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1. The mechanistic, genetic and evolutionary causes of bird eye colour variation

   https://doi.org/10.1111/ibi.13276  

   Corbett, Eamon C; Brumfield, Robb T; Faircloth, Brant C (April 2024, Ibis)

   Birds display a rainbow of eye colours, but this trait has been little studied compared with plumage coloration. Avian eye colour variation occurs at all phylogenetic scales: it can be conserved throughout whole families or vary within one species, yet the evolutionary importance of this eye colour variation is under‐studied. Here, we summarize knowledge of the causes of eye colour variation at three primary levels: mechanistic, genetic and evolutionary. Mechanistically, we show that avian iris pigments include melanin and carotenoids, which also play major roles in plumage colour, as well as purines and pteridines, which are often found as pigments in non‐avian taxa. Genetically, we survey classical breeding studies and recent genomic work on domestic birds that have identified potential ‘eye colour genes’, including one associated with pteridine pigmentation in pigeons. Finally, from an evolutionary standpoint, we present and discuss several hypotheses explaining the adaptive significance of eye colour variation. Many of these hypotheses suggest that bird eye colour plays an important role in intraspecific signalling, particularly as an indicator of age or mate quality, although the importance of eye colour may differ between species and few evolutionary hypotheses have been directly tested. We suggest that future studies of avian eye colour should consider all three levels, including broad‐scale iris pigment analyses across bird species, genome sequencing studies to identify loci associated with eye colour variation, and behavioural experiments and comparative phylogenetic analyses to test adaptive hypotheses. By examining these proximate and ultimate causes of eye colour variation in birds, we hope that our review will encourage future research to understand the ecological and evolutionary significance of this striking avian trait. 

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2. Olfaction and reproductive isolation in birds

   https://doi.org/10.1016/j.anbehav.2025.123076  

   Van_Huynh, Alex; Rice, Amber M (April 2025, Animal Behaviour)

   Olfaction is an important sense that has contributed to reproductive isolation and speciation in many taxa. However, in birds, olfaction and its potential role in communication has historically been neglected. Thus, what role olfaction plays in avian speciation is mostly unknown. Here, we aim to guide future research by highlighting the likely potential for olfaction to contribute to reproductive isolation and speciation in birds. First, we detail the best-understood example found thus far that is consistent with reproductive isolation by olfaction in birds: black-capped and Carolina chickadees. These species show interspecific differences in preen oil chemistry and conspecific odour preferences within their natural hybrid zone. We then suggest a number of promising avenues for future research and the kinds of systems, approaches and evidence that would help to advance this severely understudied area. In sum, the growing body of research into the chemical ecology of birds suggests an important role of olfaction in many areas of avian life. Future research will be necessary to determine to what extent olfaction contributes to the formation and maintenance of species boundaries in birds. 

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3. Mechanistic Hypotheses for Proprioceptive Sensing Within the Avian Lumbosacral Spinal Cord

   https://doi.org/10.1093/icb/icad052  

   Stanchak, Kathryn E; Miller, Kimberly E; Shikiar, Devany; Brunton, Bingni W; Perkel, David J (June 2023, Integrative And Comparative Biology)

   Synopsis Animals need to accurately sense changes in their body position to perform complex movements. It is increasingly clear that the vertebrate central nervous system contains a variety of cells capable of detecting body motion, in addition to the comparatively well-understood mechanosensory cells of the vestibular system and the peripheral proprioceptors. One such intriguing system is the lower spinal cord and column in birds, also known as the avian lumbosacral organ (LSO), which is thought to act as a set of balance sensors that allow birds to detect body movements separately from head movements detected by the vestibular system. Here, we take what is known about proprioceptive, mechanosensory spinal neurons in other vertebrates to explore hypotheses for how the LSO might sense mechanical information related to movement. Although the LSO is found only in birds, recent immunohistochemical studies of the avian LSO have hinted at similarities between cells in the LSO and the known spinal proprioceptors in other vertebrates. In addition to describing possible connections between avian spinal anatomy and recent findings on spinal proprioception as well as sensory and sensorimotor spinal networks, we also present some new data that suggest a role for sensory afferent peptides in LSO function. Thus, this perspective articulates a set of testable ideas on mechanisms of LSO function grounded in the emerging spinal proprioception scientific literature. 

