Hazelnuts are the nut of the hazel which is also known as the cobnut or filbert nut.
It is spherical to oval in shape and measures between 15-25mm long and 10-15mm in diameter. It has an outer fibrous husk which surrounds their smooth shell. The nut falls out of the husk when it is ripe and the kernel of the seed is edible and used raw, ground into a paste or even roasted.
So can rabbits eat hazelnuts at all and if they can how much of them can be eaten?
Lets take a look at their nutritional data in particular their acidic, phosphorus, calcium, fibre, sugar, and fat content.
Energy 2,629 kJ (628 kcal)
Carbohydrates
17
Starch 0.48
Sugars 4.34
Dietary fiber 11
Fat
60.75
Saturated 4.464
Monounsaturated 45.652
Polyunsaturated 7.92
Protein 14.95
Vitamins
Vitamin A equiv.
beta-carotene
lutein zeaxanthin
(0%) 1 μg
(0%) 11 μg
92 μg
Vitamin A 20 IU
Thiamine (B1) (56%) 0.643 mg
Riboflavin (B2) (9%) 0.113 mg
Niacin (B3) (12%) 1.8 mg
Pantothenic acid (B5)
(18%) 0.918 mg
Vitamin B6 (43%) 0.563 mg
Folate (B9) (28%) 113 μg
Vitamin C (8%) 6.3 mg
Vitamin E (100%) 15.03 mg
Vitamin K (14%) 14.2 μg
Trace metals
Calcium (11%) 114 mg
Iron (36%) 4.7 mg
Magnesium (46%) 163 mg
Manganese (294%) 6.175 mg
Phosphorus (41%) 290 mg
Potassium (14%) 680 mg
Sodium (0%) 0 mg
Zinc (26%) 2.45 mg
Other constituents
Water 5.31
source wikipedia
As you can see hazelnuts contain a huge amount of phosphorus, a lot of calcium, and are very acidic.
This means hazelnuts are really bad for rabbits to eat. Their phosphorus, calcium and acidic content means that they will really harm the rabbit if they are eaten.
So do not purposely feed hazelnuts to rabbits.
If they happen to eat a little of them then they should be fine but anything more than that, will require veterinary advice.