Please be very careful if you decide to take your Squirrel outside for sunlight. Yes, Squirrels need vitamin D but they do NOT need sunlight to get it. As McCarthy points out, Rodent Blocks contain Vitamin D. In reality and sadly so, the popular rodent blocks such as Harlan-Teklad (now Envigo) are made primarily for laboratory rodents who usually never see the light of day and still do fine nutritionally and do not develop Metabolic Bone disease or Calcium or Vitamin D deficiencies or disorders from being kept indoors. Also, there are risks associated with direct sun exposure especially in the hot Summer months. Risks associated with direct sun exposure include the development of skin cancers among other concerns. Exposure to hot summer temperatures, especially by a Squirrel who lives indoors where it relatively cool can quickly lead to dehydration, heat exhaustion, heat stroke and even death in a relatively short period of time. If you do decide to take your Squirrel outside please do not put them in an enclosure and location where they are in constant direct sunlight. They should always have shade available. They should also have readily available and accessible of water at all times. Further, you should ideally be in constant attendance to monitor the status of your Squirrel while outside. There are also additional concerns about being outside and these include potential exposure to biting and stinging insects, etc.
Again, it is truly UNnecessary for your Squirrel to be outside to get Vitamin D as long as you provide a diet that contains adequate Vitamin D such as Harlan-Teklad blocks or supplement with Henrys Healthy Blocks (HHB). Teklad blocks can be free fed but HHB should never be free fed. These blocks are in themselves supplement and as far as Vitamin D is the issue, HHB contain a relatively high concentration of Vitamin D compared with a free fed dietary source such as Teklad. HHB should be limited for the "average" adult Squirrel to two blocks per day but this can be and should be adjusted up or down a block or so depending on the size of the Squirrel and how much they may waste while they eat. Ideally when considering the nutritional needs of Squirrels, a quality rodent block such as Harlan-Teklad should be the major constituent of there diet. Henry's website contains the frequently cited Diet Pyramid (
https://henryspets.com/healthy-diet-for-pet-squirrels/) which is quite useful. Most Squirrels seem to appreciate variation in their diet and the Pyramid makes some good suggestions BUT, a Squirrel will do very well nutritionally speaking if they eat nothing but a quality rodent block such Harlan-Teklad and they will not need to run any risk of being outside get their Vitamin D.
Regards,
SamtheSquirrel