Birds despise snakes....who will raid their nests for babies or eggs.

Sounds like a juvenile Ratsnake. Rattlesnakes do not climb trees.

Yeah, I agree it would have to be a black rat snake... Timber rattlers won't climb trees and are not common in Connecticut.

The only other eastern US snake that I know of that really likes climbing trees are corn snakes, but they aren't that far north.

Such pandemonium among the small birds, on my property, has often caused me to explore, only to find a rat snake climbing a tree towards a nest.
A similar disturbance among crows has led to sighting a fox, owl or hawk.

Baby Rat snakes all have blotches on a lighter gray background. As they grow, depending on the subspecies, the coloration changes, except for the Gray Rat Snake, who keeps the baby pattern for life. Black Rat snake turns black with age (yours is a Black Rat), Yellow turns yellow with four black stripes, Everglades Rat Snake turns all yellow or orange with a suggestion of stripes, and Texas Rat gets even heavier blotching.

Timber Rattlers are rarely found in trees, always have a rattle (unless the tail has been cut off), even when tiny (at least a "button"), do not have "diamonds" or blotches, but "chevrons" (like sergeants stripes) down the back. Their adaptations do not include tree climbing, as they are a "sit and wait" predator, being heavy bodied and slow, waiting beside animal trails and tagging a small creature when it strolls by. Rat Snakes will actively pursue food, climbing trees to get to bird and rodent nests, going into burrows to find rodents, and even going into houses in search of rats and mice. This is why birds get all upset when a snake is in their tree. Rat Snakes, curiously, have a flat tummy with belly scales that can flatten out and grip bark and inconsistencies in wood, climbing quite as well as a squirrel, though slower. I had a pet Black Rat snake that I would put on a pine tree, and when I removed him, he would pull bark from the tree.

In the southeast, garter snakes....even large ones, are famous for their tree climbing skills. Garters get very large (I've seen them around four feet) in the south, where they can stay active all year round.

The first time I saw one shimmying up a Loblolly pine, I almost peed my pants, lol. But I was new to the south at that time. Many a time, I was able to watch them climbing Live Oaks, palmettos, and southern pines of all kinds.

In the spring, they would raid bird nests for eggs or hatchlings. But trees are a fruitful source of food for a predator. Tree frogs, baby squirrels, anoles, large beetles and other insects all contribute to the diet of a well fed southern garter snake.

Um, hate to contradict, but Garters don't get big, rarely eat birds or eggs (mostly worms, slugs, minnows, salamanders)and share their striped appearance with a much larger climbing snake in the South, the Yellow Rat Snake, which can approach 7 feet. Garters only get about 3.

A link to Garter Snake

Here is a link that might be useful: Eastern Garter Snake

Thanks for that link. It cites that garters do eat bird eggs and do get up to 4 feet in length. They have ample opportunity to grow that large in warm climates where they can remain active all year long. It would be very unusual for one to grow to that length in a more temperate zone climate.

Gotta agree with danetherton here. I lived on the GA/SC border for a number of years and spent a lot of time lookign for snakes in SC, GA and all over Florida. In all that time I can't remember EVER seeing a Gartersnake climb a tree. Ribbonsnakes will climb trees to bask over ponds/rivers sometimes, but I don't think I've ever seen an Eastern Gartersnake even do that that. Further west, I have seen Checkered Gartersnakes (T. marcianus) in small shrubs, etc. basking by water but I've never seen a T. sirtalis do it.

I've seen quite a few ratsnakes in trees going after eggs.

As for size, Gartersnakes are usually bigger up in the NE than they are in the SE, probably due to the SE snakes having to compete with more species.

And although a gartersnake may eat a bird egg on occasion, they are not regular nest predators like ratsnakes.

I would be surprised if this snake was a gartersnake and not at all surprised if it was a ratsnake.