First, let me say that it's day 17 for the bluebirds but they aren't ready to leave yet. Good thing cause we have some nasty weather headed our way...again. I did see one of their faces in the hole when I looked at the box...not IN the hole but from behind it.
And the chickadee parents are going into their box regularly with food so I assume at day 8 for them they are fine. If it weren't so damp I'd open the box but I'll wait a day. I'm sure they are fine.
Now to answer the question.
As we've watched these birds grow from an egg to a bird, we've seen that at every step along the way, the nestlings and the parents know instinctively what to do and when to do it. For instance, the parents knew what to feed the hatchlings (soft food), the female parent knew how long to brood them (till they could regulate their own body temperature), the nestlings knew that they had to do what we call preening, they knew to 'play dead' when there was danger, etc. etc.
But leaving the nest is a really serious move and there are conditions that have to be met before it's safe to take that flight. To make sure that the bluebird population continues to grow, we want successful fledgings.
The chicks have to be physically ready to take at least a short flight when they leave the box so that they can get somewhere safe. I try to put my nest boxes so that there's a bush or vine or tree close enough that the chicks can fly UP and into protection. Sometimes, however, the bird's wings aren't strong enough so they have to be physically be able to at least hop to a safe place. If there's a 'runt' in the brood, sometimes they will leave too early because they follow their siblings. To get physically strong enough, they have to do their stretches and flapping.
As they get older, the chicks develop a 'curiosity' about where the parents go when they bring food (maybe that's not the right word, it may be more like "I'm hungry, I'm watching that place where they come in with food"). When the chicks are small, the parent has to come all the way in to the box to feed them. They learn that if they stretch their open mouths closer to the parent (who has just come through this hole-thingy), they get fed first. Part of their physical growth has enabled them to get closer to the source of the food. As big as they are now, the parents can just put their heads through the hole to feed them.
The parents have to know that the chicks are ready and there is enough food to feed them when they have fledged (fledgings in the spring when there aren't enough insects for food may be later than ones in the summer). When the parents know it's that time, they will often withhold food from the chicks. Hunger is a strong motivator (it is with me). When they are encouraging a chick to leave the nest, the parents will sit outside close to the box and call to the chicks or sometimes even sit on the roof and dangle food in front of the chick who is looking out of the hole. This is just what is going on in the photo below from one of my 2007 broods. The same bird, Phoenix (he survived being thrown out of the nest TWICE by a House Wren and survived), pushed off and changed his mind and wanted to come back but that was only a momentary issue...he turned and flew to the vines on our deck.
I've watched the fledging process as it goes on inside the box and it's basically the same every time. Days before the event, the chicks get more active. They walk around the top of the nest, peck at spots on the walls of the box (perhaps poop), they flap their wings and often the day before or day that they will be leaving the box, they can actually get lift and have a short flight in the box! They often just have so much energy they are walking and flying around and at some point, catch a glimpse of 'the unknown', ie. the place where the food shows up.
It's hard to separate their behavior into what is instinct and what is learned. Birds learn by imitation and naturally they are going to imitate their parents. Younger birds will imitate their older siblings. Instinctually, they know that if they flap their wings their feet come off the ground. Often a chick will stand as far back as it can in the box and as high as it can on the edge of the nest and you can catch a glimpse of its face when you look at the box. Then it's what's called 'successive approximations':
(any of these stages may be skipped or repeated)
- I'm OK looking out but from back here where it's safe
- I'm OK looking out but I'm just going to peek from the side of the box
- I'll climb that metal grid only a little bit until I'm OK with that
- OK, I'll climb to the top of the grid and look out
- I feel OK with all of this so I'll put my head in the hole (but not out of it)
- It's time...I'm going to put my head out of the hole and look around
- It's really time...I gotta get my legs on the edge of the hole, my head and half my body out
- Here goes....wheeeeeeeee!!!!!!!!!!
There is one magic moment that I wait for with each brood. The chick has been looking out of the hole and is very interested in what it sees. Its parents may be out there with food and the chick is hungry! Often, the chicks inside the box will almost 'throw' themselves at the hole. In a brood of 5 or 6, this is hysterical because they'll take turns doing that. They'll stand on the sides of the nest to get as high up as they can. Some will put their heads out of the hole and look this way and that.
But the moment I watch for is when a chick looks out the hole which is the 'unknown' and looks back inside which is the 'known' (sometimes they'll fall back into the box because they're not ready). So, they get into the hole with their body halfway in and halfway out...it becomes a decision between:
known=that which they've been safe in for 17 or more days
unknown=don't know what is out there except food and their parents
You know they're going to go if their feet are under them on the outside of the hole. A moment comes when they 'decide' that it's time to try the 'unknown'. They push off with their feet and you hold your breathe to see where they land...
But we'll just wait to talk about that when I can show this to you. Oh, they don't go back to the box after they leave. Some have been know to try to fly back in when they've pushed off. I've heard a few stories of a few who have come back but that's rare.
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