Well, the rain has not ceased at all (it seems) in the past week; and I believe it is not a far stretch to label this Germany's new monsoon season!
Not being able to go wander the fields for the second weekend in a row during the tail end of spring migration has got me kinda bogged down. But being stuck indoors (especially since I'm still recovering from some spring allergy-related cold-thingy I first caught 3 weeks ago at home in Cape May, and mainly due to the weather, not been able to shake fully yet), I have spent some time thinking about my last post, and I started to wonder: how did the different species of Locustella warbler evolved their different trill songs?
The first idea I had, which seemed logical, was that the environment in which a bird lives (its surrounding habitat), must play a significant role in shaping the quality of its song(s) and calls.
My curiosity led me to a google-search frenzy, upon which I stumbled across one intriguing paper, whose pdf copy is publicly available here:
Ecology Shapes Birdsong Evolution: Variation in Morphology and Habitat Explains Variation in White-Crowned Sparrow Song (Derryberry 2009)
It was refreshing to read a technical paper that was actually both interesting AND comprehensible!
Anyway, this paper presented the first evidence of variation of song with variation of morphology and habitat within a single species (White Crowned Sparrow (Zonotrichia leucophrys) in this case).
The main findings of the paper include that both trill rate and minimum frequency decreased over time in populations in which vegetation density increased. This makes sense to a condensed matter physicist, if one allows a ridiculous oversimplified generalization of the situation: habitats with dense vegetation are like "lattices with many scattering sites", where sound waves (or any waves, generally speaking) can be reverberated and attenuated in an amount proportional to the frequency. Therefore one expects that birds with slowly repeated notes and lower frequencies (least attenuation) will transmit their songs better in those territories, thus their genes will remain in the gene pool, and so on...at least that is one theory that seems logically sound to me, and maybe even true in many cases. ;)
Besides habitat, though, she also discussed how morphology can shape a bird's song. For instance there is a limit to how fast a bird can open and close its bill depending on the size of the bill, so longer and/or deeper bills have a maximum frequency due to mechanical constraints of the bill, and so the evolution of bill size may influence the evolution of birdsong. Birds with larger bills are also predicted to produce slower trills with narrower bandwidths of frequency (i.e., "lower performance" songs)
Now back to my initial quandary. On the one hand when one considers the habitats that (Common) Grasshopper, Savi's, and River Warblers generally occupy, one sees a relation between trill frequency and vegetation density; namely, frequency is inversely proportional to density. For instance, Savi's has the lowest frequency trill and is also usually the skulkiest, singing from deep in the reeds...whereas on the other end of the spectrum (literally in this case) you have the Grasshopper's variable-but-usually-highest frequency trill, and he usually sings from exposed perches, like from a branch of a short tree on the edge of a reed bed. River warbler falls in between both in trill frequency and in density of habitat vegetation, I believe.
On the other hand, Savi's has the fastest tempo of individual notes, so that the trill is morphed into a fine buzz (to our ears at least, which cannot pick up the individual modulations), which would logically seem unfavorable in dense vegetation. River Warbler, on the other hand does have relatively slow tempo, so that one can hear the frequency modulations well. So it seems that, for Savi's at least, there must be other factors that have evolutionarily shaped its song.
So then I wonder about bill sizes. I have not seen bill size differences emphasized (or even mentioned) in any field guide as a character for differentiating these species, but just looking at the drawings in SMZ. It seems that the bill length, in relation to overall body length, is almost identical for Savi's and River Warblers, but slightly smaller for Grasshopper. Especially considering that Grasshoppers are the smallest of the three (L 12.5-13.5cm according to SMZ), this seems to indicate that they have the shortest bills, meaning they should be able (mechanically) to produce the highest frequency, by being able to open their mouth the widest...and that is what we do indeed hear! On the other hand, Savi's is next largest (L 13.5-15cm) and Rivers are usually the largest (L 14.5-16cm). This means, if bill-length-to-body-length ratio is the same, Savi's should have a shorter bill and thus be able to produce higher frequency than River...but then why don't they? Maybe for Savi's, unlike some other species, the females prefer to select males with lower trill frequencies?
When one thinks of bill evolution, the example that first comes to mind is Darwin's Galapagos Finches, right? Here, due to isolated populations with different food sources available, evolution led to the proliferation of various bill sizes, and thus splitting of species into subspecies (and later on, after more genetic divergence, splitting into completely different species). With some birds though, it seems another mechanism is behind the diversification of bill sizes/shapes...sexual selection. Because of this, males which need their courtship songs to be transmitted over as long a distance as possible, may need to vary the tempo and frequency of their song due to vegetation density constraints, as mentioned above. This, then in turn, favors certain bill mutations, and thus over many generations favors shorter or larger bills.
I would love to hear from anyone, such as a bander/ringer, who has done bill length, width, and/or depth measurements on these three species, and whether my hypothesis is correct. To the best of my knowledge (really to the best of my google-searching ability), there are no reported findings, similar to the above-linked paper, for Locustella warblers, so I think it would be interesting for recreational birders to look into, and even a nice topic for a budding PhD student of ecology/evolutionary biology! :)
Good (dry) birding everyone!
Steve
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