Who you are has a lot to do with the way that you were raised; not just by your parents but also by your immediate and extended families and the environment that you grew up in. The same is true of many birds. Growing up in a nest with lots of siblings could make birds smaller, as they need to fight with each other for food and space. But your genes also determine how big you are to some extent. It's the classic argument about nature versus nurture. Generally, both have a say in many important traits from how big something is to how fast or slow its metabolic rate is. So, what happens if you change the environment that a bird grows up in? Does being around fewer nestmates make you bigger and give you a faster or slower metabolism? To answer these questions, Nina Cossin-Sevrin, Antoine Stier, Katja Anttila and Suvi Ruuskanen of the University of Turku, Finland, working with Mikaela Hukkanen of the University of Helsinki, Finland, and Sandrine Zahn and Vincent Viblanc of the Université de Strasbourg, France, turned to the great tit (Parus major), a common songbird in the woodlands of Finland.
Between April and July 2020, the researchers studied great tits nesting in artificial nest boxes on Ruissalo Island, Finland. First, they changed how many birds were in each nest by removing two, 2 day old chicks from certain nests and placing them in the care of foster parents which had eggs that hatched the same day, increasing the number of nestlings in the foster parent's nest by two. During this process, Cossin-Sevrin and colleagues measured how big the 2 day old chicks were by measuring their wings, and returned 7 days and 2 weeks later to check how much the chicks had grown. Over the first 14 days, chicks from larger nests were smaller than those that had fewer nestlings. Surprisingly, after the birds were recaptured later in the year, there were no differences in how big the chicks were regardless of which type of nest they came from, suggesting that the number of chicks in the nest doesn't have a negative impact on the birds in the long term. However, the researchers point out that when there are lots of chicks in a nest, the parents spend extra time hunting for food to feed all the hungry mouths. So instead of having to fight over every scrap of food, the birds from larger families are being fed just as much as those from nests with fewer chicks.
Between October and November of the same year, the team recaptured some of the birds to test their metabolic rates now that they were a bit older. The researchers found a similar result: the number of mitochondria in the chick's blood cells didn't change and neither did their metabolic rate, though older animals did have fewer mitochondria than younger chicks. The team also found that the birds didn't make any more superoxide, a detrimental chemical that is sometimes produced when mitochondria make energy. This suggests that their mitochondria were working efficiently, making energy instead of producing the harmful by-products.
Perhaps most importantly, the team found that a higher proportion of birds were able to leave the nest if they came from larger broods. Just as having the genes that make chicks bigger and hardier could help them survive, being raised in the right environment can give chicks a wing up as well.