Abstract: Nocturnal vocalization in birds is more common than we may assume but is understudied. This paper seeks to understand why some birds sing at night, examining both phylogenetic hypotheses and environmental ones. Though nocturnality has a clear phylogenetic basis, nocturnal vocalization has not been shown to be restricted by phylogeny. It explores the (much more substantial) literature on environmental impacts on nocturnal vocalization, which discusses the surrounding ecosystem, weather, ambient noise, and light. It has been found that higher temperatures and lower wind at night are more likely to result in nocturnal song, but different species have different responses to weather. Similarly, it appears that more light, whether artificial or from moonlight, has a positive effect on nocturnal singing, but species’ responses vary. Ambient noise throughout the day can increase the intensity of avian night singing. This paper finds merit in both phylogenetic and environmental hypotheses, suspecting that certain species are genetically more suited to nocturnal activity that is often triggered by certain environmental factors. This paper discusses functions of nocturnal bird songs (including mating, migration, and territorial disputes) while also finding that night may be a superior singing time in some contexts. It also makes recommendations for future research. Background: Though not deeply explored, the practice of bird nocturnal vocalization has long been observed. In an 1893 issue of Science, Ornithologist J. M. Edson wrote about nighttime song: “The notes which from time to time have appeared in Science with reference to the nocturnal singing of birds demonstrate that a considerable number of species of birds are known to exhibit this eccentricity. From my own observations, I can corroborate some statements and…add one or two to the list of daylight songsters guilty of keeping very late hours” (Edson 1893). But to what extent exactly do birds vocalize at night? Using observations of species in Birds of North America and The Flight Calls of Migratory Birds, it was found that 30% of North American birds have reported nocturnal vocalizations. Taxonomically, this represents 232 species of birds and 51 families. The greatest presence is found in the Strigiformes, Gruiformes, Procellariiformes, Caprimulgiformes, and Trogoniformes (La 2012). Though we do not have as much data about the nocturnal activity of birds on other continents, our understanding of night calls and song in North America lends legitimacy to our interest in the subject, as well as studies that observe nocturnally vocalizing birds in Europe (Kulaga and Budka 2020), Africa, and other continents (York et al. 2015). Despite more recent efforts in the past decade to synthesize research on this subject, there is much that remains to be learned (Foote et al. 2018). First, let us explore the nocturnal vocalizations of nocturnal birds. A 1973 article in The Auk gives us strong insights into the vocal displays of a nocturnal bird through 400 hours of recording the breeding pairs and offspring Burrowing Owls (Speotyto cunicularia) in central New Mexico. The study found similarities between the owls’ calls and the calls of other birds in sound, etiology, and function. Their vocalizations are used for copulation, location cues, distraction, or when the birds are hungry and distressed (Martin 1973). A study on Ural Owls (Strix uralensis) in central Sweden found a similar result with vocalizations usually occurring for courtship, attack, and territorial signaling (Lundberg 1980). It seems fair to assume that vocalizations of normally nocturnal or crepuscular birds during the night are similar to and have the same functions and causes as the vocalizations of diurnal birds during the day. But what about the nocturnal vocalizations of diurnal birds that also sing and call during the day? Why do some diurnal birds sing at night, while others do not? It is important to investigate whether the trait of nighttime vocalization is phylogenetically or environmentally dependent and what triggers these vocalizations during the night. Phylogenetic Development of Nocturnality in Diurnal Birds: The phylogenesis of nocturnality in all birds has long remained untraced, and the genetic cause of nocturnality has remained unknown, but recent research has specifically set out to find the common ancestor of living Aves, which exhibits nocturnality. It has long been known that nocturnal organisms thrive in the night due to greater sensitivities to light—the system of phototransduction, the conversion of light particles into a graded receptor potential, in nocturnal birds is optimized for low light levels (Yokoyama and Yokoyama 1996). Since then, researchers have used a molecular phyloecological method of analyzing 33 genes related to phototransduction in several lineages of birds. Two genes, GRK1 and SLC24A1, have been found to enhance night-vision and act as the basis for nocturnal adaptation in birds. Researchers found evidence of positive selection of these genes along bird clade, tracing them to a common ancestor in Carinatae within Aves, an ancestor that was active nocturnally and diurnally. Nocturnal activity in birds can be concretely linked to its evolutionary origins as superior avian vision. (Wu 2019). However, a phylogenetic basis for nocturnality in birds does not necessarily suggest a similar basis regarding nocturnal vocalization. A 2012 review sought to examine the significance of a phylogenetic signal for nocturnal vocalizations. Author Van T. La built the phylogeny for 82 families, positioning them based on 19 independent DNA loci. He then calculated the percentage of nocturnally vocalizing birds in each family, testing