This summer I have observed certain dynamics in the bird dawn chorus in a specific location which is characterized by a medium-sized river (about 5 m wide), riparian forest (which is rather narrow in some strips and wider in others) and a small patch of pine forest (pines dominate the canopy) further upslope.
Agricultural fields are also found beyond the forest patch.
I began listening to the birds’ early singing in perhaps April and at the time, most of the singing (upon the outset of the chorus) was associated with the narrow riparian woodland strip on the banks of the river.
This continued through May which was when I became restricted to home for several weeks and I could not visit the specific habitat.
When I returned in late June, initially, I did not pay as great attention to the dawn chorus although I suppose that the birds were predominantly riverbank-bound, as well.
However, over the past weeks (mid-July) I have noticed that the birds do not aggregate for their dawn songs by the river and the singing actually begins upslope in the forested patch.
The change has been rather striking if I noticed it at all because I was not paying very close attention to the birds, specifically.
At first, I assumed this was related to breeding activities.
Early in spring birds were looking for partners and perhaps aggregating more closely while later they were sticking to their nest sites to brood, to provision for the chicks and to defend their nest and their young.
This might limit their distribution on the landscape and their ranging behaviour (smaller home range).
As the forest patch likely offers many nesting sites compared to the more narrow riparian woodland strip, I thought this could explain the change.
Some birds attempt at two or three broods and an alternative explanation could be that behaviour during the second brood is different from that during the first brood.
For example, some birds might have moved into more optimal nest sites that have been vacated by species that only brood once per season.
Still, it also appears that the birds are not singing (in July) in the riparian strips which are relatively wider and offer many dead standing trees and veteran trees as well as an abundant shrub layer.
Unfortunately, I am only beginning to study invertebrate behaviour.
My suspicion is that this shift in dawn chorus distribution might indicate at some shift in invertebrate (or some other food resource) distribution.
Perhaps springtime offers more plentiful invertebrate resources in aquatic and semi-aquatic habitats while in July, forest interior invertebrate abundances have exceeded the riparian invertebrate abundances.
Later on, about half an hour after the beginning of the dawn chorus, birds show up by the river, as well.
I assume that there are many species which are associated with riparian vegetation and their quiet ways early on before dawn might suggest even they depart from their nest sites to prey on invertebrates elsewhere.
I would be very interested to learn what caused the shift and I hope to come across publications explaining my observation.
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I wonder if this shift might mean that bird species that rely on resources typically obtained close to river, begin reproductive activities earlier than birds nesting in (non-riparian) forest.
Also, I wonder if the second (/third) brood tends to be raised in sites that differ from the first brood site and that follow the seasonal resource distribution (e.g., birds would first nest near earlier resources and the second nest would be selected (if a change is made at all) closer to patches where the resource abundance is more pronounced later in the breeding season.
Notes by Ieva 