Bird litter parasites lay eggs in the nests of other bird species, forcing the hosts to do the hard work of raising unrelated young. The team of scientists wanted to simulate the task of breaking an egg – a tactic used by only a minority of host birds to catch and throw out foreign eggs. Their study offers insight into some of the physical challenges faced by host birds that differ.
New discoveries are emerging in Journal of Experimental Biology.
Take cows for example. Their eggs do not look like the eggs of host birds, “yet most hosts do not reject parasite eggs,” said study co-author Mark Hauber, a professor of evolution, ecology and behavior at UI and an expert on litter parasitism. “One explanation is that the shell of a cow’s egg is too thick and firm for a small host to pierce its beak.”
Daniel Clark, an undergraduate student working in Hauber’s lab, teamed up with another professor in the same department, Philip Anderson, a biomechanics expert, to determine if the difficulty in breaking the egg of the litter parasite played a role in whether the host bird tried to expel it. punching, cutting and stabbing. Anderson previously studied the characteristics that contribute to the ability to cut and crush teeth and the penetrating power of canine hops and cactus spines.
The team used chicken eggs in the experiments because collecting and destroying wild bird eggs would be ethically problematic and difficult to standardize. The researchers wanted to determine which elements affect an egg’s ability to withstand breaking.
“The factors we specifically looked at in the newspaper were the presence of the nest, the sharpness of the bird’s beak, and the speed with which it struck the egg,” Clark said.
The team measured the energy needed to break eggs under different conditions: with and without an egg-supporting nest, with a piercing object approaching the eggs at high or low speeds, and with blunt or sharp objects. The researchers used the sharp end of the nail to simulate a sharp beak, and the head of the nail as a replacement for a blunt beak. The experiments included either a fingernail swinging rapidly on the pendulum or a material testing device that slowly pushed the nail into the egg.
The researchers said they were surprised to find that the blunt end of the nail pierced the egg better than the sharp end, especially when it struck the egg at a higher speed.
“My lab has done a lot of research into punching and cutting mechanics, but we’ve always looked at soft materials like skin or muscle,” Anderson said. “The egg shell is fragile – it looks more like ceramic than leather. If you’re trying to break something fragile, like glass, it’s more logical to use a hammer than a knife, so this result isn’t so surprising at first.”
Experiments have also found that nests absorb some of the impact energy of the nail, especially when the nail moves at a slower speed.
“In the slow-motion nest experiment, the nest was very important, but the sharpness or numbness of the nail was less important,” Clark said. “In a fast-moving experiment, the nest was less important, but the sharpness of the nail ultimately mattered a lot.”
The team also found that the act of repeatedly beating the eggs blunted even the sharp end of the steel nail.
“It shows that biological surfaces are much tougher and more durable than we think,” Clark said.
If biting a foreign egg at the bottom of the nest damages its beak, the host bird could reduce its ability to penetrate, weave its own nest, or feed itself and its young. These findings offer clues about factors that affect how – and whether the host bird will react to the arrival of a foreign egg in its nest, the researchers said.
“Our experiments help us understand the long-standing mystery of why most host cows and their conspicuous egg did not evolve to expel a parasitic egg from its nest,” Hauber said.
The National Science Foundation and the Joseph B. Hawkes Research Award from the United Kingdom supported this research.