"If you travel by air, it's very important to keep your weight down to a minimum." "You can't afford to carry a lot of fuel around, and what you do carry should be energy-packed and not too bulky." "0ak trees put just such a substance into their acorns." "It's there to fuel the growth of their seedlings, and they protect it with a hard shell, but jays know how to deal with that." "A beak is itself a concession to weight-saving." "It's much lighter than the jaws and teeth that reptiles and mammals use to process their food, yet it's also very efficient and versatile." "A jay using its beak like a pick can cut through an acorn's armour without any difficulty." "Beaks are closely matched to diet." "A goldfinch uses its beak like a pair of tweezers." "It's just the right length for extracting the seeds from between a teasel's spines." "A blue tit, on the other hand, has a stubbier beak." "That gives it the strength to crack small seeds, but it also prevents its owner from getting them from a teasel" "A greenfinch's beak is even stouter and stronger, but it's far from clumsy." "Watch how, with the help from its tongue, the bird delicately removes the outer shell of these rosehip seeds." "The strongest beak in the finch family belongs to the hawfinch." "That can even deal with the cherry-stone." "But the bird doesn't simply rely on brute force." "First, it manoeuvres the cherry-stone into the right position for easy cracking." "It gets rid of the broken shell... ..and now it starts the fiddly operation of removing the husk that covers the kernel" "Pine trees, these are in California, protect their seeds by enclosing them in cones." "When they're still green, the seeds developing within are beyond the reach of most birds." "But the cross-bill has special equipment." "It's the only finch that can twist its upper and lower bill in opposite directions." "Now, right at the bottom, it can feel the soft young seed with its tongue." "Got it!" "After a meal of pine seeds, these American cross-bills regularly fly off to a bank of exposed clay." "They're in need of digestive tablets." "Green pine cones are resinous, and resin may cause stomach upsets, but clay in the stomach will absorb the resin and so prevent any trouble." "A crossed bill, however, is not the best implement for digging." "You have to twist your head to one side to get the point into the ground." "But the birds seem to manage to get a sufficiently effective dose to allow them to take daily meals from the pine trees." "Seeds in the temperate parts of the world, however, whether they are in pine cones, cherry-stones or acorns, all have a major disadvantage as a food for birds - they're very seasonal" "In spring and in summer there's none, and then in the autumn there's a glut." "A single oak tree like this can produce 90,000 acorns in a season." "But there are lots of things apart from birds that eat acorns." "Squirrels do, for a start, so if a jay is to collect acorns in the autumn, it will have to do so quickly, before others grab them all." "It is carrying one in its beak, because its crop is full, as you can see from that bulge on its throat." "There can be as many as nine acorns in there." "But what is a jay going to do with such quantities?" "It can't eat them all." "They store them - by burial" "0ne jay, in a month, may bury as many as 3,000 acorns." "What is more, when winter comes and it's in need of food, it will remember exactly where most of them are." "In North America, oaks still form huge forests and they produce acorns on an astronomical scale." "Among those that harvest them are woodpeckers." "A woodpecker's beak is a drill, and a very efficient one, so it's not surprising that it stores acorns by drilling." "There are as many as 60,000 acorns stored in the holes drilled in this one tree." "All the members of this woodpecker family, eight birds in all, use this one tree." "To start with, the birds deposit newly gathered acorns in a hole to allow them to dry off." "Then, when they have shrunk as much as they are going to do immediately, they're given individual storage." "This larder provides food for the family throughout the year, but the birds have to be very vigilant and ready at all times to repel raiders." "There's also a lot of maintenance work to be done." "As the acorns continue to dry and shrink, they become loose in their sockets." "That would never do." "They would be easy for someone to steal They might even drop out." "0n the other hand, if they are hammered into a hole that is too tight, the shell could crack and then the acorn would rot." "So maintenance is a never-ending, year-round labour, and it takes a lot of care and judgement." "The result is a great acorn treasury that will last the whole family well into the next harvest." "These neat holes are made, not for storage, but for theft." "They're cut by another kind of woodpecker - a sap-sucker." "They're just deep enough to tap the vessels along which the tree transports its sap, and sap is largely what the sap-sucker lives on." "The bird cuts its sap-wells with an accuracy and symmetry that would do credit to the finest cabinet-maker." "Sap normally hardens quickly and seals a wound." "This doesn't." "It could be that the sap-sucker produces an anti-coagulant in its spittle, but if it does, no one yet has managed to identify it." "Even so, each little well eventually runs dry and the bird has to cut another." "These wells have been made in the trunk of a pine tree which produces sap throughout the year." "0ther trees, such as these aspens, only produce sap in quantity in spring and summer." "When the sap-sucker moves on to these, it cuts differently shaped wells." "With this spring increase in the sap supply, new birds appear in the woods." "A yellow-rumped warbler." "They're quick to drink from the wells cut by the sap-suckers." "Food is short so early in the year." "The birches are now in leaf, and the