Hornbeam

 

Hornbeam Carpinus betulus has always been one of my favourite trees. It's native to southern England but widely planted up here in the North East, often for its wonderful chrome yellow autumn foliage. It's known also for its hard timber, hard enough to blunt carpenters' chisels and saws and durable enough for traditional uses like wooden gear wheels for windmills and water mills, and teeth for rakes. Some say that the name hornbeam comes from the timber being as hard as animal horn.

Hornbeam is particularly beautiful in late autumn after most of the leaves have fallen, when the clusters of seeds still dangle like little pagodas from the twigs. Eventually the seed clusters break up and individual seeds fall and spin to earth on their winged bracts. The hard seeds, which are tiny nuts, are favourite food of hawfinches, so I always look for them at this time of year, though I've yet to see one of these spectacular rare birds here. I've only ever seen them in southern England.

The tree is also very attractive in spring, when it's covered in catkins and when the beautifully pleated leaves begin to unfold. 

A bit of a stink

Few of our native wild flowers have such beautiful fruits, but such an unpleasant name, as stinking iris Iris foetidissima. I grew some plants from seed a few years ago and planted them in dry sandy, sun-baked soil under our garden hedge, where they've thrived ever since and produce these lovely seed capsules that open in late November to reveal their spectacular seeds.

Two other common names, gladdon and roast beef plant are also in common usage and the latter refers to the smell of the crushed leaves that have a powerful aroma of beef (though to me they smell more of roast beef-flavoured crisps, rather than the real meat). In The Englishman's Flora Geoffrey Grigson listed no less than seventeen further common local names used in various parts of the British Isles. A proliferation of such names for a plant is usually a sign that people once found it useful and gladdon has a long history of applications in herbal medicine, mentioned by Dioscorides, William Turner and John Gerard in their herbals. One popular use was as a purgative, made from a decoction of gladdon root and beer.

In his Botanical Arrangement the 18th. century doctor and botanist William Withering, always a good source of contemporary anecdotes, mentions that "the juice of the root of this species is sometimes used to excite sneezing; but it is an unsafe practice, violent convulsions sometimes having been the consequence."

I rather like the flowers that are unspectacular and reminiscent of faded denim, but Withering wasn't so impressed, describing them as being "of a disagreeable purplish ash colour", also mentioning that in his day there was also a variegated-leaved form which now seems to have disappeared from cultivation.

Long term project ...

There is a saying amongst foresters that 'you should plant as though you are going to live forever'. Sound advice, since most forest trees have natural life spans that far exceed ours - provided that the trees can survive hazards like rabbits, fungal diseases, deer, climate change, gales or developers building more out of town shopping malls.

For any tree planting enthusiast who takes a really long-term perspective and wants to leave a natural legacy, now is the perfect time to plant an oak tree........ 

.......... because acorns germinate almost as soon as they fall from the tree, producing a sturdy white root that spears down through the leaf litter and continues to grow through the milder days of winter. Then, in spring, the first leafy shoot appear. So pot up a few germinating acorns now and a successful start for the seedlings is almost guaranteed. 

It's a 'mast year' for oaks here, with thousands of acorns on the ground. Most will be eaten by pigeons (I once found a dead one with 23 in its crop), deer and mice but the lucky ones might be carried away by a jay from the shade of the parent tree and buried for future use when food is scarce. The jay will probably forget where it cached its winter food supply - and all the while the buried acorns will be sending down those roots, ready for the start of their first growing season. Come to think of it, jays probably give more oak trees a good start in life than we humans do.

The mysterious attractions of black berries

Thursday's Guardian Country Diary focuses on bird cherry Prunus padus, which has produced an exceptionally fine crop of its beautiful black fruits in Weardale this year. This is probably the result of excellent flowering conditions in late spring, fewer small ermine moths that often defoliate the trees and fewer instances of pocket plum disease which can destroy the fruits before they ripen.

