What makes you decide to pick yourself up, migrate far from home, and then come back in the spring? If you are a snowbird from the human species, usually cold weather in the fall and even worse, the first frost, is a definite sign that it’s time to get the trailer ready and to go south in search of a warmer climate until springtime.

If you are an actual bird, the situation is more complex.

I had a turkey vulture roost in my backyard for many years. The timing of their migration at first was puzzling to me. They would leave for a warmer climate at the end of September, which made sense from our human point of view: The days were getting shorter, and it might be time to look for a better place to hunt during the day.

But they would always be punctual in returning the first week of January when it’s still rather cold by our Californian standard. Not as cold as December, when we often have a few days of frost, but not balmy weather every day, by any means.

That is human thinking, so we have to put on our “bird hat” to understand why they do what they do, when they do it.

The two objectives in a bird’s life are to find food and to reproduce. The reproducing was the reason for this early return. The roost is only a gathering place where the birds come to be together at the end of the day. There is safety in being among a large number of birds at a time when the defenses are down as you sleep.

The theory is also that it can help them to find food. If one bird finds prey before coming to the roost, in the morning it will go back to that prey, and the other birds will follow it. It is only a theory and is hard to prove.

The roost is never the place where turkey vultures would nest; they choose a quiet place in a different area. As soon as they arrive in January, they need to make a number of arrangements: find a mate and a nesting territory where they will have reliable food source and protection from predators. This takes time, and only after that will they lay eggs and raise their young so the new birds have enough time to be completely ready to handle the migration in September.

No wonder they always came back faithfully the first week of January – that’s a lot to get accomplished in just nine months.

Many small birds have a rather short incubation period. Incubation is the time when the female sits on the eggs until they open up and the chicks come out.

Let’s look at some common birds we see in our garden all the time.

Scrub jay: The incubation period is 17 to 18 days, and the time to fledge is 17 to 21 days. Fledging is when the baby birds are starting to be able to fly, even if they still have some more learning to do.

House finch: 13 to 14 days for incubation and 16 days’ time to fledge. These birds often have two broods a year, meaning they lay eggs twice and raise two families because each cycle is completed rather quickly.

Then you have larger birds like a Canadian goose: 25 to 28 days just for the incubation period. That is a long time to be dependent on your partner to bring you food. And then the babies require 6 to 7 weeks to become independent. With this amount of time, I’m sure you’ll agree it is necessary to have only one nest per year. Especially if you consider that Canadian geese migrate each winter.

The start of the nesting season differs for each species of bird for several different reasons, including migration, a place for nesting, availability of a partner, amount of food, length of incubation and fledge time, ambient temperature … and there might be even more I cannot think of right now.

Some large birds such as owls and hawks start courting in December and will nest in January.

How can you help them? One way is to stay away, back off and not bother them when they are building a nest. I know it is not always easy. I had a nest in my mailbox one year and another too close to the back door, so we made the effort to use only our front door for that nesting time.

There are a few things you can do to transform your backyard into a nature retreat for many animals. Plant bushes – if possible, bushes with berries, so they can find both food and protection. Have a bird bath that is refilled on a regular basis. Bird feeders are a good idea. Right now, my bird feeders are covered by a carpet of goldfinches gorging on thistle seeds the whole day long. They are beautiful to watch, and it helps them survive through the winter.

Do not have your trees trimmed in the spring; the birds need peace and quiet while they are establishing their nesting territory. Dead trees or branches are good places to keep for cavity nesters.

If you have a place that is muddy, robins love mud puddles. Do not do a thorough spring cleanup in your yard; they need some dead grass and twigs for their nests. Pet fur comes very handy if it is not treated with flea and tick chemicals. Leave this outside after you brush your dog or cat and when you have mopped all the hair from your floor. I could even tell you not to remove the spider webs outside of your house because many birds use this stretchy material for their nests, especially hummingbirds.

Speak with your gardener so he does not use his blower where nests are built around your house. You can even rope the area and add a sign as a reminder until the babies are gone.

One thing I found out: Do not leave your dryer lint outside. It might contain residue from chemicals (detergents or fabric softener) that could be harmful. We want to let nature take its course undisturbed as much as possible, but you can leave pieces of cloth cut into small strips (1 inch by 6 inches) and short pieces of yarn or string (4 to 6 inches long). I have seen nests made from all kinds of interesting objects found by birds, so you might as well leave objects that will be useful to them and safe.

You can also hang bird boxes, but that is a new topic all on its own. Check information for the different types on Google. Different birds need different bird boxes, so you must first find out what bird you have in your yard that you want to help.

So, here we are in January. The New Year has started, and I send you my best wishes for 2015. The nesting season has started, too, and I hope you will have a chance to observe the first few signs of this exciting time in the natural world that surrounds us.

Evelyne Vandersande has been a docent at the Placerita Canyon Nature Center for 28 years. She lives in Newhall.