Linked paper: Four decades of cultural evolution in House Finch songs by C. Ju, F.C. Geller, P.C. Mundinger, and D.C. Lahti, The Auk: Ornithological Advances 136:1, January 2019.
(Read part one here.)
So what did we learn about how House Finch songs have changed since the 1970s—the equivalent of a millennium of cultural evolution in human terms? Here are the main results, and how we interpret them. We have to be careful with interpretation, though, as we cannot be sure that the differences we observed represent consistent trends; it’s possible that the birds have been fluctuating through the years and we merely caught two random points.
- All the main features of House Finch song in 2012 (such as song length, pitch, and syntax) are within the same range as they were in 1975.
Because the basic characteristics of House Finch song have remained consistent across the decades, birds today would probably still recognize old recordings as being from their own species. We’re soon going to test this to find out for sure!
- Roughly half of the individual syllables that were around in 1975 were around in 2012, too. The more common the syllables were in 1975, the more likely they were to still be in use by 2012.
- However, none of the particular songs (that is, sequences of syllables) that we recorded in 1975 were sung by any bird in 2012.
These results are to be expected. Since House Finch syllables are learned whole, they can be preserved from generation to generation; perhaps birds even reinvent the same syllables over time. However, young birds individually assemble syllables into songs each generation, and there are millions of combinatorial possibilities.
- The population of songs is more diverse (there are more different syllables in use) in 2012 than there were in 1975.
- Although birds shared songs with each other in 1975, the birds in our 2012 sample didn’t share any songs with each other, despite being the same distances from their neighbors.
We know that the population of House Finches generally grew and expanded between 1975 and 2012, although a nasty outbreak of conjunctivitis was decimating the population for a while. A larger population means more neighbors to listen to and more individuals to create new syllable modifications as they learn. Both of these factors should eventually cause greater overall song variety, which is what both of these results show.
As is typical in science, we also found results we cannot readily explain:
- Birds in 2012 do not repeat their songs as reliably as they did in 1975—they are more likely to skip syllables, add new ones, or switch them around.
- Individual songs in 2012 have fewer different kinds of syllables than they did in 1975 (despite there being more total syllable types in use in the population as a whole!).
- In 2012, the syllables that are more common tend to be the ones that are more complex—they change pitch more rapidly and more often. They also tend to be higher pitched. This was not the case in 1975.
We’re developing some ideas to explain these curious results—hypotheses that will inspire our next round of field and laboratory work. The House Finch researchers in our lab are taking some exciting next steps, looking at such things as song similarity over geographic distance, changes on islands, early song development, social networks, and sex differences.
Unfortunately, Paul Mundinger passed away while this study was being conducted, and he never got to see the results. But it is because of his early work that we were able to chronicle changes in these songbirds over nearly four decades, and his song recordings (which are voluminous) will continue to provide us with interesting baseline data and prompt new research for years to come.
See more about the Lahti lab at http://lahtilab.org.