This year, I have been paying close ear to grasshopper warbler (Locustella naevia) vocalizations.
Naturally, I have been wondering how their specific song (which resembles the chirping by grasshoppers) has originated.
At first, I thought that it was perhaps a hunting strategy.
Namely, the grasshopper warblers might mimic the chirping by grasshopper males in order to attract females who then would be preyed upon.
However, grasshopper mating season (at least in much of temperate Europe) is limited to late summer – early autumn (in Latvia, end of July – early August).
This hardly constitutes a time when an insectivorous species would evolve specialized adaptations in order to acquire a rather limited range of resources (accounting for the fact that grasshopper warblers seem to prey on a variety of invertebrates and their habitat provides quite some invertebrate diversity) for a relatively brief period which even falls outside of the main reproductive stage (late April – early June).
While grasshopper warblers can produce two broods per season (the second brood coinciding with the grasshopper mating season), I do not suppose that a species would have evolved adaptations to enhance the survival of the second brood, specifically, as the second brood is not produced every year (but only under favourable conditions) and the second brood is far likelier to fail.
Additionally, grasshopper warblers are quite small and adult grasshoppers are quite small. I cannot even imagine that mature grasshoppers would be easily fed to chicks without investing in the additional effort of tearing it into smaller pieces (which would reduce the overall feeding rate).
Unless the adaptation has been aimed at ensuring the adult grasshopper warbler chances to prepare for the migration and to survive the winter (which the adults would probably begin focusing upon after the reproductive season, i.e., mainly in August which is after the grasshopper mating period), I do not see why the birds would have evolved the song with the sole purpose of attracting some females and to then prey upon them.
Certainly, grasshopper warblers have not evolved their song lately, nor in Latvia.
Which is to say, they might have evolved their song in a region and during a time in the evolutionary history where other grasshopper species might have lived with a slightly different reproductive cycle (e.g., where both the warblers and the grasshoppers might have bred at least twice per year) and where perhaps these grasshopper species would have constituted the very bulk of the warbler diet.
In our area, I do not see that the grasshopper warblers necessarily settle with grasshopper distribution on their mind although many (perhaps, majority) of the populations do dwell in shrubs bordering grasslands.
While the evolutionary history could be of significance, I also developed an alternative theory regarding the origin of the grasshopper warbler song (and perhaps songs by similar bird species such as river warblers).
My theory includes the concept of ‘grasshopper husbandry’ or ‘grasshopper herding’ and this theory could explain the evolution of the song also in temperate climates.
If grasshopper warblers could confuse the grasshoppers into believing that they were, too, grasshoppers, singing at the edge of a grassland near their own shrub dwellings, grasshopper females have migrated toward the song assuming that those were males chirping to attract their attention.
The females would not be devoured but rather spared in order to attract males, as well.
Thusly, the grasshopper warblers might have gathered and formed little grasshopper herds very close to the warbler nesting habitat.
As these grasshoppers would mate, the females would lay eggs, also, near the warbler nesting habitat.
The eggs would hatch in the following spring.
While the adult grasshoppers are highly mobile (thence, the potential for attracting them which would not work out in less vagile species), their early instars probably are not.
These instars might also be smaller (naturally bite-sized).
The grasshopper warblers could then feed the instars to their chicks.
Accordingly, the warblers might have used the song to create local grasshopper mating hot-spots near their very nest sites in order to utilize the resources (the grasshopper young) in the following warbler breeding season.
Perhaps, initially, the grasshopper warbler song only slightly resembled chirping but this already proved sufficient to confound some of the insects which then migrated toward the song forming herds.
As a result, grasshopper warblers whose song was the most chirp-like, could have succeeded in securing prey for their chicks passing on the chirpy vocalization.
In temperate regions, grasshopper warbler chirping during the courtship activity leading to and during the raising of the second brood would have coincided with the grasshopper mating activity.
If it is still a working strategy (i.e., if grasshopper warblers did not evolve such song during an earlier period when grasshoppers were a dominant prey item but later favoured a largely generalist foraging behaviour), grasshopper warblers might intensify their song during the grasshopper breeding season and they might even avoid killing adult grasshoppers (especially, the large females).
Some sources claim that both grasshoppers and grasshopper warblers are most active during dusk.
If it were true, the warblers could have adapted to the grasshopper mating diel activity.
***
It is, of course, equally important to explore the potential for manipulating cricket/grasshopper behaviour in the wintering range.
Notes by Ieva 