Most wild fruits that are dispersed by birds are red (hawthorn, rowan, guelder rose and honeysuckle, to name but a few), so how come bird cherries (and also blackberries, buckthorn, deadly nightshade and elder berries) are black? Is fruit colour of any great significance for fruit-eating birds?

Over the last few decades there have been several scientific investigations into the ways in which fruit colours attract birds, addressing the question  'why are some ripe fruits red and others black?'

So here's a selective summary of some findings to date:

1983.
A study in the US using black cherry Prunus serotina concludes that it's not the final black colour of the ripe fruit that's important in attracting birds, but the mixture of intermediate red-coloured ripening fruits and black fruits that attract birds' attention. They observe the same results with pokeweed Phytolacca americana, whose fruits change from green to pink to black, and the conclusion is that the bicoloured phase of fruit presentation is important, rather than the final ripe colour. 

1999.
Unlike humans, birds can perceive ultra-violet light so the colours that they see don't correspond to our perception. Blue, violet and black berries reflect ultra-violet light, which may make them more conspicuous to birds and indicate their stage of ripeness. The bloom on the surface of bilberries (which is caused by a surface layer of yeast) reflects UV. When redwings were allowed to choose between bilberries with the bloom on the surface and others where the bloom had been polished away adult redwings preferred berries with the UV reflecting bloom, but inexperienced juveniles didn't distinguish between the two, suggesting that the birds learn to associate UV reflectance with fruit ripeness. 

2003
Another study revisits the bicoloured fruit hypothesis from 1983 and finds evidence to support it. 

2004.
Another study with juvenile redwings, which initially prefer red fruits but learn to associate black/UV reflective fruits with palatability, suggests that learning plays a key role in fruit colour choice. The study suggests that the colour of the fruits, not the contrast between their colour and the background, is particularly most important. 

  
2004
Another study that examines the relative significance of fruit colour and the contrast between fruit colour and background colour concludes that red and black fruits contrast more strongly with background foliage colours than any other fruit colours, which may be why they predominate. Tests on four bird species indicate that it's conspicuousness relative to background colours, not fruit colour itself, that's most important to foraging fruit-eating birds.

2006
Tests with crows in a flight cage conclude that artificial  red fruits are more conspicuous to them from a distance than artificial black fruits. When the crows were offered blueberries with or without their UV reflecting bloom, they were more successful in finding the UV reflecting ones - but that also depended on how UV-reflecting the background was.

2009.
A study concludes that blackcaps preferentially eat black or UV-reflecting fruits, choosing those with the highest intensity of colour, and that these darker fruits have high purple anthocyanin pigment antioxidant levels (good for their health) and also higher energy content. 

2009
A  study shows that birds can preferentially select ripe elder berries that have red stalks (indicative of high anthocyanin content) or ripe elder berries with green fruit stalks (that make the fruits more conspicuous and indicate a higher sugar content) - all suggesting that birds don't just gobble fruit down, they can be picky eaters and make dietary choices based on fruit quality.

2011
Studies using captive blackbirds and redwings that are offered artificial fruits show that blackbirds preferentially choose red fruits whereas redwings prefer black ones, but that these preferences can change depending on previous experience - they tended to prefer fruits of a colour that they had previously fed on, so learning was important.

2011
A review of the literature on the subject concludes (a) there is evidence that fruit colour is an important signal that birds react to; (b) that black fruits high in antioxidants (purple anthocyanin pigments), which tend to be formed in the fruits in bright light under cold conditions, enhance the immune systems of blackcaps; (c) that high levels of antioxidants reduce the fruits' proneness to fungal attack, benefiting the plant and suggesting that black fruits might have originally evolved for this reason, rather than to attract birds. 


So, back to the original question :'why are some ripe fruits red and others black?'. In summary, it's all to do with - 

- the fact that these are the two colours that birds find most attractive

- that the mixture of reds and blacks in a branch of ripening fruit (think of blackberries ripening from pink to black) may be important in attracting birds' attention

-  UV reflectance (that we can't see) is very important especially in fruits like blueberry, bilberry and sloe that have a UV-reflecting surface 'bloom'.

