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Animal magnetism: modeling flocks of birds using simple attractions

Large collections of animals, such as flocks of birds or schools of fish, are difficult to model mathematically. A large flock of starlings, for example, may contain thousands of individuals covering a volume nearly 100 meters across, yet the entire group is able to fly as a unit, changing direction dynamically in response to its environment. In this sense, flocking may be similar to physical systems where order arises spontaneously.

A new study by William Bialek et al. models the flocking patterns of European starlings on ferromagnets (commonly known as permanent magnets), where individual electron spins within a solid spontaneously align, giving rise to an overall magnetic field. By making a minimum number of assumptions about how the birds behave, the authors focus on determining whether the flock's patterns of flight can arise from simple local interactions between pairs of birds. The researchers found they could model flocking behavior with a small number of theoretical parameters, and determined that the amount of interaction was independent of density of the group: the physical distance between two birds is less important than the fact that they have neighbors. 

Even though the model contains no long-range communication across the flock, it allows the entire group to change direction spontaneously.

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Four-winged dinosaur fossilized after swallowing a bird

Feathered dinosaurs existed for millions of years before the origin of birds and, for a time, there was a period where feathered dinosaurs and early birds coexisted with animals that don't fit neatly into either category. One of the latter was microraptor, a feathered dinosaur that doesn't appear to have been of the avian lineage but still seems to have had wing-like feathers on all four of its limbs. Researchers have now found that microraptor did have a close relationship with birds: it ate them.

Microraptor, as its name implies, was a small dinosaur, maybe a meter long counting its tail. Based on the number of fossils we've recovered, it was about as unremarkable as a pigeon in its day, which was about 120 million years ago. But most reconstructions suggest it looked rather unusual by modern standards. That's because microraptor featured wing-like forelimbs that retained claws, and hindlimbs that had an array of feathers similar to that on its forelimbs. A number of researchers suggest it could fly or glide, possibly using all four limbs for lift.

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Singing in the brain: wrens cooperate to make better music

Deep in the cloud forests of Ecuador, on the slopes of an active volcano, there lives a chubby, unassuming looking bird known as the plain-tailed wren. As the name implies, it does not look extraordinary, but there is something extraordinary in the way this bird sings. The plain-tailed wren is famous for its unusual duet, where the vocalizations of a male and a female meld so seamlessly that one might think it was a single bird singing.

"What's happening is that the male and female are alternating syllables, thought it often sounds like one bird singing alone, very sharply, shrilly and loudly," explained John Hopkins behavioral neuroscientist Eric Fortune.

In order to examine how sensory information from each wren is used to coordinate singing between individuals for this cooperative behavior, Fortune and his colleagues listened to more than 1,000 wren vocalizations captured in over 150 hours of recordings. They found that wrens commonly sang duets, but both males and females also sang on their own as well. The structure and sequence of syllables sung in duets and solitary singing were identical—with gaps in the individual song where the partner would normally sing. But during a solo, the duration of gaps between sung syllables varied more significantly. This suggests that sensory cues affect the duration and variability of the gaps, and that the birds do not use a fixed pattern to sing.

To learn how cooperative duet singing was encoded in the brain, the researchers captured 6 birds and monitored brain activity in the area that controls singing. They recorded up to 30 hours from each of the three female and three male wrens, and then played back isolated "units" from the recordings. These various pieces included both duets and isolated syllables, and the researchers manipulated some of them, reversing a clip in its entirety or presenting each syllable in reverse order. The researchers expected to find that the brain responded most to the wren's own singing voice, but both females and males responded best to the duet.

"We found that the brain of each individual participant prefers the combined activity over his or her own part," said Fortune. Since the wren's brain responses were stronger for duets than for any other sound, it appears that their brains are wired for cooperation. Because the neurotransmitter systems that control brain activity at the molecular level are nearly identical in all vertebrates and the layout of the brain structures is the same, the brain mechanisms observed in the wrens could hint at the same ones used for cooperative behavior in other vertebrate species.

Science, 2011. DOI: 10.1126/science.1209867  (About DOIs).

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