Dawn Chorus [1]
If you could do it all again
A little fairy dust
A thousand tiny birds singing
If you must, you must
Please let me know
When you've had enough
Of the white light
Of the dawn chorus
If you could do it all again
You don't know how much [2]
For diurnal animals, dawn and dusk are special times. Bird behavior is extremely varied but there are commonalities around how they behave at dawn, the dawn chorus. In researching this I also found mention of a dusk chorus, but as yet, little research, so we will stick to dawn. When one hears the term "dawn chorus," one thinks of birds singing, particularly songbirds. Yet so much more is happening and I'm sure this article will be little more than an introduction.
Take any enclosed region on the surface of the earth, it can be any size but say a few acres more or less. This is called a landscape. This comprises a heterogeneous variety of structures, geological, living, and living artifacts such as nests or human houses. Landscapes create dynamic patchiness in ambient air flow, temperature ranges, amount of light, and acoustic properties. [3] The living creatures in this landscape form a community, a set of species that interact. These interactions, which are termed "between=species interactions," form a continuum that is vaguely typed as dichotomies; predator-prey, competition-cooperation, symbiote-parasite, etc. In addition, there are interactions within species, which usually, in biology, means behavior. The dawn chorus can consist of many species of birds, There is at least one between-species consequence, hybridization. [4] A species can exist over a certain landscape range, and can also be subdivided into semi-isolated concentrations of individuals within the range. Bird species concentrations are dynamic over the course of a year and a major dynamic for social birds seems to be a process called fission fusion. The majority mating pattern of birds is social monogamy (monogamy with extra-pair copulations and a varying degree of extra-pair births.) [5] The fusion patterns for birds are loosely kin-based and a concentration of birds can have many kin-based subgroups. Within these concentrations of birds is the individual organism. These organisms (agents) have the property of autonomy through some level of cognitive (computational) ability, the ability to sense (receive signals), and through the processing of the signal, react to events outside itself. The autonomous agent is not passive, it can not only receive and process signals but can also send signals. This requires at least two individuals and the dynamic interactions of a concentration of a species are called a network. [6] The controversial claim is that the network can act as an autonomous computational agent. [7] In landscape theory, any two agents that can communicate form within the geographic landscape, a cognitive landscape, also called an eco-field. [8] With bird song, this is called a soundscape.
A dawn chorus is a community event of multiple species of birds (up to 300 in the tropics! [9] ) singing together. Separate species may start at different times, perhaps due to the size of their eyes allowing for variations in sensing of dawn light. [10] [11] Dawn is a time of low wind and ambient noise from other creatures, including man [12] [13] which supports consistent sound quality in heterogenetic landscapes. [14] The dawn chorus rises to a cacophony before, in most species, abruptly ending at sunrise. This is a daily year-round event, although the intensity is much greater during the breeding season. [15]
Biology at its root is the study of a historical process, Darwinian evolution. The process of biology as a science is also a historical process of technological, conceptual, and social evolution. Bird song has suffered from historical assumptions that have clouded its study. Until recently it has always been assumed that only males sing. Most early studies of bird songs have focused on temperate climates and limited species of birds. Looking at known data from all songbird species it has been found that female birds sing to some extent in 71% of known species including a majority of temperate species. Phylogenetic studies show that singing in both sexes is ancestral in songbirds which are believed to have originated in neotropical Autral-Asia. [16] In temperate climates, females in some species of songbirds have lost the ancestral ability to sing. There is also an issue with the definition of bird song itself. Bird vocalizations are broken down into two types: songs and chirps. The dichotomy comes from human perception. Humans can tell the difference because well, we just do. This folk biology is unfortunately hard to quantify because the two types seem to lie on a spectrum. One suggestion is to make vocalization an axis, a continuous variable like, for instance, the measure of male testosterone levels. [17] But how does one quantify vocalization? It has been suggested that vocalizations form discrete motifs that are inherited. For instance, during the dawn chorus, some female fairy-wren species have a response call to male singing. This call is also used by both sexes as a predator warning signal during the day. Looking at data from 11 out of 16 species of fairy-wrens it has been found that the predator call is the ancestral call and the dawn call is derived. [18] This not only supports this discrete motif idea but shows the dawn chorus as containing behaviors separate from other times of the day.
In a paper published in early 2023, Ontogeny of Collective Behavior [19], Isabella Muratore and Simon Garnier extend the idea of ontogeny, the study of an individual's lifespan from birth to death, to the collective behaviors of groups of individuals. They provide varied examples from ant colonies to motile worms to cells within an organism to biochemical processes within a cell. This is a process in which the structure of a global unit is transformed by the actions of local agents. These local agents can be what is normally biologically defined as an individual or not. An important feature of this collective transformation is its ontogeny. Its birth, maturity, and senescence. The dawn chorus can be thought of this way. As a community behavior, the agents are species of birds. As a species behavior, the agents are individual birds. There is some research on community behavior [20] and some about the temporal structure but no research looks at the full ontogeny of the behavior and nothing answers a question about its ending: "Why does it end so abruptly at sunrise?"