- that the contrast between the fruit colour and background colour is important

- bird species may have innate preferences but these can change when they learn to associate colour with fruit quality

- some birds can select fruit, on the basis of colour signals, that has higher nutritional value

. black colour, with high antioxidant anthocyanin pigment content, might have evolved in plants as a defence mechanism against fungal disease rather than to attract birds.

As is so often the case, there are no easy answers; scientific research rarely delivers simple explanations but always produces a whole raft of more interesting, more sophisticated questions. 

Which is why science is so addictive - the natural world is always far more complex than anyone can possibly imagine.

As for the birds in my garden:

- the blackbirds and thrushes never touch yellow-berried holly but eat all the red berried holly as soon as it ripens.

- waxwings and bullfinches only eat the yellow fruit of 'Golden hornet' crab apple when it goes brown and begins to rot. If you want to attract birds to a garden in autumn, don't plant yellow-fruited trees and shrubs

- blackbirds go mad for the blue, bloom-covered UV reflecting, high anthocyanin containing berries of red flowering currant and Berberis - there's purple-stained bird poo all over our garden right now!


Click here for two more long-standing botanical mysteries

Sources:

Willson,M.F. and Melampy, M.N. (1983) The effect of bicolored fruit displays on fruit removal by avian frugivores. Oikos 41: 27-31.

Siitari, H; Honkavaara, J; Viitala, J (1999). Ultraviolet reflection of berries attracts foraging birds. A laboratory study with redwings (Turdus iliacus) and bilberries (Vaccinium myrtillus). Proc.Roy. Soc (B) 266,2125-2129

Jennifer M. Cramer, Maria L. Cloud, Nathan C. Muchhala, Anastasia E. Ware, Brent H. Smith and G. Bruce Williamson (2003) A test of the bicolored fruit display hypothesis: Berry removal with artificial fruit flags. Journal of the Torrey Botanical Society 130, 30-33

Honkavaara, J; Siitari, H; Viitala, J (2004) Fruit colour preferences of redwings (Turdus iliacus): Experiments with hand-raised juveniles and wild-caught adults. Ethology, 110, 445-457

Schmidt, V; Schaefer, HM; Winkler, H (2004) Conspicuousness, not colour as foraging cue in plant-animal signalling. Oikos 106, 551-557

Schaefer, H. Martin; Levey, Douglas J.; Schaefer, Veronika; et al. (2006) The role of chromatic and achromatic signals for fruit detection by birds. Behavioural Ecology 17,784-789

Schaefer, H. M.; McGraw, K.; Catoni, C. (2008) Birds use fruit colour as honest signal of dietary antioxidant rewards. Functional Ecology 2, 303-310


Schaefer, H.M. and Braun, J . (2009)  Reliable cues and signals of fruit quality are contingent on the habitat in black elder (Sambucus nigra). Ecology 90, 1564-1573.


Larrinaga, Asier (2011) Inter-specific and intra-specific variability in fruit color preference in two species of Turdus. Integrative Zoology 6, 244-258


Schaefer, H.M. (2011) Why fruits go to the dark side. Acta Oecologica 37, 604-610.

Botanical artillery...

Cranesbills are wild flowers with a particularly violent method of dispersing their seeds, that involves a catapult mechanism and is particularly well developed in bloody cranesbill Geranium sanguineum.

Cranesbills belong to the genus Geranium, a name that's derived from the Greek word geranos, meaning 'crane' (the bird, not the mechanical device) and nicely describes the central axis of the fruit, which resembles a crane's beak.

There are five egg shaped seed containers at the base of that long beak, which is formed from five long strips of plant tissue that are firmly attached to the tip of the beak and develop tremendous internal tensions as they dry out. They are botanical springs and when their inner tensions reach a critical point they break free at the base and curl away from it with enormous force......

.... so that they curl upwards with the egg-shaped containers attached, hurling out the seed which lies in each container, in much the same way as those dog ball throwers that dog walkers use. After the seeds have been dispersed this elegant candelabra-like structure is left behind.

