In Southwestern France, a group of fish have learned how to kill birds. As the River Tarn winds through the city of Albi, it contains a small gravel island where pigeons gather to clean and bathe. And patrolling the island are European catfish—1 to 1.5 metres long, and the largest freshwater fish on the continent. These particular catfish have taken to lunging out of the water, grabbing a pigeon, and then wriggling back into the water to swallow their prey. In the process, they temporarily strand themselves on land for a few seconds.
Other aquatic hunters strand themselves in a similar way, including bottlenose dolphins from South Carolina, which drive small fish onto beaches, and Argentinian killer whales, which swim onto beaches to snag resting sealions. The behaviour of the Tarn catfishes is so similar that Julien Cucherousset from Paul Sabatier University in Toulouse describes them as “freshwater killer whales”.
In Australia, a pair of superb fairy-wrens return to their nest with food for their newborn chick. As they arrive, the chick makes its begging call. It’s hard to see in the darkness of the domed nest, but the parents know that something isn’t right. Whatever’s in their nest, it’s not their chick. It doesn’t’ know the secret password. They abandon it, flying off to start a new nest and a new family somewhere else.
It was a good call. The bird in their nest was a Horsfield’s bronze-cuckoo. These birds are “brood parasites” – they lay their eggs in those of other birds, passing on their parenting duties to some unwitting surrogates. The bronze-cuckoo egg looks very much like a fairy-wren egg, although it tends to hatch earlier. The cuckoo chick then ejects its foster siblings from the nest, so it can monopolise its foster parents’ attention.
But fairy-wrens have a way of telling their chicks apart from cuckoos. Diane Colombelli-Negrel from Flinders University in Australia has shown that mothers sing a special tune to their eggs before they’ve hatched. This “incubation call” contains a special note that acts like a familial password. The embryonic chicks learn it, and when they hatch, they incorporate it into their begging calls. Horsfield’s bronze-cuckoos lay their eggs too late in the breeding cycle for their chicks to pick up the same notes. They can’t learn the password in time, and their identities can be rumbled.
Our intelligence clearly surpasses those of our primate relatives, even though other apes and monkeys also rank within the highest tiers of animal smarts. Likewise, the corvids – the group of birds that includes crows, ravens, rooks, magpies and jays – have very sophisticated brains for birds, but one species reputedly outclasses the rest. It’s the New Caledonian crow.
Found in a Pacific island, this crow wields tools in a way that none of its relatives can match. It uses sticks to “fish” for grubs buried in dead wood, and can chosen the right tool for different jobs, combine tools together, and improvise from unusual materials. These abilities have fuelled the New Caledonian crow’s reputation as the top of the corvid class – an unusually intelligent member of an already intelligent family.
But what if it just has the right face?
If I wanted to fish for a grub, I can use my dextrous hands while moving my face around so I can see what I’m doing. The crow only has its beak, which is attached to its face. But Jolyon Troscianko from the University of Birmingham has shown that it has two features that make the job easier: an unusually straight bill, and an extreme overlap between what both eyes can see. These physical traits set it apart from other crows and corvids, and give it an edge when using tools.
It’s said that beauty is in the eye of the beholder, but that’s only half-true for the Gouldian finch. Jennifer Templeton from Knox College, Illinois has found that these beautiful birds only display their famous fussiness over mates if they’re looking with their right eye. If the right is shut, and the left eye is open, the birds have more catholic tastes. As Templeton writes, “Beauty, therefore, is in the right eye of the beholder for these songbirds.”
The Gouldian finch, found in northern Australia, looks like a bird painted by Gauguin. Its palette includes a purple chest, yellow belly, green wings and cyan highlights. But it’s the head that really matters. They come in red or black (there’s a very rare yellow variant too, but we can ignore that here), and they strongly prefer to mate with partners of their own colours. This isn’t abstract fussiness – genetic incompatibilities between the black-heads and red-heads mean that their offspring are often infertile and feeble. Indeed, these two variants could be well on the way to becoming separate species.
Red and black finches are so easy to tell apart that scientists could be forgiven for neglecting how they do so. But Templeton suspected that the act of choosing a mate was more complicated that anyone had thought.
Here’s an amazing fact: Adult robins have a magnetic compass in their right eye that allows them to sense the direction of the Earth’s magnetic field, and navigate when all other landmarks are obscured. Here’s an even more amazing fact: Baby robins have two such compasses, one in each eye. They lose the left one as they grow up.
Robins kick-started the study of magnetic senses in the first place. In the 1950s, a German biologist called Hans Fromme showed that robins would always try to escape from a cage in the same direction when it came time to migrate. Even though they had no visual bearings, they headed south-west, as if sunny Spain lay just beyond their cages. In 1966, the husband and wife team of Wolfgang and Roswitha Wiltschko showed that a powerful magnet could disrupt this constant vector, sending them skittering in all sorts of directions.
