Why house sparrows lay both big and small eggs

phys.org | 8/9/2018 | Staff
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Why does the egg size of house sparrows vary so much? Isn't it always an advantage to be big?

Perhaps unsurprisingly, baby sparrows that hatch from large eggs are consistently bigger than their small egg counterparts. They can store up more reserves if food becomes scarce. So you would think that it's always a good idea to lay big eggs, because your offspring would seem to have a greater chance of survival.

Research - Biologists - University - Science - Technology

But research biologists from the Norwegian University of Science and Technology (NTNU) noticed that the difference in volume from the smallest to the largest egg can vary by 50 percent. Between 2003 and 2009, they examined the egg sizes of an insular population of house sparrows on Hestmannøy in Nordland County. Now, the results have been published—and they are surprising.

"We looked at the survival of the young and the chances of recruitment," says postdoctoral fellow Thomas Kvalnes at NTNU's Centre for Biodiversity Dynamics (CBD). The researchers didn't simply look at whether the young survived, but also whether they survived to be able to produce their own young when they became old enough. They found two main attributes: First of all, big eggs are advantageous for the short-term survival of the young—but only when there's a lot of rain. During rainy periods, parents rarely venture out to get food because insects are less accessible. In that case, it's an advantage for the nestlings to have more energy reserves from the egg.

Researchers - Connection - Temperature - Eggs - Temperatures

But researchers also found another connection: Temperature affects whether it is advantageous to hatch from large or small eggs. In higher temperatures, the young are better off if they hatch from smaller eggs. "At low temperatures, extra reserves from large eggs are important, while at high temperatures, the same reserves probably lead to faster growth than is optimal for long-term survival,"...
(Excerpt) Read more at: phys.org
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