Bloody cranesbill is the official county wild flower of Northumberland.

A plant with 100 names

Until a few years ago we had just one plant of cuckoo pint Arum maculatum in our garden but every year more appear - probably because the blackbirds are eating the ripe fruits and dispersing their seeds. I reached this one before the blackbirds found it.  It's well known that cuckoo pint berries are dangerously poisonous to humans but birds seem immune to its toxins, as they are to those of many other poisonous plants, including deadly nightshade.

Cuckoo pint is a fascinating plant in many ways. Most of the aroid family, to which it belongs, are native to warmer climates and include the spectacular titan arum and the voodoo lily, whose stench is breathtakingly awful . Almost all aroids have fascinating pollination mechanisms and cuckoo pint is no exception, trapping small insects such as owl midges in its inflorescence until they pollinate the flowers - you can find photos and an explanation by clicking  here.

Cuckoo pint is just one of over a hundred regional vernacular names for this plant, which is unusual in being a rare example of a single species in the British flora that has had a whole book devoted to it, under one of its aliases - Lords and Ladies by Cecil T. Prime, published in 1960 as a special volume in the Collins New Naturalist series. It's well worth seeking out a copy of this scholarly but highly readable book, in which Prime describes every aspect of the plant's biology and history, including its use in the production of starch for stiffening the extravagant  ruffs worn by Elizabethan courtiers. During this period starch was produced from cuckoo pint's tubers when starch from wheat was in short supply.

Later the industry was relived on the Isle of Portland in southern England and Prime describes the hazards of the laborious extraction process, quoting John Gerard: "The most pure and white starch is made of the roots of Cuckow-pint; but most hurtful to the hands of the Laundresses that hath the handling of it, for it choppeth, blistereth, and maketh hands rough and ragged and withall smarting". 

The hurt was caused by needle-sharp crystals of calcium oxalate which are present in high concentrations in the tubers and in other parts of the plant - you can see photographs of similar crystals in another plant by clicking here.

It must have been incredibly laborious work - last year I dug up a cuckoo pint tuber from the garden and found that it was about the size of an acorn; vast numbers would need to be harvested to produce starch in commercial quantities. Remarkably, the industry continued in Italy (in Tuscany) using the closely related Arum italicum right up until 1919.

Scrumping bears

Whenever I look over a garden wall and see this sight it takes me straight back to youthful scrumping adventures. This is an old and very fine apple tree, in a garden in Durham city, that holds onto its apples right up until Christmas. Fine food for fieldfares, because the fruit is too high up to be pickable (these must be around 15 metres above ground) and will most likely be smashed when it falls and hits the deck. I wish I knew what variety it is.

Genuine native crab apples are quite rare and their fruit stays green and incredibly bitter, but there are plenty of hedgerows around here with feral culinary apples that must have sprouted from discarded apple cores. The skin colour of this one, which is just about to rot, is particularly attractive. 

Molecular biologists, comparing DNA sequences, have shown that the cultivated sweet apple Malus pumila is not descended from crab apple M.sylvestris.  Dr. Barrie Juniper at Oxford University has suggested that the unknown ancestor of culinary sweet apples appeared about 10 million years ago in the Tien Shan forests of Central Asia, where brown bears may have played a role in its evolution. Apple trees always cross pollinate, so apple seedlings are genetically variable and never exactly resemble their parents. Juniper has argued that wild brown bears, known to have a ‘sweet tooth’, would have selected the sweetest, largest fruit as part of their autumn diet. Tough seeds of these superior fruits passed through bears’ digestive tracts unharmed and were dispersed widely. Once humans arrived on the scene their horses, also partial to apples, distributed apple seeds in their droppings as tribes migrated westwards. With the advent of agriculture, about 10,000 years ago, humans began selecting even sweeter varieties from the pool of genetic variability in feral apples and learned to graft them, to perpetuate the sweetest varieties.