The Wiltschkos have been studying the magnetic sense of robins ever since. In the 1980s and 1990s, they showed that their compass depends on light. They need some of it, and blue-green wavelengths in particular, to find their way. And in 2002, they showed that the compass lies in just one eye – the right one. If they wore a one-sided goggle that blocked their left eye, they could navigate just fine within their featureless cages. If their right eye was blocked, they headed in random directions. It’s not just robins. They right-eye compasses that the Wiltschkos discovered also exist in Australian silvereyes, homing pigeons and domestic chickens.
It’s June in the Arctic tundra, and male pectoral sandpiper hasn’t slept for weeks. He’s too busy trying to have sex. The females will only be fertile for three short weeks, and they’re very choosy. A male has to spend his time chasing the females and displaying with his puffed-up breast, while fighting off rivals and maintaining control of his territory. With so much at stake and so little time, there is simply no time for sleeping.
You might have thought that this constant activity would take its toll on the male. Sleep, after all, is important for our physical and mental wellbeing. Males who go without it for too long should be too tired and addled to make successful suitors. But not so – John Lesku from the Max Planck Institute for Ornithology found that the males who slept the least actually sired the most offspring.
In his lab at Indiana University, James Goodson keeps violet-eared waxbills – a stunning but notoriously aggressive type of finch. Males and females form life-long bonds but they don’t play well with others. “Most of our animals are housed in male-female pairs, but were you to introduce another adult into their cage, most of them would attack immediately,” says Goodson. But some of Goodson’s birds don’t fit the stereotype. They almost never attack intruders.
These birds weren’t born docile. They became that way after Goodson stopped a special group of neurons in their brains from releasing a chemical called VIP. This single act turned fighters into pacifists and confirmed, in dramatic fashion, that there’s a special class of cell that drives aggression in these bird brains.
As first light tickles the air, songbirds croon to proclaim their territories and woo potential mates. There are many possible explanations for the timing of the beautiful dawn chorus, including the fact that sound travels further during the early hours of the day.
But Michael Beaulieu and Keith Sockman from the University of North Carolina have found another for the list. It’s based on a very simple observation: dawn is often very cold. And female Lincoln’s sparrows find songs sexier if they hear them in the cold.
When Rachel Carson wrote her famous book Silent Spring, she envisioned a world in which chemical pollutants killed off wildlife, to the extent that singing birds could no longer be heard. Pesticides aside, we now know that humans have challenged birds with another type of pollution, which also threatens to silence their beautiful songs – noise.
A man-made world is a loud one. Between the din of cities and the commotion of traffic, we flood our surroundings with a chronic barrage of sound. This is bad news for songbirds. We know that human noise is a problem for them because some species go to great lengths to make themselves heard, from changing their pitch (great tits) to singing at odd hours (robins) to just belting their notes out (nightingales). We also know that some birds produce fewer chicks in areas affected by traffic noise.
Now, Julia Schroeder from the University of Sheffield has found one reason for this. She has shown that loud noises mask the communication between house sparrow mothers and their chicks, including the calls that the youngsters use to beg for food. Surrounded by sound, the chicks eat poorly. “City noise has the potential to turn sparrow females into bad mothers,” says Schroeder.
A migrating robin can keep a straight course even when it flies through a cloudy night sky, devoid of obvious landmarks. That’s because it can sense the Earth’s magnetic field. Something in its body acts as a living compass, giving it a sense of direction and position.
This ability – known as magnetoreception – isn’t unique to robins. It’s been found in many other birds, sharks and rays, salmon and trout, turtles, bats, ants and bees, and possibly cows, deer and foxes. But despite more than 50 years of research, the details of the magnetic sense are still elusive.
Unlike light, sounds or tastes, which come and go, the Earth’s magnetic field is always ‘on’. To study how animals sense it, scientists first have to cancel it out using magnetic coils, and set up their own artificial field. The field also pervades the entire body, so there’s no obvious opening, like an eye socket or ear canal, where a magnetic sensor would most likely lie.
In birds – the best-studied of the magnetic-sensing animals – scientists have narrowed down the location of a possible sensor to the eye (sometimes, just the right one), the beak, and possibly the inner ear. But it’s been far trickier to find the individual cells responsible for sensing magnetic fields. Now, Stephan Eder from the Ludwig-Maximilians-University in Munich has developed a way of doing that. It’s deceptively simple: look at cells under a microscope surrounded by a rotating magnetic field, and spot the ones that start to spin.