for phylogenetic dependence of nocturnal vocalizations by using the statistical method of Pagel’s lambda. Pagel’s lambda being equal to zero would indicate the complete absence of a phylogenetic signal, while it being equal to one would indicate the complete presence of a phylogenetic signal (a significant evolution of nocturnal vocalization). La states as his result: “The phylogenetic signal in nocturnal vocalizations is insignificant (Pagel’s λ = 0.41, P = 0.78), which suggests that nocturnal vocalizations are not restricted by phylogeny” (La 2012). These results do not provide any support for the hypothesis that nocturnal bird song is a trait with clear phylogenetic origin; instead, it seems more likely that it is environmental factors that are the basis of nighttime vocalization. But this finding seems somewhat at odds with our discussion of the phylogenesis of nocturnality. Since there is a (primarily visual) genetic basis for nocturnal activity in specific bird species, it seems fair to also believe that there is a similar (perhaps the exact same) genetic basis for nocturnal vocal activity in bird species as well. (In other words, if some birds are better adapted to being awake during the night, these are the same birds that are vocalizing at night, thus establishing a genetic basis for this vocalization as well.) Using a bird’s vision to predict its vocal activity is hardly an unreasonable idea: British researchers in 2002 found that eye size was predictive of the time at which birds sing at dawn (Thomas et al. 2002). A study on the nocturnal vocalizations of diurnal birds in eastern Poland takes this question to the field. Researchers selected 54 recording locations in two types of habitats—(1) areas with meadows and arable fields and (2) forested areas. Though they tracked the effects of various environmental factors on the birds’ singing, they also identified the species that were the sources of this nocturnal singing. They recorded 88 bird species, representing 12 orders and 32 families, and 24 of these species vocalized at day and night. Thirteen of these species were songbirds, and the species with the greatest intensity of vocalizations were the Thrush Nightingale (Luscinia luscinia) and the Common Cuckoo (Cuculus canorus). The researchers found that nocturnal singing in diurnal birds was common in multiple genetically distinct families (e.g., Gruiformes, Alaudidae, Scolopacidae), while also finding that birds in other families (e.g., Laniidae, Paridae, Motacillidae) do not vocalize at night at all. “This suggests,” the researchers write, “that night singing by diurnal birds might have evolved independently several times” (Kulaga and Budka 2020). Environmental Factors as a Basis for Nocturnal Song: An alternative explanation for the vocalizations of birds hypothesizes that whether or not birds vocalize at night depends on their surrounding environment. The researchers of night vocalizations in eastern Poland found that birds’ level of nocturnal vocalization changed in different types of environments. They found that “night singing by diurnal birds was strongly limited to the species that inhabited open environments” and night singing by diurnal birds was less common in more forested areas. They present two hypotheses for this difference: (1) varying levels of nocturnal predators and (2) varying levels of light. The impact of a bird’s surroundings during the night may explain differences in nocturnal vocalization across different environments. But research conducted in a Cameroonian tropical rainforest compared with research near rivers in the Himalaya may prov ide an alternate explanation: the soundscapes of more densely vegetated environments tend to have less reverberation and less turbulent air, making vocalization less effective and, thus, not necessary (Slabbekoorn 2002). One environmental variable that has been linked to changing levels of night singing is related to weather. Ovenbirds (Seiurus aurocapilla) and White-throated Sparrows (Zonotrichia albicollis) have been found to sing fewer songs at lower temperatures and in windier environments, which appears to be consistent with other birds as well (Foote et al. 2017). Precipitation also limits night vocalization, which is in part due to reduced territorial disputes. Both wind and rain impede transmission, rendering vocalization much less useful. Nocturnally-vocalizing birds have been found to vocalize less under cloudy conditions as well (Morrel 1991). Increasing humidity and ground moisture are associated with greater vocalization as well—this is hypothesized to be a function of easier foraging conditions for birds at night. Fog may have a positive correlation with nocturnal vocal activity due to the increased need for spatial awareness and location cues. Interestingly, it has been hypothesized that it is advantageous for nocturnally active birds to increase their vocalization under increased atmospheric pressure, this perhaps being due to better transmission of calls (Digby et al 2014). It should be noted that certain weather phenomena can have varying effects on different bird species: Canadian researcher Daniel J. Mennill recorded the acoustic signaling behavior of Common Loons (Gavia immer) around three lakes in eastern Ontario, Canada. He found that loons have a unique signaling strategy themselves: “Common Loons showed significant differences in vocal behavior with changing weather conditions, producing more calls at cold temperatures, with low wind speed and air pressure” (Mennill 2014). There is no substantial global model for how weather affects species-specific nocturnal vocalization. Researchers have also begun to investigate how the ambient noise in an environment may impact levels of nocturnal vocalization. In 2007, conservation biologists from the University of Sheffield recorded noise levels during the day at 67 different locations around the city. They then identified the locations where urban European Robins (Erithacus rubecula) vocalized at night. They discovered that the locations where the robins sang were among the noisiest in the whole city, suggesting that an external noise in an environment may change the time at which birds vocalize (Fuller 2007). Even variations in ambient noise within one city can itself cause variations in night singing of birds. A similar study was conducted in urban, suburban, and rural environments in Salamanca, Spain on the European blackbird (Turdus merula). Colleagues at the University of Salamanca recorded blackbird songs and ambient noise at 13 sites. Many have explored the ways in which birds adapt their songs’ frequencies and amplitudes in response to ambient noise, the avian practice of adapting the time of vocalization itself is worth understanding further, this knowledge also being applicable to bird conservation efforts (Mendes et al. 2011). Similarly, more attention is being paid to variation in nocturnal vocalizations in response to light. Even since the beginning of the twentieth century, researchers have commented on birds’ sensitivity to light and its role in their dawn and dusk songs (Allard 1930). At early hours, birds are able to sense dim levels of light and begin to vocalize. Further, research in the past few decades suggests that this sensory relationship to light may go deeper than we were predisposed to think: light can neurologically alter birds through neurogenesis and literally change vocalization patterns (Dawson et al. 2001). During the night, birds experience two sources of light: (1) lunar and (2) artificial. Both have been suggested as possible causes of night singing in birds. Light cycles, throughout the day and throughout the year, have long been known to influence biology. Light from the moon may stimulate nocturnal activity as well as nocturnal song. However, initial research that has investigated how moonlight may impact vocalization has failed to establish a strong relationship between the two. Northern Mockingbirds (Mimus polyglottos) were anecdotally known as diurnal birds that may often gsing during late hours, and researchers at Coastal Carolina University observed mockingbirds on six sites around their campus, measuring the portion of the time they could hear singing during visits throughout the day. A statistical analysis showed no relationship between song output and light for their entire dataset, though they could not “rule out an effect of light at very low light levels.” The researchers recognized that measuring duration of group singing (rather than each bird individually) is not a perfect measure of the intensity of nocturnal vocalization and that light may have an effect during other parts of the year (Hill et al. 2005). More recently, we have become aware of the full moon’s ability to enhance foraging and other activities during nocturnal hours, while also recognizing inconsistencies across species with regard to night singing. The White-throated Sparrow (Zonotrichia albicollis) has been found to sing less often during nights with a full moon, but the Marsh Wren (Cistothorus palustris) will sing 780 songs per hour during the night throughout the lunar cycle. Last year, a study examined the nocturnal singing of the Willie Wagtail (Rhipidura Leucophrys) in a sanctuary in Australia during a lunar cycle in breeding and non-breeding seasons. The University of Melbourne colleagues recorded vocal activity from 10 pm to 4 am, finding that Willie Wagtails’ nocturnal song rate increased positively with lunar illumination (Dickerson et al. 2020). Similarly, a study found that the vocal performance of the White-browed Sparrow Weaver (Plocepasser mahali) increased by 67% from a new moon to a full moon. These are two studies that begin to establish empirical evidence for a theoretically reasonable hypothesis—that certain diurnal birds vocalize during the night at greater levels of moonlight. Other research has found that light pollution itself can affect the singing behavior of birds. A 2006 study found that birds begin their songs earlier in more brightly illuminated areas. The same study also tested American Robins’ (Turdus migratorius) initiation of morning song, finding that: “Robin populations in areas with large amounts of artificial light frequently began their morning chorus during true night” (Miller 2006). This being said, other studies have had trouble linking changes in vocalization to artificial night lighting (Kempenaers et al. 2010). The type of environment a bird inhabits has been shown to affect its nocturnal vocalization patterns. The surrounding ecosystem, the weather, the noise, or even the ambient light in a particular area all may have an effect on whether a bird sings during the night. Functions of Nocturnal Song: It is important to investigate not just what factors (environmental or phylogenetic) result in some species of bird vocalizing at night and others not, but also why birds might vocalize at night instead of during the day. One of the most important reasons is related to migration: birds use flight calls at night to stay in contact with their flocks. Calls during migration are known to provide directional information to lost birds (Hamilton 1962). Other night migratory songs can have different effects—some are thought to be used to select stopover areas (Mukhin et al. 2008). Birds also vocalize at night because of territorial conflict. Territory defense is important during nocturnal hours in large part due to the risk of incoming migrants who land at night. Visual cues, which may be used during the day to mark territorial boundaries, are less effective at night, forcing birds to vocalize to communicate. The same study of the Common Loon (Gavia immer) mentioned earlier found that territorial loons were known to vocalize significantly more at night rather than during the day (Mennill 2014). There have been similar findings for other species as well: Cetti’s Warbler (Cettia cetti), Western Meadowlark (Stumella neglecta), and Willet (Tringa semipalmata) have all been found to vocalize at night over territorial disputes. Diurnal birds may continue to vocalize at night in order to attract a mate. Writing about the nocturnal singing of Marsh Wrens (Cistothorus palustris) in 1985, Barclay et al. found that nocturnal singing may have the same functions as diurnal song (Barclay et al. 1985). A 2008 study on the Black-throated Blue Warbler (Dendroica caerulescens) claimed that a birds’ songs after breeding can determine whether birds return to the same breeding site during the next breeding season (Betts et al. 2008). One strategy researchers have employed to determine the role of mate selection in nocturnal song has been to determine whether unmated birds sing more at night than mated birds. In 2002, Swiss researchers recorded the diurnal and nocturnal singing of unmated and mated males (Luscinia megarhynchos) during their entire breeding cycle and found that there were low levels of diurnal singing early in the breeding season. They also found that unmated males were much more likely to sing at night as the breeding season progressed. These discoveries suggest that for some bird species, nocturnal vocalization may be necessary in the mating process (Amrhein 2009). Similar studies have found similar results for the Corn Crake (Crex crex) and Limpkin (Aramus guarauna) (La 2012). Others have suggested that night songs may be used to maintain a pair bond. There is little literature on this subject, though one account of Grasshopper Sparrows (Ammodramus savannarum) suggests that they sing more pair-bond vocalizations than territorial ones during the night. This could mean that birds use the night as a way to maintain these bonds (La 2012). Research has also mentioned the effects that male singing can have on female reproduction––that it can cause increases in egg and clutch size. Our understanding of nocturnal vocalization’s ability to stimulate female reproduction is minimal. But another study on Luscinia megarhynchos found that one of the points at which nocturnal vocalization is greatest is just before egg laying, which may mean that birds sing at night for the stimulation of reproduction, a learned habit that benefits future members of the species. However, this is simply a hypothesis. An alternative explanation for this peak in the Common Nightingale’s vocalization is that females are still fertile at this juncture, and males may be engaging in nocturnal mate guarding. Limited visibility at night provides female birds the ripe opportunity to reproduce with new male birds, heightening male birds’ incentive to guard their mate (La 2012). Birds have long been known to engage in communal roosting, which has three primary benefits: “a reduction in thermoregulation demands, a decrease in predation risk, and an increase in foraging efficiency” (Beauchamp 1999). Studies have found nocturnal vocalization due to communal roosting in “the American Crow (Corvus brachyrhynchos), Chimney Swift (Chaetura pelagica), Leach’s Storm-Petrel (Oceanodroma leucorhoa), Least Tern (Sternula antillarum), Manx Shearwater (Puffinus puffinus), Ring-billed Gull (Larus delawarensis), and Thick-billed Murre (Uria lomvia)” (La 2012). Vocalization at night provides significant advantages to birds, who may not simply extend communication into the night but instead use night singing as an alternative to diurnal singing in specific contexts. One of these advantages is that the night may allow birds to carry out their activities without the danger of predators. A 2004 study on the Black-breasted Wood-Quail (Odontophorus leucolaemus) found strong evidence that singing poses the risk of being identified and captured by predators (La 2012). Birds may try to limit or restrict their diurnal singing in favor of singing during darker hours where predators will have less success at capture. The study on diurnal birds in eastern Poland as mentioned earlier reached similar conclusions (Kulaga and Budka 2020). The relative quiet and atmospheric conditions allow birds to sing with (1) less competition with other bird songs and humans, (2) better transmission of sound, and (3) ample opportunities to teach song to offspring. This paper has already established that birds in noisier areas during the day will increase their nocturnal song output. This illustrates the effect of external noise in disrupting bird song. Acoustic research has found that night improves transmission of vocalizations. The Common Loon (Gavia immer) can call 1 kilometer farther during the night (La 2010). A 1996 review paper entitled “The Influence of Surface Atmospheric Conditions on the Range and Area Reached by Animal Vocalizations” gives insights into the atmospheric changes that occur as day transitions to night that positively impact sound transmission. As the sun sets, the ground cools rapidly, creating a temperature inversion, “a layer of air in which temperature increases with increasing height above the ground, as opposed to the more common condition for the lower atmosphere in which temperature decreases with height.” During an inversion, the colder temperatures toward the ground cause the downward refraction of acoustic energy, amplifying noise heard on the ground. Low wind speeds at night also aid in transmission of sound (Larom et al. 1997). The loon is hardly the only bird that has been shown to benefit from the enhanced sound conditions at night: the Rufous-collared Sparrow (Zonotrichia capensis), the Altamira Oriole (Icterus gularis), and the Yellow-breasted Chat (Icteria virens) are suspected to vocalize at night for this reason (La 2012). A study on the Rufous-collared Sparrow (Zonotrichia capensis) found that its nightly songs were more complex and longer at night (Lougheed and Handford 1989). Better night singing ability may be related to an underexplored possible function of nocturnal vocalization: to teach songs to offspring. Birds have only a small window of time to learn their songs—a process that is made more difficult by risks of predation or interfering noise. This gives an explanation to the continued nocturnal vocalization researchers have recorded even after the breeding period is over; these are birds teaching their offspring (La 2012). Conclusions: Nocturnal vocalization has been observed in a wide variety of birds in many geographical areas. Despite these observations, nocturnal vocalization has been historically understudied relative to diurnal song. A review of observations of North American birds estimates that almost a third of these bird species are known to practice singing at night; the phenomenon is worthy of attention and academic research. Less is known about the nocturnal vocalization habits of non-North American and non-European birds. The contexts in which nocturnal birds vocalize at night are similar to the contexts in which diurnal birds vocalize during the day, such as courtship, attack, and territorial singing. Recent research using phyloecological methods has made great strides in identifying the evolutionary origins of nocturnality in birds. There is a phylogenetic basis for nocturnality in birds related to phototransduction. This analysis has identified a common ancestor of nocturnality in birds, Carinatae, which was both diurnal and nocturnal. However, efforts to establish a phylogenetic story for specifically nocturnal singing have proven difficult. Statistical analyses that have measured the phylogenetic signal for night vocalizing have suggested that vocalizations are not restricted by phylogeny. Instead, others have put forward environmental theories to answer the question of varying levels of night singing among diurnal birds. The acoustic properties of a soundscape (e.g., degree of vegetation) could affect the intensity of night singing. Particular weather events and temperatures affect vocal activity of birds at night––often in different ways depending on the species. These changes may be due to the effect weather has on other bird behavior, like foraging or fighting. Noise in an environment has been shown to have a significant impact on vocalization. Research, particularly in urban areas, has shown that birds sing during the night at locations that are louder during the day. Some of the most comprehensive research on the environmental factors of nocturnal song is with regard to light, both lunar and human-produced. We have similarly found that different species respond to light differently, but there is solid evidence linking heightened levels of nocturnal singing to especially well-lit nights. This paper favors the conclusion of “Nocturnal singing by diurnal birds in a temperate region of central Europe” in its answer to the question of whether birds sing at night because of their phylogeny or environments. The paper showed that, despite many birds singing diurnally in one environment, only specific species sang in the same environment, indicating that some species are inherently more likely to sing at night. The fact that there is no significant phylogenetic signal may mean that nocturnal vocalization has evolved independently multiple times. It seems clear that environments often have a great impact on nocturnal song, but that impact can also vary from species to species. This paper’s hypothesis is that birds that are more biologically-suited to nocturnality are more likely to sing at night at all, and this probability can be greatly amplified or depressed by various environmental factors. Territorial conflict, mating, and migration are proven functions of nocturnal vocalization. Singing at night due to pair-bonding, stimulation of reproduction, and communal roosting are interesting hypothetical functions. A key motive of nocturnal vocalization is that night environments are quieter and improve sound transmission, meaning the functions of some avian sounds in the night may be the same as their diurnal songs and calls, and birds are simply selecting an alternative time. This paper recommends more attention be placed on nocturnal vocalization in continents other than North America. Academic literature that continues efforts to understand the genetic basis for nocturnality could aid our understanding of night singing. Developing a phylogenetic model for the possibly repeated evolution of nocturnal singing is deserving of attention as well. There is a lot of seemingly conflicting research on the impact of weather and other environmental elements on night singing. Understanding to a greater degree the purposes of why some species may sing at night and others do not may be achieved by close, interspecific monitoring of territories with diurnal birds that vocalize during the night. Research that examines nocturnal vocalization in relation to breeding schedule is also quite helpful to our understanding. The current literature that focuses on bird singing disproportionately examines diurnal, rather than nocturnal, song. We should better understand the contexts in which nocturnal vocalization occurs.