Berries and even some seeds.

"Some preferred winter food sources include dogwood, hawthorn, wild 
grape, and sumac, and hackberry seeds. Supplemental fruits eaten include 
black raspberries, bayberries, fruit of honeysuckle, Virginia creeper, 
eastern juniper, and pokeberries."

https://en.wikipedia.org/wiki/Eastern_bluebird

I assume the abundant sumac on the edges of my property and beyond is 
the reason they're such frequent visitors in wintertime.

Jane
(Shoreham)

On 2/21/2017 6:21 PM, Sarah Fellows wrote:
> Continuing bluebirds are now sitting on opposite bluebird boxes in
> the morning.Since we saw one in a box last week at 9pm, I assume the
> birds have taken up winter residence here. I have lived here for 36
> years without a bluebird in winter in Williston. What do they
> eat???!!!!!!!  No insects apparent  except snow fleas  ,when the sun
> is out , on the snow. That would be a very small meal! Good thing
> snow is beginning to melt. Sally Williston
>
>> On Feb 20, 2017, at 10:56 AM, Jane Stein <[log in to unmask]>
>> wrote:
>>
>> Vermont has always hosted a fair number of bluebirds through the
>> winter.  They tend to wander around in small groups.  I have two or
>> three or four visiting for a day, rarely as long as two days, every
>> week or so. There may be more of them this winter, and/or they may
>> be wandering a little farther. But there's nothing unusual about
>> bluebirds in VT in the wintertime.  I don't know how far north they
>> usually are, but I've seen them at Dead Creek in mid-winter going
>> back a couple decades.
>>
>> Jane (Shoreham)
>>
>>
>>
>>
>>> On 2/20/2017 10:38 AM, Kevin Thorley wrote: We have bluebirds in
>>> the Southridge neighborhood of Williston as well. I have seen up
>>> to three at one time in trees in the meadow between Metcalf Drive
>>> and Williston Road. The three I saw appeared to be one adult and
>>> two juveniles. I saw these three birds on two different days in
>>> late January.
>>>
>>> Kevin
>>>
>>>> On Sun, Feb 19, 2017 at 5:44 PM Sarah Fellows
>>>> <[log in to unmask]> wrote:
>>>>
>>>> Today at 5pm , I noticed a male plump beautiful bluebird on a
>>>> fence post by my house. Then I saw another big fat male on the
>>>> electric wire at my house.They were about 50 feet apart. Both
>>>> of them swooped down and apparently caught things on the
>>>> surface of the snow .They both left at 5:20. Last week was the
>>>> first winter bluebird at my house. NOW, this!!!!!!!!!! What is
>>>> going on?
>>>>
>>>> Amazed in Williston
>>>>
>>>>
>>>> Sally Fellows
>>>>