sap-sucker moves on to them, and makes wells of yet a different shape." "A northern oriole - another hungry migrant only too willing to benefit from the labours of others." "And a hummingbird." "It used to be thought that hummers timed their arrival to coincide with the opening of the spring flowers from which to drink nectar." "But they arrive well ahead of that." "Their appointment is with the rising of the sap and the work of the sap-sucker." "Sap is an excellent food - energy rich and easily flicked up with the tongue." "But it can only be collected from many of these northern trees during part of the year." "So when winter approaches, warblers and hummingbirds fly south again to where it's summer all year long." "Here in Mexico, the sap is taken, not only by birds, but by insects." "The trunks of many of the trees seem to be sprouting long hairs." "At the end of each, there's a tiny drop of liquid." "The hair is a tube projecting from the rear of an insect lying beneath the bark drinking sap." "But the insect gets more sugar than it needs, so it excretes the excess." "And that is what the hummingbirds, with exquisite accuracy, manage to collect." "Many different warblers take it too." "The liquid, rather flatteringly called "honey-dew", is so much sought after that some birds take up residence in a particular tree and will drive away any others that try to feed there." "Their meals, however, come in such small instalments that feeding has to be almost continuous." "It takes about an hour for a drop to accumulate at the end of a tube, so to get enough to sustain themselves, the hummingbirds have to travel from one to another on a regular round throughout the day." "Plants produce other edible things as well as seeds and fruit and sap." "They sprout leaves, but in truth leaves are not very good food." "They're very bulky and need a lot of digesting." "So animals that live on leaves, like these cows, for example, tend to be rather hefty creatures with massive batteries of grinding teeth and special capacious stomachs." "Cows, having grazed, lie down and bring up each mouthful for a second grinding chew." "No bird does that - you can't chew with a beak." "Geese have to use a different technique." "They're big birds, as they have to be to accommodate such bulky meals." "But instead of digesting the grass intensively, they eat a very great deal of it and get rid of what they can't digest almost immediately." "The appetite of geese is apparently never-ending, and a flock of them, like these barnacles, will work its way across a meadow nibbling non-stop with almost feverish speed." "And pooping all the way." "It makes a terrible mess, but it does mean that after feeding for several hours... ..the geese are not weighed down by great quantities of undigested grass..." "..and can get into the air without much difficulty." "In South Africa there's a rather smaller leaf-eater - the mousebird." "They do make some attempt to digest their meals a little more thoroughly." "They start feeding early in the morning." "Then, with as much nibbled leaves on board as they can manage, they sit for hours on end with their distended stomachs turned towards the sun, so that its warmth helps with their digestion." "You can't beat a siesta after a heavy meal" "In all the bird kingdom, there's only one species that is really specialised for leaf-eating." "This is it - the hoatzin of South America." "Like a cow it has two compartments to its stomach, the second of which is full of bacteria that help ferment its meals." "In consequence it's a bulky bird and positively clumsy in the air." "More a lumbering cargo plane than a super jet." "Birds, by stripping leaves, eating seeds and drinking sap are exploiting plants - stealing from them." "But many plants exploit birds by using them as couriers." "The arrangement is such an ancient one that both employers and employees have evolved special ways of transacting their business." "The plants attract their couriers with flowers." "They pay them with nectar, which is easy and cheap to produce, because it's no more than water and sugar and that's what I've got in here." "And these Australian rainbow lorikeets love it." "Here in Australia there are some plants that are in flower throughout the year, so it's possible for birds to specialise as nectar feeders, as these lorikeets do." "Their tongues, instead of being hard and leathery, have a feathery brushy tip so they can lap up the nectar." "And the plants, when they have a need for a messenger, advertise the fact by producing flowers with particularly bright petals." "Having collected all the nectar immediately available on one tree, the lorikeets move off in a flock to feed at another, carrying the pollen they collected with them." "But they take nectar from many different kinds of flowers, and if, as here, the plant they next visit happens to be a different one the pollen they're carrying will be wasted." "The plants way of reducing that risk is to recruit an exclusive service with couriers who, during their flowering season, will visit them alone." "Here in South Africa, this species of heather encloses its nectar in a "floral safe"" "which only a particularly shaped beak can unlock." "This orange-breasted sunbird has a beak of that shape, but even so, it has to probe really deeply to reach the heather's nectar." "And every time it does, it triggers a little explosion of pollen." "When the bird drinks at another heather plant, some of that pollen will be brushed off and the heather will have achieved its end." "But bird and flower can fit one another more closely than that." "0n Mount Kenya, there's a sunbird with an even more strongly curved bill" "The golden-winged sunbird." "And this is its employer - the lion's claw flower." "The feathers on the sunbird's head look golden, like those on its wings." "But not so - they're black." "The gold colour is entirely due to