Commonplace Beauty

I think it's one of the fundamental truths of natural history that you can find beauty in all living organisms - even a commonplace dock Rumex sp. plant like this - if you look a little closer. The ripe seeds are attractive to look at and also favourite food of bullfinches in autumn.

Add a cobweb and some early morning sunlight and a whole dock plant becomes attractive.

Violent violets, with Seeds as Slippery as a Bar of Soap ....

Summer is over and it's time for plants to disperse their seeds. Some just release them on a feathery parachute, to drift away on the breeze. Others hook themselves into fur or feathers and let animals carry them passively. Others are enclosed in succulent fruits and pass through an animal's digestive system.

And then there are those with a violent tendency, with exploding seed pods and catapult mechanisms or, in the case of some members of the genus Viola (pansies and violets), a mechanism that's easier to demonstrate than describe. 

It works like this. Grip a wet bar of soap as tightly as you can, then watch the soap fly out from between your fingers. Much the same thing happens when a pansy disperses its seed. It's globular seed capsule splits into three boat-shaped sections that fold outwards and curl inwards at their edges, exerting ever-increasing pressure on the smooth-coated seeds that they enclose ..... until the seeds are violently shot outwards.

A Tree-Spotter's Guide to Fruits and Seeds: Part 2

Yew Taxus baccata, one of only three native conifers in Britain (the others are Scots pine and juniper) is a conifer that doesn't produce cones. The vast majority of its relatives carry seeds on the surface of whorls of woody bracts that form a cone, but yew produces single seeds surrounded by a red fleshy aril. In the picture above you can see an aril that has yet to swell and envelope its seed, just above and to the right of the seed in the centre of the image. Thrushes are particularly fond of the succulent aril. All parts of the tree are very poisonous apart from this conspicuous coat but the thick hard wall of the poisonous seed (which can take two years to germinate) means that it can pass through the bird's gut with no ill effects. There are separate male and female yew trees, so any that are not bearing seeds at this time of year are probably male. There is a rare mutant of yew (which I have never seen) known as var. lutea that bears yellow arils. In Sussex you can find one of the finest of all yew forests, at Kingley Vale National Nature Reserve, filled with venerable trees whose dense shade prevents anything from growing beneath them -well worth a visit if you are ever in that part of the country. Some churchyard yews in various parts of Britain are reputed to be over 2000 years old.

Usually privet Ligustrum vulgare grows as a shrub and is often clipped into a neat hedge, but it it's allowed to grow unchecked it will grow into a small tree, flowering prolifically, attracting butterfly and bee pollinators and producing these rather attractive indigo-hued berries. It's a member of the olive family (Oleacea), as is ...

... the common ash Fraxinus excelsior. Ash trees can be either male, female or hermaphrodite and it's only the latter two forms that bear these familiar ash 'keys', in large bunches. Ash seeds are dormant and can take up to 18 months to germinate. They begin to fall in late autumn and require a winter chill before any will germinate - and even then only a small percentage of the crop sprouts in spring. The rest are immature and the embryos inside continue to develop throught the summer, on the tree or on the ground, and will not germinate until the following spring.

Some potential ash seeds never mature, if the flowers are attacked by a microscopically-small gall-mite called Eriophyes fraxinivorus (also known as Aceria fraxinivorus). Trees like this one alongside the River Tyne at Wylam can become completely infested and produce few seeds.

For more posts on tree ID click here

Elder - the Elixir of Life?

There seems to be a fine crop of elder Sambucus nigra berries in our local hedgerows this year. John Evleyn, the 17th. century writer, gardener and diarist, was a great fan of this ubiquitous hedgerow tree. “If”, he wrote, “ the medicinal properties of the leaves, bark, berries,&c., were thoroughly known, I cannot tell what our countrymen could ail, for which they might not find a rememdy from every hedge, either for sickess or wound. The inner bark of elder applied to any burning takes out the fire immediately; that, or in season, the buds boiled in water-grewel for a breakfast, has effected wonders in a fever; and the decoction is admirable to assuage inflammation. But an extract may be composed of the berries, which is not only greatly efficacious to assist longevity, but is a kind of universal preventive against all infirmatives whatever; and of the same berries is made an incomparable spirit, which drunk by itself, or mingled with wine, is not only an excellent drink, but admirable in the dropsy. The ointment made with the young buds and leaves  in May with butter is most sovereign for aches, shrunk sinews &c., and the flowers macerated in vinegar not only are of a grateful relish, but good to attenuate and cut raw and gross humours”. All of which may go some way to explaining why my maternal grandmother, who used to make some pretty potent elderflower champagne and elderberry wine, lived to a ripe old age...... although more recent research and opinion has been more circumspect about the safety of some of the folk medicine attributed to this plant. 

