Aaron's project: What's in an egg?

 My name is Aaron Aguirre and I am an undergraduate at Michigan State University studying Fisheries and Wildlife. This summer I was lucky enough to be Cara’s assistant as she conducted her research.

As part of helping Cara with her work, I was also responsible for developing my own topic of study and designing an experiment to conduct throughout my time here. Delving away from bird songs and calls, I decided to put more of a focus on the other ways in which house wrens communicate with each other.  Something Cara had said to me sparked my curiosity in the eggs of these birds. She told me that the last egg of a female’s clutch will often be much lighter than the others because male house wrens seem to be attracted to lighter eggs and more willing to invest in caring for them. After collaborating with Cara I decided to study if the brightness of a female’s eggs relates to their physical condition as well as if stress on the females will affect the brightness of their eggs. 

 Above: An example of the photos Aaron used to collect data on eggs.  As you can see above, the last egg is often brighter than the ones before.  A previous graduate student in the Getty lab, Lindsay Walters, studied the connection between egg color and male investment.


My interest in this topic stems in a similar way from why Cara is studying house wrens. When people think “bird communication,” they will probably think about the multitude of bird songs and calls they hear in the early morning. Similarly, most will think that male song birds are the gender that are competing and proving themselves to the females; a nice song tells the female that a male is in high condition worthy of being their mating partner. This is why I find my topic to be so interesting; it is the unexpected. By looking at the eggs, I am seeing how females are communicating to the males and how they show males that they are physically fit and worth investing time and energy into ensuring the success of their offspring.

This season by the numbers

  Despite the population being roughly half the size of a typical year, we've accomplished a lot this season so far!  So far there have been 33 females and 34 males at our field site (that we know of).  All but one are currently banded.  We've collected 77 blood slides and samples from these birds and run 98 aggression trials.

These females have been busy.  They've laid 266 eggs (and they're still going!).  We've measured all and photographed most.  159 eggs have hatched and 82 of these offspring have successfully fledged.  We've measured babies 524 times and there are many more still to come!  

Nature and Nurture

What makes you who you are?  The color of your eyes or the shape of your nose come from the genes you inherited from your parents.  Something like your personal beliefs come from your social and cultural environment.  However many things, such as your risk of developing a particular disease, are influenced by both your genes ("nature") and your environment ("nurture").  Scientists studying animals must keep in mind the influences of both nature and nurture when studying a particular trait or behavior.

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

Nestlings

11-day-old baby nestling

 

Raising baby house wrens is hard work.  After a ~7 day egg laying period and a ~12 day incubation, the house wrens hatch and start growing quickly.  The babies gain around a gram each day, reaching adult weight in just 10 days!  By day 18 they can fly and look just like adult wrens.  This is a lot of work for the females.  Not only does she help the male feed them every several minutes throughout the day, but she cuts down on her feeding time drastically when she is incubating and may lay 2 or sometimes almost 3 times her body weight in eggs during the season.  Whew!

 

One of the explanations for why females are generally less aggressive than males is that aggression interferes with reproductive demands on females.  This trade-off exists for some males.  The Ketterson lab has found that  male juncos given testosterone implants (the hormone linked to aggression in males) are not as good at feeding their nestlings.  We don't know yet if testosterone is responsible for aggression in female wrens.  Regardless of the hormonal mechanism, fighting with other females could take time and energy away from the busy work of raising babies.  

 

I originally predicted that more aggressive females would have smaller babies.  However, I was surprised to find that babies from aggressive moms actually end up bigger!  In other migratory bird species, the size of the nestling at fledging has a big impact on it's chances for survival.  If these babies have a survival advantage and aggression is passed down in house wren genes, female aggression will continue in the house wren population.     

Osprey

The baby osprey are getting close to fledging.  The house wrens are already well into their 2nd brood, but the osprey are just finishing their first.  It takes a lot more food to make a baby osprey!  There are three babies in this osprey nest, although one is significantly smaller than the rest.  You can see one of the babies on the left side of the nest in the pictures below.

Some larger bird species frequently lay one more egg than they can typically raise successfully.  This extra egg acts as an insurance policy.  Usually this baby is too small to compete with it's siblings.  However, if something happens to one of the larger babies, the parents still have a chance to reproduce.  If it's a particularly successful year, the parents can raise an extra baby!

 The male osprey comes in for a landing

One fledgling on the left, adult female in the middle, and adult male on the right