeds that had simply fallen because they were buried 2–6 cm deep.
• Who made them: the authors state that in Belgium squirrels are the only animals that store pine cones and beechnuts this way.

Hoarding behaviour itself was watched directly, by finding radio-tagged squirrels and observing them.

5. Tracker or observer certification

Not reported. The cache recovery estimates rest on recognising squirrel dig holes and buried caches, but no qualification or experience in tracks and signs identification is described.

6. Other technologies, skills or innovations

• Radio transmitters, used to find squirrels so their behaviour could be watched for 30 to 60 minutes at a time
• Energy budget calculations linking behaviour to the energy value of each seed type

7. Implications for conservation (from the paper)

Competition with introduced grey squirrels for buried seeds, especially in late winter and early spring, may lower red squirrel survival or breeding. This could be a critical part of understanding why red squirrels are being replaced by grey squirrels in much of Britain.