Elder shoots grow remarkably vigorously in their first year and I have vivid childhood memories of cutting these, hollowing out the pith and using them as pea-shooters. The hollowed-out twigs have also been used to make flutes and the generic name Sambucus supposedly comes from the Greek (?) sambuca, a musical instrument – although the word was originally applied to a stringed instrument rather than one that you blow. Elder pith, dissected from the centre of the stems, figured in my education when I was at school, for demonstrating electrical charges and for holding plant specimens that were then thin-sectioned for microscopy by hand, using a cut-throat razor (can you imagine Health and Safety allowing that in a school today!)

Recently elder as a natural resource has undergone something of a revival, with the popularity of elder flower cordials and elder flower presse, which has created an unprecedented demand for the inflorescences. Nice to see that at least some of the potential of our native biodiversity is being realised…..

Trees on the Move

Just outside the fence on the south-west corner of our garden there's a large Norway maple whose winged seeds are scattered all over our vegetable patch by south-westerley gales in autumn. All winter hundreds of seeds have been laying on the soil surface, buried by snow and dusted with frost crystals, and recently they've all begun to germinate, pushing out their first root into the soil. If we didn't cultivate this patch of ground every spring we'd have a small forest of Norway maple by now; some, behind the greenhouse, have escaped the hoe in earlier years and are on the way to becoming small trees.

Like many plants, the dormant seeds of Norway maple are incapable of germination when they're shed and it takes a winter's frosts to break down the dormancy compound - abscisic acid - inside the seed and allow germination to begin, just as the temperatures begin to rise in spring.

For more posts on tree ID click here

The Alien that Conquered Britain

The conker season is here again – amply celebrated today in an article in the Times newspaper (see http://www.timesonline.co.uk/tol/news/uk/article6860922.ece). Horse chestnut Aesculus hippocastanum is our best-loved alien species which, during the 400 years since it was first introduced into Britain from the Balkans, has graced our parklands and become the focus of a traditional sport that has left generations of school-kids nursing aching knuckles after misjudged swings of an opponent’s conker.

The seeds of horse chestnut are truly exquisite objects. Their deep umber hue has given its name to a colour and their glossy texture contrasts wonderfully with the spiky husk that they are enclosed within. Sadly, it’s a beauty that doesn’t last – after a day or two those glossy seeds become leathery and lose their rippled patterns. In the USA Aesculus species are known as buckeyes, on account of the resemblance between those glossy brown orbs and the eyes of deer.

I may be wrong, but I suspect the horse chestnut seed is the largest amongst trees in Britain. They invest a lot of resources in giving their germinating seeds the best possible start in life, by providing them with a very large food store.... but at the expense of long-distance dispersal (take a look at http://beyondthehumaneye.blogspot.com/2009/10/travelling-light.html for a completely different plant seed dispersal strategy).

One of the perils of producing seeds with a large food store is that they simply become food for hungry animals (think of all the acorns that are guzzled by squirrels, jays and wood pigeons) - which may be why conkers contain high levels of toxic substances called saponins which are natural detergents and generally damaging to animal digestive systems. Ground-up conker flesh can provide a deterrent against slugs, which have a strong aversion for saponins and tend not to cross a barrier of conker meal. This might be the basis for a natural method of slug control, but for the fact that saponins are water soluble, so that any attempt to use them as slug deterrents would be washed away in the first shower of rain.

