This spring I have been observing nightingales and grasshopper warblers singing rather side by side in riparian habitats.
As the two species dominated the ‘nocturnal stage’, I could make comparisons between them quite easily.
I noticed that nightingales selected habitat patches that enhanced the acoustic potential of their vocalizations (by amplifying the sound and carrying it far along the river but also possibly upward (as the river forms an open space between other habitats that are frequently forested and that would not allow for the song to ascend as efficiently).
I did not notice a similar habitat selectivity among the grasshopper warblers, and the warblers appeared to be more concerned with the size of the woody plant area (shrubs or trees) or with the size of the canopy of an individual shrub they had settled in (nest safety considerations, undergrowth structure providing vegetation for invertebrates but also enabling the warblers to move around and to hunt their prey).
It did not seem to me that the grasshopper warblers had made particular adjustments for their vocalization to travel far and high, although their chirping was certainly intense and powerful, and probably very well suited for finding an individual in a dense canopy maze within larger groves of, for example, osiers as well as to be heard at the perimeter of such grove.
It is possible that nesting is safer deeper in the grove (not on the very edge) but that the shrubs muffle the sound and, therefore, it is important that the sound is ‘forceful’ enough to reverberate all through the grove and out of a dense canopy.
On the whole, I came to a conclusion that the nightingales were making an effort to be heard from afar (either overland or in the air) while the grasshopper warblers were producing vocalizations aimed at other birds who are already in the vicinity.
Accordingly, nightingale song, under the right conditions, could be heard from even 1 km away (of course, I cannot assess from how high up it could be perceived).
The grasshopper warblers, meanwhile, could not be heard from further than perhaps 200 metres away but, within the distance of 50 – 100 metres, they could not be missed and, at close range (singing next to one another in similar habitat patches), they could easily compete with the nightingales.
This led me to wondering about several aspects of habitat colonization and social behaviour.
I have not yet studied the research papers concerning both species very thoroughly but I would venture to guess that grasshopper warblers form flocks during migration and tend to travel as flocks and settle in habitats, also, in flocks.
As a result, the males would not be in any great need to solicit attention by females outside of their social group (local population) and their effort would be dedicated at finding a mate within the flock which has already localized in a specific area (perhaps of 50 – 100 m radius).
If such species is, additionally, philopatric, perhaps mates are sought, firstly, on the level of a specific grove/habitat patch and then on the level of neighbouring groves/patches (if all fails, extending the search over three or several adjacent patches).
Such vocalization behaviour (and acoustic quality) traits would lead to certain risks.
For example, inbreeding risk might be high (especially, if the individuals tend to return to the same breeding grounds which I suspect they do) and a local population might quite easily go extinct if it is not connected to another local population, i.e., if the groves or habitat patches are not close enough one to another (as the song does not really travel much further than 100 metres and the individuals themselves might be unwilling to leave their flock and the known nesting habitat).
Meanwhile, nightingales seem to be attracting mates on a far vaster scope.
I believe many songbird species travel in flocks and nightingales might not be an exception.
However, it appears that nightingale males can easily intercept mates from flocks to which they, personally, do not belong as these females are flying over.
Also, any females from downstream or upstream habitats can easily disperse toward singing males who can be heard from ca. 1 km distance.
Thusly, even if nightingales are philopatric and do form local flocks (populations that are composed of rather the same individuals and their offspring), their mingling potential is far greater and inbreeding risk could be considerably reduced.
Such local populations would not go extinct if two flocks were separated by some distance which surpasses the grasshopper warbler connectivity-ensuring distancing thresholds.
Perhaps young nightingales (first-time breeders) are also better enabled to find mates.
If they are outcompeted in the breeding habitat where they were born, they can venture around finding a new habitat and they do not even have to invest as much energy in the venturing (flying) as vocalizations can be heard from afar.
However, such social structure might lead to higher aggressive display levels as the local flock itself would be more dynamically changing and the individuals would not have learned their neighbours’ identities.
If the influx of younger individuals was greater, such individuals might not yet have acquired the skills of peaceful territory conflict resolution.
The relative constancy of traffic in and out of local populations during the courtship season might also increase tension as the breeding area appears to be in a flux while the grasshopper warblers simply have to pair up within a given area without as many incoming/outcoming individuals.
I was also wondering how such differences influence the potential for new habitat colonization.
Nightingales (and other species that produce far-reverberating vocalizations and whose social structure is more dynamic, i.e., they are not attempting to find mates only in a given area, within a local population) could be better equipped to colonize new habitats, especially, if these habitats are not well-connected with the habitats that are already settled by other nightingales.
New, somewhat isolated habitats could be settled, for example, if they are still within a hearing distance from the current populations (e.g., ca. 1 km overland), or a male who has discovered a newly available habitat might attract other males and females who are flying over (thusly, the location of the habitat might not have to be close to an existing population but it should be situated beneath a migratory route used by an existing flock).
Meanwhile, species that vocalize at closer range and that perhaps pair up within a given social group (e.g., grasshopper warblers) might find it more difficult to colonize new, remote habitats as a single individual would not be able to attract a mate, nor other warblers would be intrigued by the opportunity to explore the habitat.
In such species, the spreading of a population would be more gradual (without great ‘leaps of faith’, i.e., suddenly settling some half a kilometer away from an existing population).
Perhaps, as such species might also be more dependent on existing habitats and suffering greater risk of extinction on occasion the habitat was lost or reduced.
If such species tend to search for mates within the bounds of a localized population, they might also attempt to re-settle as a social group which means that they would need to find a space where an entire flock could localize (rather than mingling into other populations, on an individual basis).
Travelling as a flock might be disadvantageous as vacancies within populations could be more easily found than an entire grove/habitat patch open for settling.
However, if the vocalizations by other grasshopper warblers are not heard from as far and from as high, it might be tricky to locate existing populations in the first place, and keeping together might ensure mating opportunities for a greater number of individuals if the flock was successful at finding a suitable patch.
I believe these considerations are important in the context of species protection as vocalization behavior and acoustic traits of the calls might determine the risks of extinction as well as the potential of joining other populations and colonizing new habitats (e.g., recovering after habitat loss and exploring new habitats).
Notes by Ieva 