Birds get angry with each other, as well as show aggression when faced with danger. This behavior is marked by a series of actions.

Hummingbirds were shown to fight over mating interests, using their beaks as weapons. Differing anatomies between male and female members of long-billed hermits was thought to be a sign they consumed different foods. But, the beak of males has now been shown to be a weapon in the fight for mates.

"Historically, bird beaks have been the prime example of adaptation through natural selection, such as in the textbook example of Darwin's finches. But we show here the first evidence that bills are also being shaped by sexual selection through male-male combat," Alejandro Rico-Guevara of the University of Connecticut, said.

Male swamp sparrows have been recorded in fights to the death with each other, but the animals try to avoid fights, according to a 2013 study. Like a pair of would-be-brawlers in a bar, the animals try to scare the other off before engaging in a fight. These birds do this by flapping their wings, making noise, and trying to look larger than they are when relaxed.

Red-tailed hawks are known to attack human beings. During the 2010 nesting season in Connecticut, the animals attacked students and adults, swooping down, scratching targets with razor-sharp talons. Outdoor gym classes had to be moved indoors to protect students from the birds, defending their nests.

Even Harry Potter fans may want to get out of the way of snowy owls, which were featured in the popular movies. These birds stand up to 18 inches tall, and their wingspan can reach four feet. When these birds of prey sense a threat to their nest, as from an approaching human, they will launch themselves toward the intruder. Snowy owls use their talons to attack the head and face of those seen as a threat, including ripping out the eyes of people intruding on their territory.

Barred owls can also prove to be dangerous, even to humans. As the owls move into the Pacific Northwest from the southern United States, attacks on hikers are increasing. Four Texans were bloodied in an attack from a barred owl in British Columbia in 2007.

The southern cassowarry may be the most dangerous bird of all. These natives of the rain forest are not afraid of humans, and have ripped people apart who were perceived as threats.

Sparrows and other birds stand to lose a lot in fights with other creatures - even their lives. However, the need to protect shelters and mates makes the fights worthwhile, when needed.

"It's high stakes for these little birds. They only live a couple of years, and most only breed once a year, so owning a territory and having a female is high currency," Rindy Anderson, a biologist at Duke University, said

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