For the last mornings I have been trying to wake up very early in order to meet also the more evasive nocturnal and crepuscular species.
This had led to an additional observation regarding the timing of the beginning of the morning dawn chorus by the local bird populations.
I have read that birds usually start singing about an hour before sunrise but for the past mornings I have noticed that this beginning time fluctuates and it is not necessarily related to the morning temperature.
I am not able to gather hard data because I do not carry a phone with me and therefore I do not really know precisely at what time the birds start singing.
However, I can claim with some certainty (judging by the time I leave the house and also by the general brightness of the sky before sunrise) that, for example, today the birds began singing about half an hour later than yesterday.
Meanwhile, the temperature was about one degree warmer than it was yesterday and the birds, theoretically, speaking should have started singing earlier today.
Also, yesterday followed a sequence of colder days which meant that even if the air was about as warm as today, the sensations and the water as well as soil temperature might have been different when comparing yesterday morning and the morning of today because yesterday was a relatively hot day (after the cold spell) which would have warmed up the ground and the water.
Thereby, it is rather confusing that birds would begin singing later today if, theoretically speaking, it was warmer by accounts of air temperature as well as possibly soil and water temperatures that affect the insect and worm populations.
The previous evening temperature was also warmer yesterday than it was the day before yesterday.
Perhaps this means that the birds got to feed for longer last night compared to the night before and therefore their energy stores were sufficient to ‘sleep in’ (they were not forced to wake up early and restore the lost energy).
Another consideration gives support to this hypothesis – the night was altogether warmer yesterday than the day before yesterday which means that the birds did not have to spend as much energy for thermoregulation purposes.
I wonder if birds ‘sleep in’ if their energy stores are sufficient and do not need to be replenished speedily in order to compensate for the losses over the cooler night – since the last feeding bout which in colder evenings might have been concluded earlier than in warmer evenings.
This is the breeding season, however which is an important consideration because chicks need to be feed regularly and decisions that adult birds might make regarding feeding behaviour are likely different when they are taking care of their own energy stores or when they are, additionally, taking care of the young.
I cannot agree that birds start singing an hour before sunrise, either.
In my experience, start singing way before an hour prior to sunrise (at least one hour and a half before).
It is also interesting how different bird species begin singing at different times.
After I made this observation, I tried to look up more information and I found that larger birds tend to ‘wake up’ first which might be related to their body size enabling them to commence activities while it is still colder (while the smaller birds cannot conserve their temperature as well which is why they have to wait until their air becomes warmer).
Feeding ecology is of importance, as well – not all prey species wake up at the same time and insects can be late to rise if the temperature is low.
It would be interesting to study differences in bird species that prey on various resources, e.g., burrowing worms, meadow insects and larva, flying insects, insects that live on the water surface or fly low above the water, fish, small rodents, small birds etc. in order to discover how the prey twilight activity in mornings and evenings determines at what time the respective bird predators wake up and begin their activities (including singing).
It might be interesting to learn if the dawn chorus participation is determined by feeding behaviour, i.e., the behaviour of the prey species.
For example, very generally, the earlier rising species (robins, blackbirds, song thrushes, doves) tend to feed on worms and snails compared to species that are cited as later rising (wrens, warblers) and that feed on smaller insects.
While there is a size difference, as well, this feeding behaviour (which in itself is likely a result of the bird’s body mass having different energy requirements and varied possibilities with respect to feeding strategies – larger birds are more able to extract worms out of the soil compared to smaller birds that have to choose smaller prey according to their size) could determine the potential commencement of activity in the morning because, e.g., earthworms, snails and many beetle and spider species are nocturnal and therefore they are available earlier before dawn and their availability might even become scarce as the temperatures rise.
Smaller daytime insects probably need the air to be warm enough in order to start their activities and thereby to attract the respective bird species.
There might be research already discussing these differences in feeding behaviour, size and diel activity of the bird species but so far I have not encountered any specific publications.
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Coincidentally, there are basic theories regarding the reason behind the dawn chorus, e.g., mate attraction in those individuals who have not yet bred or who are about to breed for the second or third time, or competitor warning.
It has been claimed that early morning is the most appropriate time for such displays because sound waves carry far and before the air warms up, the activities may be restricted so the time can be put to better use.
I have even read arguments that early morning singing can demonstrate viability in males (as breeding partners or competitors) because the sound they are able to produce during this period when their energy is at the lowest, serves a a strong indicator of their fitness (because birds also sing during daytime or during evening and their singing is not restricted to dawn).
However, I find it difficult to fully believe that mating and competition are the only reasons behind dawn singing.
I have been considering other hypotheses, for example, that bird singing might serve to wake up the insect communities.
There is not much research dedicated to insect hearing or vibration perception capabilities although there are some studies that have looked at certain genres of music and their effects in invertebrates (e.g., Barton, B.T. et al., 2018, Dieng, H. et al., 2019).
These studies have looked at the impact of human-produced music (or urban sounds) on predation and breeding rates in invertebrates but if invertebrates react to such sounds at all, they might also react to bird singing by, for example, waking up (although activity commencement is, of course, rooted in environmental factors such as temperature etc. which might be less variable in summer).
I have also been wondering about the effects of the singing activity on the bird’s own organism.
Singing in humans can result in production of oxytocins and endorphins as well as improve immune functions, lower stress etc.
Singing could help overcome the more unpleasant states that are related to the night period (coldness, lack of food etc.) although it might be counterproductive to expend energy when the stress factors have been too energetically demanding to become reversed through song (e.g., during winter).
Oxytocins are involved in social group cohesion which might enable the individual to undertake the family responsibilities.
Notes by Ieva 