Hummingbirds are some of the smallest and most unique birds on the planet. Their tiny size and incredible flying abilities make them stand out from other bird species. But perhaps one of the most fascinating things about hummingbirds is their unusual skull anatomy, which allows them to hover and feed on flower nectar.
Hummingbird Skulls are Very Lightweight
One of the defining features of a hummingbird’s skull is that it is extremely lightweight. The skulls of hummingbirds weigh less than half a gram on average. This tiny weight is one of the key adaptations that gives hummingbirds their unrivaled agility in flight. With a lighter skull, their necks do not have to support as much weight, allowing them to quickly turn and orient their beaks into different flowering plants.
There are a few ways that hummingbird skulls achieve this lightweight construction:
- Their skulls lack teeth and associated alveoli (tooth sockets) which reduces overall mass.
- Many of the skull bones are hollow or have air pockets, particularly in the beak/bill area.
- The bones themselves are thin and delicately constructed.
By minimizing weight while still retaining strength and protection for the brain, a hummingbird’s cranial anatomy allows for exceptional aerial maneuvers using very little energy.
Nares are Located at the Tip of the Beak
The nares are the external openings of the nasal cavities in birds. In most species, these openings are located somewhere along the upper mandible. But hummingbirds have evolved to have their nares positioned right at the very tip of their often long, slender beaks.
This allows hummingbirds to breathe through their nose while their beak is plunged deep into a flower. By not having to open their mouth, they can more effectively collect nectar with their tongue and avoid getting pollen all over their nasal passages.
Skulls Have Many Cranial Kinesis Adaptations
Cranial kinesis refers to the ability of some animals to move certain joints of their skulls in order to open their mouths wider. This is very beneficial for swallowing larger food items. Hummingbirds exhibit an advanced form of cranial kinesis that allows them to angle their beak tips up and down as well as side to side.
This allows hummingbirds to precisely manipulate and position their beaks to gain access to nectar in differently shaped flowers. Specific points of flexion in their skulls give hummingbirds great control over how they collect nectar.
Tongues Attach to the Tip of the Hyoid Bone
The hyoid bone is one of the bones that supports structures of the tongue and throat. In most birds, the tongue attaches closer to the base of the hyoid bone near the chin. But in hummingbirds, the tongue attaches directly to the forward-projecting tips of the paired hyoid bones.
By anchoring the tongue to the very front, hummingbirds can fully extend their tongues out past their beaks which lets them reach deep into tubular flowers.
Skulls Allow for Expanded Cervical Vertebrae
A hummingbird’s neck vertebrae (cervical vertebrae) are adapted to enhance their range of motion for feeding. The opening at the base of the skull (foramen magnum) is much wider than in related species. This allows for the first cervical vertebra to articulate broadly with the skull and initiate extensive neck movements.
Other processes on the cervical vertebrae also act as attachment points for expanded neck musculature that controls their very flexible necks. This is all made possible by a skull that can accommodate these wider vertebral articulations.
Jaw Musculature Has High Proportions of Fast Fibers
The chewing muscles that close a hummingbird’s jaw are made up of an unusually high number of fast oxidative-glycolytic fibers. This fiber type allows the jaw muscles to contract very quickly and repeatedly with minimal fatigue.
This enables hummingbirds to snap their beaks shut at very fast speeds when catching insects. It also facilitates the rapid tongue flicking that helps them collect nectar. The adaptations in their skull contribute to more optimized leverage and space for these fast-acting jaw muscles.
Skulls Exhibit Sexual Size Dimorphism
Along with differences in plumage, female and male hummingbirds often have slightly different skull proportions. In species where males participate in courtship displays, they tend to have longer, more tapered beaks than females. This is thought to increase resonance and amplification of the chirps they use during these displays.
Females, on the other hand, tend to have shorter, more robust beaks which may improve access and handling of food resources to better support the high energy demands of reproduction.
Skulls Help Reduce Visual Obstruction
Hummingbirds have remarkably acute vision. To help maximize their visual field and avoid obstruction, their skulls are flattened on top and along the sides. This contouring keeps their skulls from protruding too far in front of their eyes, which would otherwise block their sight during rapid forward flight.
The reduction of visual obstruction by their skull is also aided by having very small orbits positioned towards the sides of the skull rather than the front.
Cranial Anatomy Aligns with Nectar-Feeding Behaviors
When looking at hummingbird skulls, it is clear that they are highly specialized and designed around accessing liquid nectar from flowers. The long, tapered beaks, extended tongue support, lightweight construction and kinesis all point to evolutionary adaptations for feeding on nectar.
Even as hummingbirds originated from insect-eating ancestors, these numerous modifications to skull architecture show how natural selection shaped them into the dedicated nectarivores they are today.
Conclusion
Hummingbirds have evolved truly remarkable changes to their cranial anatomy in order to thrive on their unique nectar diet. Their skulls allow them to hover with control, plunge their beaks into different flowers, flick their tongues at high speeds, and precisely manipulate their feeding structures without excessive weight or obstruction. Understanding the form and function of hummingbird skulls provides a fascinating look into the intersection of behavior, ecology, biomechanics and evolution.