Black-capped Chickadee (Poecile atricapillus) [21]
Black-capped Chickadee range [22]
The black-capped chickadee is a small North American songbird. It is non-migratory, living all year in deciduous and mixed forests. They spend the winter in flocks of mostly mated pairs. Males and females are not sexually dimorphic. Males form dominance hierarchies within these flocks. In spring the flocks dissolve as mated pairs build nests in holes that form in trees and males defend a territory. males within a flock tend to create territories bordering each other. [23] Black-capped chickadees are socially monogamous in their mating system. This means that females engage in extra-pair copulation and a percentage of eggs raised have extra-pair paternity. I was not able to find a specific range for this species. Extra-pair copulations tend to be with males of the winter flock, particularly with more dominant males than their partners. Females leave the nest and their territory before dawn to mate. [24] [25] During the dawn chorus males sing directly into the nest hole (believed to be a form of mate guarding) and have song matches with males in other territories and floating non-mated males. During the day these song matches can escalate into posturing and fights at the edge of territories but during the dawn chorus, most birds are stationary. After fledgling, young birds join mixed flocks or leave the region, there are no kin-based aggregations. [26]
Male and Female Superb Fairy-wren (Malurus cyaneus) [27]
Superb Fairy-wren range [28]
Superb Fairy-wrens are sedentary territorial songbirds in Eucalyptus forests and human habitation in eastern and south-eastern Australia. They are sexually dimorphic, the male is brilliantly colored. They are infamous in they also take the term "social," as in social monogamy, to an extreme. Over 61% of eggs hatched by a female do not belong to her mate. The birds are cooperative breeders with many male helpers and occasionally one or two female helpers at the nest. Both mates and helpers defend a territory and help at the nest. A female helper will occasionally join with some of the male helpers to split their territory. When the mated male dies the next subordinate helper takes over unless it is the female's son. [29] Like the chickadees, the female leaves the territory before dawn to mate with other males. She flies two or three territories out, thought to avoid any incest from neighboring territories. [30] During the dawn chorus, both male and female vocalize to each other, and afterward the female mates with her mate and then with at least one subordinate, but never with a relative. All species of fairy-wren are cooperative breeders, but helpers differ greatly in the numbers of males and females. [31] [32]
As can be seen, any global theory of the reason for the dawn chorus must deal with the great heterogeneity of bird behavior. In a paper published in 1998, The Dawn Chorus and Other Diel Patterns in Acoustic Signaling, Staicer, et al. proposed 12 possible hypotheses for the dawn chorus. [33] In 2020, in The Bird Dawn Chorus Revisited, Gil and Llusia reviewed the current status of these hypotheses. [34] Out of the 12 hypotheses, four have shown experimental evidence and one has shown theoretical evidence, meaning that a theoretic model predicts a dawn chorus and a weaker dusk chorus. Some of the hypotheses have not yet been tested. The five with evidence are:
hormones - certain hormones released at dawn instigate singing. It has been shown with lesion studies that loss of hormone levels can have a completely different behavioral effect on different species of birds. [35]
female fertility - species differences [36] [37]
territory and status signaling - species differences [38] [39]
honest signaling of individual differences - this is supported by the next theoretical evidence [40] [41]
excess energy stored for unpredictable night conditions is released in dawn signing - a theoretical model. Birds can lose an excess of 10% of their body weight during the night and more during unusually cold nights so any excess allows for singing from a healthy bird that survived the night. [42]
More phylogenetic studies will perhaps give a better perspective.
The dawn chorus is not just a special time during the breeding season but a special time for communication within the structure of the flock. What got me interested in the dawn chorus was a study using new technology to record interactions between individual New Caledonia Crows in close to real-time. [43] Crows are passerines but they do not sing. They do learn and are adept at tool use. As an aside in the paper, the authors mention that the crows stay in kin groups outside of mating season during most of the day but during dawn, the data showed much more interaction between groups. Social network theory talks about "the strength of weak ties" and how contact with individuals that are outside of one's group can spread information throughout an assembly of groups. In a flock, a central individual can have contact with all the others but it is the peripheral individual that pushes information across to other flocks. [44] [45]
The dawn chorus is a cross-species collective behavior deeply connected but also separate from the behavior of a community or individual species during the remainder of the day.
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