New virus hits Great Tits

The arrival of a new strain of avian pox has been in the news of late, thanks to the publication of a piece of research with which I was involved. The sight of any diseased bird can prove distressing but to see one with a huge growth covering part of its face is particularly upsetting. This, unfortunately, is what we have been seeing over the last few years in some of our Great Tits and it seems likely that we will see more individuals suffering from this virus as it spreads further across the country.

Avian pox is a familiar enough sight, with small lesions not uncommonly reported from Blackbirds, House Sparrows and Dunnocks, among others. This strain of pox virus is less severe and birds often recover. That recently found in British Great Tits is far more unpleasant and the large growths can restrict vision and movement, making the birds more prone to predation and to secondary infection.

My work at the BTO, carried out in association with vets at the Institute of Zoology, researchers at the RSPB and at the Edward Grey Institute in Oxford, has revealed that the virus being seen in our Great Tits is the same as that found in certain continental populations in the past. Not only that, but we have been able to trace its spread since the first case was diagnosed in 2006. Using information collected by BTO Garden BirdWatchers, who keep a weekly count of the birds and other wildlife using their gardens, we have been able to document the disease as it has crept northwards across the UK from an origin somewhere near the south coast. The pattern of arrival, coupled with our knowledge of Great Tit movements, underlines that the virus was almost certainly transported here by a biting insect, possibly a mosquito. British Great Tits are largely sedentary in their habits so it is very unlikely to have arrived via an already infected bird.

You may recall the arrival of blue tongue disease in East Anglia and the link to the continent via a plume of warm from the south bringing with it the biting midges that transport the disease. Something very similar may have happened in the case of the avian pox that we are seeing in these unfortunate Great Tits. Transmission between birds is primarily via biting insects but the virus may also be spread by bird-to-bird contact or via contaminated surfaces, so you should maintain suitable hygiene practices around your garden feeding stations. Hanging feeders and bird tables should be washed weekly, using disinfectant before being rinsed and then air-dried. If you see lots of diseased birds then consider ceasing feeding altogether for a few weeks. 

Living below ground

When you think of blue tits and great tits you tend to think of familiar garden visitors, species attracted to bird tables and wooden nest boxes placed on the trunk of some garden shrub or small tree. Of course, many more of these birds make a living in the wider countryside, feeding on defoliating caterpillars in woodland and nesting in natural cavities.

Within woodland, natural cavities occur in many shapes and sizes, and only some of these will prove suitable for hole-nesting birds like the tits. There can be competition for natural cavities and it is perhaps unsurprising to discover that great tits will oust blue tits from suitable cavities and that, in turn, blue tits will oust coal tits, such is the dominance hierarchy that exists. The size of the entrance hole and the location of the cavity influence the degree of competition, largely because of the size differences between the three species. Being the smallest of the three, the coal tit can use cavities with a small entrance hole, cavities into which the other two species cannot squeeze. In particular, coal tits will use those cavities that occur low down, often no more than a narrow slit opening up into something a little larger within. Great tits require the largest entrance hole and blue tits fall in the middle.

The other day we were out in the forest, leading a group of volunteers learning the art of nest recording, when we came across two good examples of natural cavities used by nesting tits. The first was a coal tit nest, placed deep into a ‘snag’ line, the row of conifer stumps that is pulled together when a block of commercial plantation is harvested. The parents were busy feeding the chicks, taking in food and carry off faecal sacks. These sacks keep the nest clean and remove tell-tale droppings that might give the nest location away to a predator.

The great tit nest was more unusual (at least for a great tit), the birds having found a cavity in an old tree stump that disappeared down below ground level. We only stumbled across the nest when the sitting female was flushed by our approach – great tits usually sit fairly tight when on the nest. In her absence we were able to use a boroscope – essentially a tiny camera on a long and flexible cable of the type used by plumbers – to peer into the darkness. There, on the small video screen, was the image of six eggs in a neat nest cup. This nest would be fairly secure from most predators, though it might be at risk from a weasel or from flooding but with luck it should survive.

