We've found in previous years that more aggressive house wren moms have bigger babies. Why is this? We know more aggressive moms don't feed their babies more often (at least as far as we've observed). Maybe aggressive females have genes that produce bigger babies. Maybe they lay eggs with more hormones that make babies grow faster. Maybe they are better at foraging for high quality food or they interact with their babies in some other way that promotes growth.
To try to disentangle the influences of nature and nurture, this summer I'm doing a "cross-foster" experiment. This is common experimental approach where scientists swap eggs or babies between nests. Birds like house wrens treat anything in their nests as their own offspring. This allows scientists to look at the influence of one genetic background in a different social environment. This summer we're swapping some clutches before incubation starts. If more aggressive females have genetic offspring that are bigger, the cause is likely in the genes or in the eggs. If more aggressive females instead have bigger social offspring, the cause is likely something in the environment or the female's behavior. However, the answer may be somewhere in between.
Two clutches that were swapped earlier this summer