pollen which is stamped on it when the bird thrusts deeply into the flower." "The devices used by plants to restrict their payments to their employees may, if taken to extremes, defeat the object of the exercise." "This South American plant has gone, literally, to great lengths to shield its nectar from all but its established partner, and that has encouraged burglary." "This is the black flower-piercer." "It knows exactly where the nectar is stored, and it knows a quick way of getting it, too." "Its tongue is flicking into the nectary at the top of the flower's trumpet so that nectar is channelled down its lower bill into its throat." "The datura has even longer flower trumpets, but they are robbed just as easily." "Here in South America, hummingbirds are the main collectors of nectar, and they will collect it any way they can." "Using a break-in by a flower-piercer is as good a way as any other, as far as they're concerned." "The trumpet of the datura flower is so long that you might think that nothing could drink from it - legally, as it were." "And only one bird can - the sword-billed hummer, which has the longest beak in relation to its body of any bird in the world." "A plant only flowers for a short period each year, so a nectar drinker has to have a succession of suppliers." "A sword-bill also drinks from passion-flowers." "The South American climate is so equable and the number of plant species so huge that there are always some flowers to be found." "Accordingly, hummingbirds have evolved highly specialised equipment for nectar feeding." "They have developed a unique way of flying that allows them to hang stationary in the air while they drink from a dangling blossom." "Their tongues have become threads that flick in and out a dozen times a second, but they're virtually useless for collecting any other kind of food." "Where most plants tend to bloom at the same time of the year, neither the suppliers nor the drinkers of nectar can be so specialised." "So the coral tree, blooming in Thailand, has no alternative but to be generous and offer its nectar in a free and open way." "This delectable seasonal treat attracts all kinds of birds from far and wide." "Such a large and varied clientele is pretty well bound to do the job required of them." "After they are pollinated, plants produce seeds and then many engage other birds to distribute them." "That, by and large, is heavier work, and the payments they offer for that are made with a different currency - fruit." "Hornbills are on their way to do a job for a fig tree in the Indonesian rain forest." "In northern Europe and America, waxwings gorge themselves on autumn berries." "A plant wraps its seeds in the minimum flesh needed to persuade the bird to swallow them." "These berries have so little that it's quickly stripped off in the waxwing's stomach, and then the waxwing can get rid of the indigestible seed." "The story is the same all over the world." "In New Zealand, kokakos are great berry eaters and distribute the seeds of many of the plants of their native forest." "In South America, tiny wild avocados are the special favourite of one of the most dazzling of birds - the quetzal" "The avocados may be small, but they're still too big for the quetzal to swallow." "So the stones are ejected, not from the back end, but from the front." "The bird has had a good meal, and the avocado has had some of its seeds carried to a new site." "There are other things for birds to eat in a forest apart from the products of plants." "They may be difficult to find;" "they may be even more difficult to catch, but they're well worth having because they're full of nutrition." "Things, for example, like this." "The morpho - a big and powerful butterfly." "A jacamar - a cousin of the kingfisher's." "A butterfly's wings aren't very digestible and have to be stripped off before the bird can swallow the fat, nutritious body." "Winged termites erupting from their holes in the ground and flying away to establish new colonies." "A whole host of birds relish these." "Ants are trickier meals." "They, after all, can sting." "It takes a specialist to deal with them." "This is the rufous woodpecker of Southern India." "Its beak is just as efficient at demolishing an ant's nest as it is at drilling into wood, and the bird seems totally indifferent to the ants' sting." "The feathers of its tail, like those of all woodpeckers, are particularly stiff so that they can serve as a prop." "The most nutritious morsels are the soft, fat, stingless grubs that can be found in the very centre of the nest." "Insects are almost everywhere on every tree;" "on twigs, in buds, crawling around in crevices of the bark, and many birds find quite enough to sustain themselves just by looking carefully." "But some work harder - and get greater rewards." "The nuthatch, in European woods, is indefatigable." "It will eat many things, including seeds during the autumn and winter, which it can crack with its workman-like beak, but in summer, insects are a major part of its diet and its beak serves equally well for picking them out of the bark." "The greater spotted woodpecker is a little more specialised." "It particularly likes the grubs of wood-boring beetles, and the first thing to do to find them is to chisel away the bark." "Its tongue extends for an inch and a half beyond its beak and has a harpoon at its tip." "When that hits a grub fair and square, it sticks." "Tree-boring insects are never safe when woodpeckers are around." "But woodpeckers never got to the Galapagos, far away from anywhere in the Pacific." "Insects did though, and their grubs bore into trees here, just as they do everywhere else." "But no Galapagos birds have the physical adaptations with which to reach them." "Galapagos finches, however, are both intelligent and ingenious." "Their beaks are perfectly adequate for stripping away bark." "There's a grub under there