Living with Birds ''What is autumn for you? '' by Nick Baker Tweetapedia

"What is autumn for you?" by Nick Baker

October 17th, 2024
7 minute read

What is autumn for you?; by Nick Baker

As soon as we start feeling ‘autumnal’ you can bet the birds will have been feeling it for several months already. The concept of autumn is of course entirely of our making – a period of time defined as somewhere between the two other human constructs of summer and winter.

What is autumn for you? A time of leaf colour change and conkers, of mould and fungus, misty mornings, hedgerows laden with the fruits, seeds, berries and drupes, and maybe the odd dew laden spider web? Whatever it means to us as individuals, it sits neatly on the annual calendar as the traditional time of harvest, and we notice it in its moment. 

For birds, of course, the time we know as autumn doesn’t really exist as a date in the calendar. Their own seasonal changes are triggered by much more subtle things. This preparation starts almost as soon as breeding stops way back in the summer. Hormones, those internal harbingers of change, are constantly in flux within every bird’s body, preparing them for the season ahead.

Think of a bird as a little machine packed with intricate circuitry. Triggered by various environmental changes, internal genetic switches are flicked that re-programme the bird’s function. No longer singing, fighting, egg-laying beasts, they become all about survival. Changes in metabolism allow the laying down of fat stores, accompanied in some species with a ‘change of mind’. Their brains literally alter in structure, with certain parts swelling while others associated with sex, courtship, territory and reproduction generally shrinking. More about that later.

These changes are reflected outwardly in all manner of behaviours we might notice. It’s the reason that birds that were once territorial rivals and competitors suddenly become flock-mates, feeding and even roosting together, and thus benefitting from safety in numbers. The cues for all these changes are still poorly understood and vary between species. However it’s thought that food availability, shorter day lengths and lower temperatures all play their part. The reality is that in many species it may take several of these environmental switches to be flicked simultaneously.

Costume change

One of the most physical changes is an external one. As soon as Nuthatch eating berriestheir breeding cycle has come to an end, birds will swap out old worn feathers for new.This has been happening since way back in mid to late summer for most species. It’s a process that is energetically expensive, but occurs at a time where there is food a plenty in the countryside at large. This is when birds hit the early crops of berries hard. The cherries and elderberries (and unfortunately the gooseberries, black currants and blackberries as well) often get snaffled before they are fully ripe. I’m often reminded that I’m too late for my own personal harvest by the preponderance of purple bird droppings about the place.

By now the fresh feather coat is nearly complete. Growing these new feathers and replacing those lost over the year, birds appear to be at their most perfect, not just aesthetically but aerodynamically too. This is essential for those species that are about to migrate. It also means with the coming of the cooler seasons they’re afforded a robust and warm protective layer. 

On the move

Migration essentially means movement from one place to another, and many species exhibit this behaviour to some extent at least. A few species might stay firmly in place. These are the residents like your garden robin, although they might be joined by other members of its species that have arrived from further afield. Others go in for short-distance migration. Birds that breed at altitude or inland – such as curlew, skylark and kingfishers – may migrate to lower lying areas or coastal sites, where the climate and the feeding are better.

Oyster Catcher There are some that surprise us by being medium distance migrants: they’re easily missed as they are often familiar species. A couple of notable superstars that regularly migrate a few hundred miles include a ringed blackbird that for several years migrated from Norfolk to Devon. Close to where I live, an oystercatcher ringed on the Exe estuary in Devon makes an annual pilgrimage to the Scottish highlands to breed and then back again for the winter months. Then there are the birds we think of most when we talk about migration. These species hop between countries, continents even hemispheres – often twice a year – from warblers, flycatchers, swifts, swallows and martins to terns and even swans. 

‘Viz-miggin’ is a neat term that stands for visible migration. Pay attention to the skies, the woods, hedges and garden feeding stations. Once you’ve tuned in, you’ll notice bird movement everywhere. From mixed flocks of tits, treecreepers and nuthatch to fieldfare, redwings and other thrushes. Even movements of seabirds past promontories and waders on mud flats.

Birds re-distribute themselves over the planet’s surface as a way of accessing more food. Take a strongly migratory bird such as the willow warbler. If they stayed in their sub-Saharan wintering grounds there would be competition from other resident birds, plus an even more critical issue of less daylight.

A move North opens up new opportunities, with the promise of plenty of concentrated food. Insect activity is also crammed into a more concentrated season: just think of the aphids on your roses. In addition to more hours of daylight in which to find it, catch it and stuff it into the gullet of their hungry nestlings. Now, however, their breeding season is over. Insect numbers are also waning as they too are influenced by the same sorts of seasonal triggers as the birds. As the days get shorter and temperatures cooler, insects retreat and the process goes into reverse, with many birds tracking back south. 

Migration at this time of the year, is less urgent than the equivalent in the spring and is frequently more subtle, with resident birds moving out in dribs and drabs and those that breed in more Northern latitudes heading to us to stay or just pass through. All these birds are moving around for much the same reason. Their hormones have clicked from breeding as a priority to survival and maintaining fitness so they can make another go at it next year.

Bird brain

Coal TitFor me perhaps the most incredible change cannot be seen. One illustration of this can be seen in birds that engage in a behavioural strategy call scatter hoarding. The brain inside the head of that coal tit swinging from your feeders might be the size of a raisin, but it has an incredible capacity for memory. Watch how these tiny birds don’t linger on the feeders. They dash in, grab a seed and zig quickly off again, especially when there are larger birds around. Each seeds is then carefully secreted in a crack or crevice elsewhere around the garden. It’s been calculated that there may be thousands of such hidden food items. How it remembers the location of each cache is down to some brain-bending neurological gymnastics.

The brain is an expensive thing to run. If any part of it isn’t being used, it makes sense to shut it down - and this is exactly what birds do. Contrary to popular belief, the specialised cells that comprise a brain can be rearranged, deconstructed and then rebuilt according to the needs of the bird. In a process that’s a bit like formatting a computer hard drive, areas of the bird’s brain that are geared towards singing, nesting or reproduction simply shrink until next spring. They’re surplus to requirements and just not needed right now.

Instead, resources are diverted to other parts of the brain that will be more useful during the tougher months. The hippocampus is the bit of the brain that’s associated with learning, spatial awareness and memory. This swells by up to half its original size, and along with it the birds’ cognitive ability to find food and memorise where it has hidden any surplus. Come the spring, this process will reverse. New circuitry associated with breeding will grow and the hippocampus will shrink back again. The overall size and volume of the brain don’t change, however. If you were able to peer into a coal tit’s skull and speed up the annual cycle, different parts of the brain would bulge and swell, pulsing in time with the needs of the seasons. 
Nick Baker, naturalist, author and broadcaster

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