Teacher teacher!

It is on those days when the first touch of spring-like warmth can be felt that many of our resident birds are spurred into song. Perhaps the most familiar and noticeable of these early springtime songsters is the Great Tit, whose ringing ‘teacher-teacher’ song can be heard from woodland, hedgerow and garden from February onwards.

With a broad black stripe running down his chest and widening to meet his legs, the male Great tit is arguably the most handsome of our tit species. This stripe plays an important role in Great Tit behaviour. While you and I might be able to distinguish between a male and female (her stripe is narrower and peters out about halfway down the belly), female Great Tits can detect subtle variation in the male stripe, which acts as a badge of social status. A male of high rank will make a better mate, providing access to more resources and so increasing the female’s chances of rearing a brood of chicks. Great Tits invariably have just the one brood a year.

Although the basic ‘teacher-teacher’ structure of the song is familiar to most birdwatchers, there are variations on this theme, with older birds tending to have a more diverse repertoire. One old male was found to use some forty different variations of the ‘teacher-teacher’ theme. The amount of song will be greater on warmer days and will increase as spring takes hold more firmly. Song output declines once the female begins incubating her clutch of eggs. Incubation does not begin until all of the eggs have been laid and the clutch is complete.

Interestingly, the female will begin to roost in the nest cavity from once the first egg has been laid. Although she sits on the nest she will not be generating sufficient heat to start incubation. This only happens once her brood patch develops. This is an area of bare skin, rich in blood supply and hot to the touch, that forms just before the clutch is completed. The combination of the brood patch and shifting patterns of time spent off the nest means that she can regulate the temperature of her clutch, keeping the eggs at 35.4 degrees C.

As if the effort of producing the eggs was not enough, the real work will begin once the eggs hatch and the chicks emerge. Each chick weighs just over a gramme when it emerges from the egg and it will increase to some fifteen times this weight when it is ready to leave the nest. One researcher calculated that the effort of feeding a brood of Great Tits was equivalent to a human parent bringing home more than 100kg of shopping every day for just over two weeks!

Garden feeders reveal an army of diners

Spending some time at my parents’ house over Christmas has enabled me to take stock of a different garden feeding station. Although many of the bird species putting in an appearance at the hanging feeders are familiar, others would constitute a red-letter day were they to appear in my garden at home. Species like Bullfinch, which are such regular visitors to my parents’ rural patch, seem to like the fact that it is well connected by a network of hedgerows to the series of small woodlots that are a feature of this rural Wealden landscape. By comparison, the attractions of urban Thetford are unlikely to suit the shy and retiring nature of this boldly marked finch.

One feature that has struck me over the past few days is the sheer volume of birds passing through the feeding station. It is clear from the composition of the visiting flocks that a good number of birds take in the garden as part of a wider foraging circuit that also covers much of the local woodland. The nature of the woodland itself is evident from the visiting birds; the stands of beech and oak contributing the Blue and Great Tits, while the blocks of conifers that still adorn much of the local heathland are no doubt the source of many of the Coal Tits and Goldcrests.

Some idea of the numbers of birds involved can also be gained from a short spell of ringing. As a licensed bird ringer, I am in the fortunate position of being able to catch wild birds and mark them with specially developed metal rings. Each ring carries a unique number and, should the bird be subsequently found, this provides an insight into its longevity and movements. Many people assume that ringing birds only tells us where they go (think of Swallows wintering in Africa) but researchers also use the information to examine survival rates and productivity, two vital components that ultimately influence whether a population increases or diminishes over time.

In this particular instance, however, one of the by-products of ringing in my parents’ garden is an understanding of the numbers of birds passing through the feeding station. When you look out the window there, you may see half a dozen Blue Tits around the feeders at any one time and it would be easy to assume that it is the same half dozen individuals that you see at various times throughout the day. Stick up a net and fit the individuals that you catch with rings and it soon becomes apparent that you are talking about a population of many dozens of Blue Tits passing through the feeding station over the course of a short winter’s day.

