Time Scavengers

Evolutionary Origin of Nocturnality in Birds

by Yonghua Wu

Summarized by: Ana Jimenez Bustos is a geology undergraduate student at the University of South Florida. She plans to attend graduate school in a field related to volcanology, possibly planetary geology. Once she has her degree, she would like to teach and continue to do research in volcanology or planetary geology. Outside of school, she enjoys eclectic, noisy music, her dog Miranda, and loves reading and learning about birds and parrots. 

What data were used? This study compiled data from scientific literature that analyzed genomes, physical characteristics such as eye sizes, ear structure, and anatomy of fossils of ancient and modern birds. Molecular, genetic, morphologic, and evolutionary data was used to determine whether the origin of these nocturnal habits (or the habit of being active at night) was based on a common ancestor or if it evolved along the way. The active and inactive genes (genes that are ‘turned on’ or ‘turned off’ in creatures’ bodies) of eyes involved in light reception and transport were used to try to understand when birds began to live, hunt, and forage in the dark. The study analyzed the compilation of these articles’ conclusions to try to determine whether nocturnality in birds was a trait inherited from a common ancestor or if it evolved side by side in different bird species.

Methods: This study used an array of existing scientific literature to study the evolutionary origin of nocturnality in extant bird species. By analyzing existing scientific literature, the study drew conclusions regarding ancient and modern bird habits.  

Results: It is likely that the nocturnal habits of birds evolved from a common ancestor, representing some of the earliest birds. This hypothesis is supported by the morphology of existing birds, such as large eye to body ratio when compared to other vertebrates because larger eyes allow more light into the retina for clearer nocturnal vision. In addition, these birds have a relatively advanced hearing apparatus that could have evolved from the need to communicate in the dark. The lack of certain organs like the parietal eye in crocodilians and birds (today found only in lizards), which is a light sensitive organ connected to the part of the brain responsible for hormone regulation, suggests that birds had a nocturnal origin, as this organ would have been rendered useless in the dark; the ancestors of birds lost this organ millions of years ago. 

In addition, certain genes that are related to detecting movement (specifically, GRK1 and SLC24A1) are thought to have been present in the common ancestor of birds. These genes would have helped to avoid predation in low-light conditions and support the hypothesis that their ancestor was at the very least both diurnal and nocturnal. 

Specific adaptations to nocturnal life present in modern birds likely evolved independently from each other. Owls’ asymmetric ears, for example, evolved to precisely locate prey in the dark. This trait was likely not preset in the owls’ ancestors. The deactivation of specific genes related to color vision in nocturnal birds was also likely an evolutionary adaptation to the lack of need for color vision in birds that hunt and forage in the dark. This mutation is present in owls, kiwis, and nocturnal parrots. Some modern birds such as nightjars (Caprimulgiformes) have also evolved a tapetum, which is an extra layer in the back of the eye that reflects light back into the retina. This structure often gives eyes a “shiny” look when flashed with bright lights and can help to give animal clearer vision at night. 

Genetic and morphological evidence suggests that it is possible that birds evolved nocturnal habits in parallel to each other, but it is still possible that the common ancestor of all modern birds was both diurnal and nocturnal. Since the activity patterns of modern birds’ ancestors are still mostly unknown more analysis is needed to understand the habits of ancestral birds. 

Why is the study important? Nocturnality of mammalian creatures is largely understood to have evolved to avoid competition and predation from creatures that lurked during the day, but the origin of nocturnality in birds is not so well understood. Did ancient avian (bird) ancestors also have diurnal and nocturnal habits or was it a trait that was picked up along the evolutionary road? Did this nocturnality evolve in several different species or was it inherited from a single ancestor? Studying extant nocturnal birds and birds that have a combination of diurnal and nocturnal habits may help shed light on the evolutionary history of these behaviors. Understanding these behaviors in birds (or avian dinosaurs) can also help understand the behavior of non-avian dinosaurs like other theropods such as Tyrannosaurus rex the distant past. 

The big picture: This study addresses the origins of nocturnal behavior in birds. It suggests that these habits were present in extremely distant relationships going all the way back to the time of the non-avian dinosaurs. Understanding the habits of modern-day bird ancestors can help understand how ancient birds, and even dinosaurs like Tyrannosaurus or Velociraptor, lived in the past. Previous studies have been absolutist in their approach by classifying ancient birds and their ancestors as either nocturnal or fully diurnal, but the complete story may be significantly more complex and requires more studies to fully understand. Analyzing molecular, morphological, and phylogenetic relationships together can provide a better picture of the origin of these behaviors.  

Citation: Wu, Y. (2020). Evolutionary origin of nocturnality in birds. ELS, 483-489. doi:10.1002/9780470015902.a0029073