Activity shifts from the outside world to the inner privacy of the nest, and at the same time the spring veil of fresh, new leaves comes up to hide the dramas unfolding within. So, we might see less of our garden birds; but that is merely a sign that the breeding season has reached its most important stages.
In early May, a good many garden birds will be incubating their eggs. An egg must be heated if the development of the chick is to proceed, and this is achieved, as we all know, by the parent or parents donating their body warmth for a period of time.
The duty incubator moults a few feathers on her or his belly to create a ‘brood patch’ (or patches) of bare skin, and much of the business of incubation is making sure that all the eggs get exposed to it.
We might suppose that incubation is an easy, relaxing task, akin to sitting on a sofa and watching television. But it’s a lot more active than that. A sitting bird is forever shifting about, sometimes moving eggs with its feet, sometimes standing up and turning them with its bill, each manoeuvre to make sure the heat has been divided equally.
Most garden birds begin their incubation only when they have laid all their eggs. There is good sense in this; it means that the chicks will all hatch out roughly in synchrony, and the brood can be dealt with as a brood, rather than as a group of individuals.
In the same way, if you’re preparing a dinner party with baked potatoes on the menu, you put them all in the oven at the same time, to be ready for each guest at one sitting.
As far as our bird parents are concerned, synchronous hatching means that all the chicks will need feeding in the nest at roughly the same time and that they will all leave the nest within hours of each other.
But if such a strategy might lure you into thinking that broods of young are little peaceful democracies, where the devoted parents feed everyone equally and where the plenty of spring is liberally shared, you are in for a shock.
From the moment they hatch, young birds enter into a continual cut-throat battle with their siblings over the food the parents bring in, like related humans fighting over a contested will. Every time an adult arrives at the nest, each chick stretches up as far as it can, and the one who reaches highest wins and gets fed.
You get the same reaction when you ask a class of school kids to put up their hands if they want sweets – everyone tries to put their hand up first and highest. But in a brood of tiny birds, it’s not a game; a weak youngster continually losing out may not survive.
Much as the chicks in a nest may be irrepressible and competitive, they are also acutely vulnerable. Predators always lurk nearby, disease stalks through populations, and infestations of ticks or lice may overwhelm and destroy the brood.
But their fate is just as likely to be decided by the weather. May is a fickle month, and should it bring a prolonged spell of cold, damp conditions, unimaginable numbers of chicks will never make it out of the nest.
The danger posed by cold and damp is wrapped up in food supply. Cold makes a small bird’s food less active and harder to find than it would be on fine, dry days; wind and rain may make the search more difficult still, making the leaves tremble and the caterpillars or other invertebrates hang on for dear life. Deliveries to the chicks inevitably become fewer.
But that’s only half the problem. In cold weather, the chicks need to be brooded just to stay alive – they cannot maintain their own body temperature and must be warmed under an adult’s brood patch.
Yet this ties one adult to the nest for prolonged periods, leaving the other carrying the entire, impossible burden of finding food for everybody.
To make matters worse, there will come a point when even the brooding bird must feed itself, against its best instincts leaving the nest and potentially abandoning the chicks to the life-sapping chill. Inclement weather brings dilemmas, hardships and shattered prospects.
But warm southerlies in May whisk one bird northwards to Britain that likes to do things differently. You know it’s late spring or summer when the sickle-shaped swift is here.
A connoisseur of fine, still air, the swift makes sure it sweeps over cricket fields and not football fields. It zips over rooftops and screams; it breezes into the garden scene and breaks sensibilities like an insensitive in-law staying for the weekend.
One sensibility it breaks is to leave the young unattended for long periods of time. The nestlings, you see, are remarkably resistant to starvation. As soon as they hatch, they begin donating a portion of every food delivery to an emergency fat supply, a personal pension in food reserves to help them through lean days.
Studies have also shown that they lower their metabolic rate during those times that the adults are away longer than usual collecting food, entering into a sort of mini-hibernation. These adaptations are way beyond those of the garden’s smaller birds.
The swift also breaks the garden mould by maintaining a breeding strategy of jolting, ruthless practicality. Unlike the rest, it begins incubation the moment the first egg is laid and, as a result, the young do not hatch synchronously; there will be a day or more between them, and each brood will contain up to three birds of strikingly different age.
This inevitably hands the eldest chick an enormous advantage over its siblings: with the largest body and most conspicuous mouth, it will always be fed first when it is hungry, and only when satiated will it deign to give way to its siblings.
The older chick will always be given the first fruits of May’s crop of flying insects, and the others get what is left over.
This might seem to be cruel, and in a way it is. But the swift, with an unpredictable food supply affected by every twist and turn of weather, cannot gamble. That first chick is an insurance policy; it gets the first of what food is available and has a good chance of survival.
A second chick is a speculation: if food supplies are poor, it will die, but if they are good, it will flourish. If swifts lay a third egg, and the chick manages to survive, that is a huge perk and the result of a bounteous season, the jackpot in a summer lottery.
And a lottery, indeed, is precisely what seems to determine the outcome in the hidden world of a garden bird nest.
