This blog was first published in April of 2018. It was inspired by RRP Director John Howe, who was reviewing footage of Mom and Dad feeding eaglets and was delighted by the dexterity of their beaks and tongues. Skip down to the paragraph below the table for more information on feeding eaglets!
We know that bird beaks are specialized for feeding and daily tasks. Birds of prey have strong, curved beaks with sharp edges to help them tear meat. Falcons specialize even further, adding a tomial tooth to help them kill prey. Dabbling ducks have tiny, comb-like structures on their beaks to strain small animals, insects, and plants from water and mud, while piscivorous ducks have saw-like structures to help them hold on to struggling fish. But what about bird tongues or, more specifically, bald eagle tongues? Are they specialized as well?
Let’s start by taking a look at an eagle’s tongue!
At first glance, bald eagle tongues look somewhat similar to ours. They are pinkish, relatively narrow, fit nicely between the sharp ridges of their beaks, and are flexible. They are short enough that eagles can’t easily bite their tongues, although they can stick them out. Unlike us, bald eagles have two barbs, or rear-directed papillae, to help lift and pull food items to the back of their long mouths: a relatively common feature in bird tongues.
Although birds of prey don’t have specialized tongues, their tongues (and beaks) are packed with mechanoreceptors that respond to pressure, distortion (think of an eagle stretching its tongue or biting into bone), temperature, texture, and vibration. Merkel cells are sensitive to very fine changes in touch, pressure, and temperature. These tonic cells keep providing information to tell eagles exactly where their beaks and tongues are and what they are doing. Phasic Herbst and Ruffini cells sound an alarm to let them know that something has just happened. Herbst cells are sensitive to vibration, pressure changes, and texture changes (I rubbed against something, I grabbed or dropped an object, I touched a hard smooth object). Ruffini cells are sensitive to stretching, distortion, and temperature (I just bumped something warm like an eaglet’s beak and mouth).
These sensors aren’t evenly distributed throughout a bird’s beak. While I couldn’t find any studies on birds of prey, studies have been done on quail and ducks. Sensory cells in a quail’s beak are distributed like this:
Once a bird has allowed something past the tip of its beak, it has a lot of decisions to make about how to respond to it and what to do with it. Bald eagles may not distribute their sensors as ducks and quail do, but they are able to slice through fur and scales, separate meat from bone, and feed young appropriately sized bits or large chunks. Their large beaks are much more sensitive than they look!
Thanks to their beak blinders, bald eagles have a blind spot directly in front of them, which means they can’t always see their young during feeding. So how do parents select food, find eaglets, and get food into their mouths? Immediate and ongoing sensory feedback helps parents and young align beaks, and tells new hatchlings to open their mouths wide for food delivery! Once both beaks are in the correct position, eagle parents use their sensitive, flexible tongues to carefully push food into an eaglet’s waiting mouth. Changes in pressure, temperature, and texture help the eagles change course in mid-delivery, detect dropped food, find eaglets, and pull back as needed. They also use their tongues and beaks to detect and avoid feeding young hatchlings chunks of bone, large pieces of fur, or sharp fins that could cause them to choke.
How do eagles manipulate food so well? Check out the photo, which shows a side view of Dad’s tongue taken last year. We can clearly see that its fleshy tip rests on a muscular stalk. As described in Laura Erickson’s excellent blog on bird tongues, this stalk controls Dad’s tongue to manipulate food items. Eagles can stretch and tip their tongues forward to feed eaglets, pull and tip them backwards to move food into their mouths, or move their tongue to one side or another as needed. Their flexible two-stage tongues and rear-directed papillae give them incredible control, and the sensory cells packed into the tip of their beaks and tongues provide the feedback they need to feed eaglets…even when they can’t quite see where eaglets are. Annoyed with an eagle parent over dropped food or missed beak-loading opportunities? Imagine trying to feed eaglets using your just tongue and nose! The video below shows just how delicate they are when it comes to feeding hatchlings.
Did you know?
Bird banders can use the color of a bird’s mouth and tongue to help age birds. Younger birds may have spots or bands to help parents with food targeting. Colored tongues in some adult birds can aid mating displays and/or signal a warning to other birds.
Understanding a species’ behavior and diet helps us understand how its tongue and mouth evolved. Check out Laura Erickson’s blog More About Bird Tongues Than A Normal Person Would Want To Know for a great look and description of some bird tongues! You know you want to learn more!
Things that helped me learn and write about this topic