When most of us hear birds twittering away in the trees, we hear it as background noise. It's often hard to separate out one bird from another. But when you can, you begin to hear just how complex birdsong can be, a complex way of male signaling to a female how THEY are the best, and THEY are the one they should clearly pick. You hear ups and downs and trills and repeating themes.
We used to think that birdsong was a relatively simple gene by environment interaction. The big males with the big songs get the best females, and then it's a matter of also getting the best food, and the then healthy bird teaches its offspring to sing, and the health offspring goes on to display the best song. The song is therefore an "honest signal" of the bird's fitness, it's got good genes and good food and it is ready to MATE, baby!
But how much of it is really training and how much is genetic? To find out, we go to what may possibly be the cutest of research subjects...the zebra finch.
(Just lookit those CHEEKS! And it's so PLUMP!!! So cute. Source)
Woodgate et al. "ENVIRONMENTAL AND GENETIC CONTROL OF BRAIN AND SONG STRUCTURE IN THE ZEBRA FINCH" Evolution, 2013.
To look at the relationship between genes and environment in song learning, the authors turned to the zebra finch. Many other studies have also looked at the zebra finch and how it learns song, and how environmental pressures (like say, not enough food) change the way the song is displayed. But those experiments usually bred the birds and looked at the environment...they didn't look at the teachers. The father birds, who were "teaching" their offspring to sing.
These authors did a "partial cross-fostering" design, where they "fostered" some birds with other fathers, while keeping some with the original dad. You can do this by moving the eggs to other nests, pulling the switch so the birds never realize they have a "cuckoo", and then keeping careful track of who exactly is who. They exposed some of both sets, the parent raised and the fostered, to environmental stress, giving them less food (but still enough to live on) compared to other groups. Then they looked at the offspring: how much did they weigh? How much and HOW did they sing? What were their brains like? How did all of this compare to their parents?
What you can see laid out nicely here is how much of each thing: genes (black), environment (white), and genes plus stress (grey) determines how the offspring look and behave, from body weight (far left, and mostly genetic), to different aspects of birdsong (right side of the graph, where you can see the bars are mostly white).
What they were able to show was that body mass? That's mostly genetic? Brain mass? That's a combination of genes and environmental stress (whether or not they were food restricted). But song? Instead of being mostly heritable, as first thought, it turns out that song is much more about who raised you, impacting the peak frequency of song, the number of syllables in the song, the length of each song phrase, and the proportion of unique syllables (whether they repeat some notes over and over, or have a wider range). The only thing that has more genetic input is the maximum frequency at which the birds end up singing. For everything else, it's all about who taught you to sing.
But it gets more complicated when you get inside the brain. There are specific areas of the brain in zebra finches associated with song, the high vocal center (HVC), and the robust nucleus of the arcopallium (RA). The HVC volume was a combination of genes and environmental stress, but the RA size correlated much more with how the birds ended up actually singing, with more contribution from who trained the birds to sing, rather than genetics or stress.
It's a stunning example of how much behavior in the animal world can be a product of training and the environment in which an animal is raised. We often think of animals as being more "hardwired" then we are, more stuck with the results of their genetics and environment, with relatively little role for social aspects. It's very striking to see such a big social role for something as important as bird song, which, after all, determines whether you'll ever be sexy enough to get those genes passed on!
So the next time you pick out a complex song from a bird, don't ask "who's your daddy", ask instead "who raised you to sing that way?"