Early Breeding

Dr Johnson famously noted that “when two Englishmen meet, their first talk is of the weather”, a pattern of behaviour that is as relevant today as when it was written. The social anthropologist Kate Fox believes that this “weatherspeak” is not a genuine discussion about the weather at all but a social facilitator, enabling relative strangers to overcome their social reserve and to actually talk to each other. I was pondering on this the other day whilst watching a pair of collared doves establishing their pair bond prior to mating. Our “weatherspeak” is akin to many of the ritualised behaviours exhibited by animals during periods of social interaction. The more questioning among you may be wondering why two collared doves were indulging in such bonding in the middle of winter; well, it’s all to do with the weather.

As I am sure you will have noticed (and, indeed, commented upon to friends and acquaintances) it has been unseasonably mild of late and this has prompted a degree of premature breeding activity amongst some of our bird species. Over the last few days I have received reports of active nests of song thrush and blackbird (both with young chicks), and of other birds seen carrying nesting material. Such reports are not unique and most years there are at least some early nesting attempts taking place in January or even December. The onset of breeding in our garden birds is under the control of hormones, which are themselves triggered by a range of factors, the most important of which appears to be the change in day-length. The hormones set loose changes within the individual bird, including the development of the reproductive organs. Perhaps uniquely among the “higher animals” the reproductive organs of birds shrivel to almost nothing outside of the breeding season but, come spring, they may enlarge by several hundred times. The controlling influence of change in day-length means that, for most species, it is still too early to be preparing to breed. However, there are some species which always seem to feature in early nesting attempts. Along with the odd blackbird, robin, wood pigeon and thrush, there are the tawny owl, which normally starts laying eggs during February, and the collared dove, which seems to keep going all year round, so long as the weather holds.

Over the coming weeks you may start to hear other birds proclaim ownership of breeding territories. A warm day in February or early March might well prompt a great tit to send forth its “teacher teacher” call or a mistle thrush might sing from the tallest of trees. Certainly, if you come across nests with eggs or young I would love to hear of them.

Long nights hard on small birds

The long nights that we experience at this time of year can make things difficult for many of our birds. A combination of low temperature, reduced amounts of daylight in which to find food, and a general decline in the amount of food available, can all have a dramatic impact on smaller species. Small birds may need to expend more energy to keep warm and, with only small reserves of body fat, they have to spend much of a typical winter’s day searching for food. This food is vital to replace losses from the night before and to prepare for the night ahead. Great tits may easily lose five percent of their body weight overnight, with the amount of weight lost being greatest during periods of cold weather.

Watching my bird feeders has revealed a pattern to the order in which the different species arrive in the morning. As one of some 2,000 licensed bird ringers nationwide, I am frequently up before dawn to set the nets in which I catch small birds for ringing. At this time of day, I can already hear the calls of blackbirds and robins up and about before it is truly light. With the nets set, I operate a regular pattern of checking and emptying the nets every 20 minutes or so, fitting rings and taking various measurements before releasing the birds. The first birds to be caught are usually blackbirds and other thrushes, followed by various tits and the occasional dunnock. It is not until an hour or so after dawn that the greenfinches, goldfinches and chaffinches begin to appear in any numbers. Birds continue to arrive to feed through until mid-morning then it all goes rather quiet, with numbers not increasing again until the hour or so before dusk, presumably when individuals come in to top up before going to roost.

Intuitively, you might expect those species that most urgently need to top up their fat reserves to arrive first but other factors also come into play. For example, roosting behaviour may be important. Some species roost communally, sharing body warmth and reducing the amount of energy they have to expend to keep warm. While wrens can use nestboxes or roosting pouches, long-tailed tits prefer to roost together on a branch in the centre of a thicket. Dominance also plays a role, with less dominant species forced to feed at times when feeders are not being dominated by more aggressive birds like greenfinch or starling. Such factors influence the basic pattern and will change over time as birds respond to changing weather conditions. Recent nights have been relatively mild but, as we move further into the winter, lower temperatures may bring birds to our feeders that much earlier relative to dawn.