Magic of the Hemispheres

Remember those rotating globes of the world that used to be in every school classroom? I recall having a miniature version given to me as a Christmas present by parents who probably hoped it would sharpen up my performance in geography classes. The globe separated around its equator into two hemispheres, just like this scarlet pimpernel Anagallis arvensis seed capsule, which separates and releases a shower of seeds from within it you gently squeeze it. For the plant in flower, see http://cabinetofcuriosities-greenfingers.blogspot.com/search/label/Anagallis%20arvensis)

Botanically, this form of capsule is called a pyxidium - useful to remember if you've got an x to get rid of in a game of scrabble. Plantains (Plantago spp.) have a similar arrangement in their rows of minute capsules, although they're smaller and less aesthetically pleasing.

A Study in Scarlet.....

....and crimson.

Why are so many berries – including the species below - red?

Guelder rose

Hawthorn

Sorbus (intermedia?)

Bittersweet

The standard answer is that bird vision is particularly sensitive at the red end of the spectrum, which is why they’re attracted to red fruits. Bird pollinated flowers in the tropics tend to be red too. It’s undoubtedly true that there is a correlation between this colour and attractiveness to birds, but it’s also now known that birds are also able to see ultraviolet, at the opposite end of the spectrum – a wavelength that our eyes are not sensitive to. So we can’t make an absolute direct comparison between the colours that we see and the colours that birds see – when we see purple, for example, it’s a combination of blue and red light reflected from opposite ends of our spectrum of visual perception. Bird purple would be a combination of ultraviolet and red – whatever that might look like. Although we can’t have absolute knowledge of the colours in bird-world, what researchers can do is to present birds with colour choices to see what they prefer...... and that produces some interesting results. It turns out that it’s not just the colour of the berries in our visual spectrum that's important - it's also the contrast between the colour of the berries and the background, including UV light reflected or absorbed by leaf sufaces, that’s also important – the contrast between berries and the background affects the choices that birds make, not just the colour of the berries. Also the waxy  ‘bloom’ on many black, blue and purple fruits (like bilberries) reflects UV light, which makes them conspicuous to birds – rub the bloom off and birds tend to be less attracted to them. Finally, experience counts too - juvenile and adult redwings show different preferences, so learning has a role in associating fruit colour with the best food sources. As in so many branches of science, research hasn't yielded clear-cut answers yet, but it has produced some very interesting questions. The story has as many twists and turns as a Shirlock Holmes mystery. What is certain is that simply trying to interpret what birds (or any other animals) see according to what we see is unlikely to give a true impression of the way the world looks to them.

Exploding Acorns

Knopper galls (above), uninfected normal acorn below 

These weird objects are knopper galls, caused by a tiny wasp called Andricus quercuscalicis, that I found on oaks growing beside the River Tyne at Wylam this morning. The wasp lays its eggs in flower buds which, instead of developing into acorns, grow into these popcorn-like galls that provide a home for the wasp larvae and pupae through the winter. The young galls are bright green and covered in sticky sap, but as they age they turn brown before dropping off. If you collect a few and keep then in a dry container over winter you can watch the minute wasps emerge through the pore in the gall in spring. The causative gall wasp first arrived in Devon sometime in the early 1960s, having slowly spread across Europe from Turkey, and colonised rapidly once it arrived in England, so it’s now well established throughout most of Britain and well into Scotland. It first arrived here in the North East in the 1980s. The conspicuous damage that it does to acorns led to widespread speculation that it would destroy the acorn crop and would be a major threat to the future of oaks in Britain, but infestation levels vary a lot from year to year and it’s unlikely that this minute wasp poses a long-term threat to our national tree. Whenever you see a knopper gall (usually on pedunculate oak Quercus robur), there’s sure to be a Turkey oak Quercus cerris (identifiable by its hairy acorn cups) somewhere nearby, because the wasp spends half of its life cycle on this introduced tree, which is widely grown in parks, arboreta and large gardens, and the remainder on native pedunculate oak.

Turkey oak acorns