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4. Local community composition drives avian Borrelia burgdorferi infection and tick infestation

   https://doi.org/10.3390/vetsci9020055  

   Lilly, M.; Amaya-Mejia, W.; Pavan, L.; Peng, C.; Crews, A.; Tran, N.; Sehgal, R.; Swei, A. (January 2022, Veterinary sciences)

   Globally, zoonotic vector-borne diseases are on the rise and understanding their complex transmission cycles is pertinent to mitigating disease risk. In North America, Lyme disease is the most commonly reported vector-borne disease and is caused by transmission of Borrelia burgdorferi sensu lato (s.l.) from Ixodes spp. ticks to a diverse group of vertebrate hosts. Small mammal reservoir hosts are primarily responsible for maintenance of B. burgdorferi s.l. across the United States. Never- theless, birds can also be parasitized by ticks and are capable of infection with B. burgdorferi s.l. but their role in B. burgdorferi s.l. transmission dynamics is understudied. Birds could be important in both the maintenance and spread of B. burgdorferi s.l. and ticks because of their high mobility and shared habitat with important mammalian reservoir hosts. This study aims to better understand the role of avian hosts in tick-borne zoonotic disease transmission cycles in the western United States. We surveyed birds, mammals, and ticks at nine sites in northern California for B. burgdorferi s.l. infection and collected data on other metrics of host community composition such as abundance and diversity of birds, small mammals, lizards, predators, and ticks. We found 22.8% of birds infected with B. burgdorferi s.l. and that the likelihood of avian B. burgdorferi s.l. infection was significantly associated with local host community composition and pathogen prevalence in California. Addition- ally, we found an average tick burden of 0.22 ticks per bird across all species. Predator and lizard abundances were significant predictors of avian tick infestation. These results indicate that birds are relevant hosts in the local B. burgdorferi s.l. transmission cycle in the western United States and quantifying their role in the spread and maintenance of Lyme disease requires further research. 

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5. Forebrain nuclei linked to woodpecker territorial drum displays mirror those that enable vocal learning in songbirds

   https://doi.org/10.1371/journal.pbio.3001751  

   Schuppe, Eric R.; Cantin, Lindsey; Chakraborty, Mukta; Biegler, Matthew T.; Jarvis, Electra R.; Chen, Chun-Chun; Hara, Erina; Bertelsen, Mads F.; Witt, Christopher C.; Jarvis, Erich D.; et al (September 2022, PLOS Biology)

   Townsend, Simon W. (Ed.)

   Vocal learning is thought to have evolved in 3 orders of birds (songbirds, parrots, and hummingbirds), with each showing similar brain regions that have comparable gene expression specializations relative to the surrounding forebrain motor circuitry. Here, we searched for signatures of these same gene expression specializations in previously uncharacterized brains of 7 assumed vocal non-learning bird lineages across the early branches of the avian family tree. Our findings using a conserved marker for the song system found little evidence of specializations in these taxa, except for woodpeckers. Instead, woodpeckers possessed forebrain regions that were anatomically similar to the pallial song nuclei of vocal learning birds. Field studies of free-living downy woodpeckers revealed that these brain nuclei showed increased expression of immediate early genes (IEGs) when males produce their iconic drum displays, the elaborate bill-hammering behavior that individuals use to compete for territories, much like birdsong. However, these specialized areas did not show increased IEG expression with vocalization or flight. We further confirmed that other woodpecker species contain these brain nuclei, suggesting that these brain regions are a common feature of the woodpecker brain. We therefore hypothesize that ancient forebrain nuclei for refined motor control may have given rise to not only the song control systems of vocal learning birds, but also the drumming system of woodpeckers. 

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