somewhere - it can hear it." "But how, without the long tongue of a woodpecker, can it get it out?" "It needs a tool - a spine from a cactus." "A success - but only a partial one." "It has only extracted little bits of the grub." "Nearby, a bird from another clan of finches uses a slightly different technique." "It selects a rather stouter tool that can be used, not so much for stabbing as for levering." "That has shifted the grub a little nearer the hole." "It's not quite within reach, but it's still got that lever handy." "Give it another go." "And that's got the rest of it." "Another remote and isolated island, on the other side of the Pacific" " New Caledonia." "It gets a lot of rain and so it has a much bigger and richer forest than the Galapagos." "But it's so far from any of the major continents that woodpeckers have not got here either." "This fallen tree-trunk is studded with holes;" "the work of wood-boring beetles." "Their size suggests that they're made by much bigger insects than their equivalent on the Galapagos." "The New Caledonian crow - and crows are among the most intelligent of birds." "0nce again, the sound of a grub gnawing away in its burrow betrays its presence." "And once again, since this grub-hunter hasn't got the long tongue of a woodpecker, a tool is needed." "To contact this grub, the stick will have to be thrust in really deeply." "A spectacular catch!" "Some of these crows become so attached to one particular tool that they carry it about with them." "This log is clearly a good source of grubs and a whole group of crows have come here to feed." "Their technique is neither to stab nor to harpoon, but something more subtle - to irritate." "This grub has got big jaws and, if attacked, it can give a powerful bite." "And that's what the crows rely on." "A younger bird joins an experienced adult to see how things are done." "Now the pupil has a chance." "It hasn't got all the details exactly right." "It will be about a year before it masters the skill." "There are insect grubs everywhere, of course." "The only problem for insect-eating birds is getting at them." "But sometimes other creatures are of help." "You might think that this is a recent partnership, but I'll bet when our prehistoric ancestors first dug for tubers and planted seeds in Europe one of these little robins appeared within a couple of days." "0ther animals must have done the same job for them before human beings did." "0nce, not so long ago, wild pig were common all over Europe, and they're great diggers and rootlers." "So maybe the robin's boldness and friendliness with other kinds of animals started in prehistory, even before human beings arrived in Europe." "Such partnerships exist all over the world - even in the most unlikely places." "This little bird lives on a small island in the Indian 0cean, one of the Seychelles:" "so small and so isolated that few mammals got here before human beings." "It's not closely related to the European robin, but it behaves just like one." "When Europeans first came to the Seychelles and saw it, they called it a robin because of its similar habits." "But what partner did it have before human beings came along?" "Could it be this?" "0nce there was a large population of these giant tortoises on several islands in the Seychelles." "They weigh several hundredweight and those huge legs dig into the ground with every step." "There we are!" "These little birds, now even rarer than the tortoises themselves, are still their regular companions as they plod around the island." "A swamp in South America." "An abundance of water and a warm tropical sun make it a paradise for insects of all kinds." "A kind of fly-catcher - a cattle tyrant, and another obliging partner - a capybara, a large semi-aquatic rodent." "As the capybara moves around it inevitably disturbs insects of some kind or another, and what better place for an insect-eater to spot them than sitting on the back of one." "Would the view be any better from there?" "Perhaps it would be." "A few of these partnerships between birds and other kinds of animals have become very intimate indeed." "The hide of a hippo may not seem a particularly rich source of insects, but there are little ticks to be had in the various cracks and crannies, and ox-peckers go there to search for them." "They have extremely sharp claws, with two toes pointing forwards and two backwards, so that they can cling at any angle - even on a slippery hippo." "Land animals with hair on their hide are likely to be more productive ground." "0x-peckers pay particular attention to their ears." "That's the sort of place where you find ticks." "And if there is one, the ox-pecker will remove it." "And they also eat earwax." "Dandruff is another part of their diet." "Their beaks are flattened so that, with their head held sideways, they can comb through their hosts' hair." "0x-peckers spend all their life on, or closely beside, their animal hosts." "They court and mate on their backs." "And when they fly off to make a nest, as they must do, they pluck hair from their hosts' backs with which to line it." "But do they do anything in return?" "They remove irritating, even damaging, insects that their hosts can't dislodge for themselves." "But the bird's main diet is blood." "Sometimes they get it by swallowing ticks that are bloated with blood." "But they also take it directly, pecking at an animal's wounds to keep them open." "When their hosts get irritated, they go back to their toiletry duties, before once again snatching a sip." "So, in spite of having such a specialised life, living on the bodies of mammals, ox-peckers manage to get quite a varied diet." "A maggot here, a tick there, a little sip of blood, perhaps a little tasty earwax." "But there are some birds that literally live on mammals - alive or dead." "They eat them; and those are the birds we'll be looking at in the next programme in this series."