Passerines
In grad school, my friend introduced me to the concept of the banana not-banana sort. The idea being you can sort any item into either a banana or not-banana. A banana would go into the banana category, whereas a cup of coffee, a chair or a bus would be categorized as not-banana. A similar (admittedly more useful) classification exists for sorting birds, the Passerine non-Passerine sort. Placing birds into one of two groups yields fewer specific individual names to recall, and turns up some quite interesting morphological and behavioral similarities within each. Today we will focus on the Passerines, and save the non-Passerines for a future post.
Passerines are a large group representing 2 in every 3 birds who are members of the order Passeriformes. Sparrows, Crows, Warblers and many more are examples of the diverse species within this order. The grouping of seemingly diverse species together suggests there should be commonalities between them, though it’s not immediately obvious to me what a Crow and a Sparrow have in common that a Sparrow and a Cormorant do not. After some research, I found out that on a genetic level Passerines share a common ancestor in their evolutionary past. The Passerines branched off ~47 million years ago and surprisingly are most closely related to our modern day Parrots and Falcons. From this common ancestor we find one key similarity between the Passerines, a lack of controlling muscles in the front three toes of the foot. This lack of fine motor skills gives Passerines less dexterity than birds of prey and gives rise to this family being referred to as “Perching” birds. This shared genetic ancestry and lack of specific foot muscle is difficult to realize from simple field observations, but there is another key differentiator that we are put in contact with on a regular basis.
Generally speaking Passerines are considered to be more highly evolved birds. This manifests through their unique and enlarged brain infrastructure as well as more highly developed vocal organ (the syrinx). The result is the ability for Passerines to make a larger array of and more complex noises vs non-Passerines. These complex vocalizations give another colloquial name for Passerines as “Songbirds”. Out in the field we may notice this divergence in the croaking of Cormorants vs. the song of Sparrows. There are some very interesting differences within the Passerines in how songs are learned or inherited, but we’ll save that nugget for another day.
So far, we have learned that Passerines are “Perching birds” and “Song birds”, both should help us make distinctions in the field. What other features can we find that distinguish the Passerines? A useful resource to explore this question is the AVONET database (see Appendix below). This database contains information detailing mass, wing measurements, tail lengths, habitats and other info for over 10,000 species of birds! Using this data we can dig a little deeper on this question. Looking through the bird mass data species-by-species, we find Passerines generally weigh about 4 times less than non-Passerines. This equates to 36g for the Passerines vs 159g for non-Passerines on median. The cartoon graph below shows the distribution of mass data for the two bird types. Within these distributions we see some interesting exceptions to this general mass rule. Notably, Hummingbirds weigh very little (and somewhat sing) but are non-Passerines, and Crows weigh a lot and are!
The average diet between these two groups also appears to diverge. Preferred diet skews more towards invertebrates for Passerines, with ~63% of the Passerine species favoring this niche vs 26% for non-Passerines. These two factors are likely not independent, given there is correlation between bird diet and mass (birds that favor invertebrates tend to have a low mass), but never-the-less is an interesting tidbit. The Passerines further show an equal proclivity for an Omnivorous diet as non-Passerines, and an aversion for an aquatic diet. In terms of the habitat they occupy, Passerines favor Forests and Woodlands, and show much less preference for Wetland and Marine areas. This seems to make sense given the foot biology in Passerines evolved for perching rather than wading or swimming.
One final interesting observation is a generally smaller Kipps distance in Passerines, as depicted in the cartoon histogram below. Admittedly I was not familiar with this measurement before, but the significance of it to this discussion peaked my interest. In the same article the AVONET database is introduced is a depiction of Kipps distance, which boils down to the length from the carpal joint to the longest primary feather, minus the distance from the carpal joint to the first secondary flight feather. This is much more easily drawn then described in words - see the illustration below! With this nugget of information, the question I had is why this metric is so determinant of Passerines? Short Kipps distances generally correlate with more rounded wings and greater maneuverability. The smaller size of these birds and their proclivity to perch certainly benefit from this, as well as having the agility to evade larger non-Passerines such as the Raptors. I can’t also help thinking if this increased agility helps with their invertebrate diet, being able to swiftly change directions in the air to pluck flying insects for example. I doubt we’ll be out in the field and get a good look or assessment of the Kipps Distance, but the increased agility might be noticeable.
The Passerines share many similarities despite encompassing such a wide range of species! Proclivity for perching, wide array of vocalizations, a preference for an invertebrate diet, increased flight agility and a generally small size are all traits we can look for in the field. I’m excited to see what similarities the non-Passerines share, perhaps we’ll learn about a new distance too!
Appendix
Interesting scientific articles on Passerines include:
Schmitt, C. J. & Edwards, S. V. Passerine birds. Current Biology 32, R1149–R1154 (2022).
Oliveros, C. H. et al. Earth history and the passerine superradiation. Proc. Natl. Acad. Sci. U.S.A. 116, 7916–7925 (2019).
Tobias, J. A. et al. AVONET: morphological, ecological and geographical data for all birds. Ecology Letters 25, 581–597 (2022).
Mooney, R. Birdsong. Current Biology 32, R1090–R1094 (2022).
Graphs that the illustrations above are based on:
