Sandhill cranes are large birds known for their loud, rattling calls that carry over long distances. Their screeching sounds are a distinctive part of their behavior and ecology that have several important functions. In this article, we will explore why sandhill cranes screech by looking at the context in which these vocalizations occur, their acoustic properties, and the benefits they provide cranes. Gaining insight into crane vocalizations can help us appreciate the behavior and natural history of these iconic wetland birds.
What is a sandhill crane?
Sandhill cranes are a species of large wading bird found primarily in open wetland habitats across North America. Adults stand about 4 feet tall with a wingspan over 6 feet, making them one of the largest flying birds on the continent. Their plumage is gray overall with some reddish coloring on the forehead, cheek, and nape. Sandhills have long dark legs for wading through shallow water and long necks and bills adapted for probing in mud to catch small prey like insects, worms, snails, crayfish and plants.
There are 6 subspecies of sandhill crane that occupy different geographic breeding ranges and undertake migration journeys between northern nesting grounds and southern wintering sites. The abundant greater sandhill crane subspecies nests across the northern tier of the U.S. and Canada. Each spring and fall, groups of sandhill cranes numbering in the tens of thousands congregate in key staging areas across the Great Plains and western states during migration.
Sandhill crane calls and screeches
Sandhill cranes have a very loud, rattling call that carries for miles. Their territorial and alarm calls are commonly described as a loud, trumpeting “kar-r-r-oo” or guttural “kruck.” This is often rapidly repeated multiple times in succession when the birds are alarmed or communicating over distance across open wetlands.
During the breeding season, paired cranes engage in distinctive duets, synchronized calls between male and female. Their calls are coordinated and can rapidly alternate between mates within 75 milliseconds. The male’s booming display call is lower pitched, and the female makes a higher-pitched one or two-note call in response. The vocal duets help pairs maintain the pair bond and defend their territory.
Both males and females give a loud, rattling screech in aggressive encounters such as defending a mate or the nest against intruders. This is called a guard call and has an unusually rapid frequency modulation that gives it a jarring, mechanical sound. The guard call is thought to signify a readiness to attack if necessary. During migration, cranes may screech loudly when conflicting with other cranes over resources.
Acoustic properties of crane screeches
Researchers who have analyzed sandhill crane vocalizations note their unusually low fundamental frequency, wide frequency range, and rapid frequency shifts that generate the rattling, screeching quality. The guard calls are only about 0.15 seconds long but shift wildly in pitch from around 600 Hz down to under 300 Hz and back up again. This rapid frequency modulation is driven by very quick changes in the tension of the syrinx organ in the trachea where the sound originates.
The frequency lowering is thought to intimidate rivals by signalling the caller’s large size while the rapid shifts add harsh, unpredictable elements found in alarm calls. The brief but loud screech can carry up to 3 km due to its low frequencies resistant to attenuation. The guard call’s wide frequency range also distinguishes it from flock contact calls at a longer range. This combination makes the screech stand out to receivers.
Functions of sandhill crane screeches
Researchers have identified several primary functions of sandhill crane screeches based on when they are observed:
Territorial defense
Sandhill cranes establish breeding territories around their nesting wetlands each year. Males begin advertising and defending territories up to four weeks before females arrive in the spring. Territorial calls help delineate occupied areas while guard call screeches are used to drive off intruding cranes of both sexes. Studies of banded cranes show that pairs return to the same nesting marshes year after year, with vocal displays playing a key role in maintaining site fidelity.
Distraction displays
When cranes perceive a threat to the nest or young chicks, the parents may perform distraction displays. These involve picking up objects, throwing weeds or sticks, jumping, bowing, and intense screeching directed at the intruder. This mobbing behavior draws attention while the chicks hide, discouraging predators like foxes, coyotes or raptors.
Alarm calls
Sandhill cranes emit their rattling screeches when alarmed by human disturbance or the presence of predators. This functions to notify flock-mates of danger and prime them for vigilance or escape behaviors. Cranes screech in response to aircraft overflights, gunshots, vehicles, and humans approaching their wetland habitats. Hunted populations like the endangered Mississippi sandhill crane subspecies have learned to view all humans as threats. Their loud alarm calls signal a readiness to flee to safety.
Aggressive encounters
The screeching guard calls are key to defending resources. Cranes fight using their wings, beaks and feet, sometimes leaping forward with spearing motions. Flocks may skirmish over access to feeding grounds, mates or spring staging areas where tens of thousands of birds converge. The rattling calls signal escalation so disputes can often be settled with displays and calls instead of physical attacks. This reduces energy costs and injuries from fights within the flock.
Maintenance of social hierarchies
Sandhill cranes form small social groups within winter flocks, with stable dominance hierarchies. More dominant birds give deeper pitched territorial calls. The guard call screeches are used to establish and maintain status, signaling aggression in conflicts over resources. Studies show higher-ranked birds feed first and gain better access to prey like corn in agricultural fields. The rattling screeches communicate social rank and help maintain group structure.
Pair bonding
Sandhill cranes form long-term pair bonds that can persist for years or life. Pairs spend almost all their time together. Their coordinated duets are part of maintaining their pair bond. Pairs will lodger screech to display aggression towards intruding cranes that get too close to their mate. This helps strengthen the pair bond and signals a readiness to defend each other from social challenges.
Unique evolutionary adaptations
Researchers who have studied crane vocalizations note they are unlike those of most other birds due to their specialized syrinx anatomy and rapid changes in resonance. Sandhill crane syrinxes have stiffened cartilage rings around the bronchial openings. They can manipulate these with their trachea muscles to induce sudden shifts in resonance that generate the rattling, quavering quality.
This syrinx anatomy is an evolutionary adaptation also found in whooping cranes and some swans within the crane family. It provides sandhill cranes with a uniquely effective alarm and guard call compared to more melodic vocalizations in related wading birds like herons or egrets. The loud, jarring screeches stand out over wetland soundscapes, transmitting information on identity, motivation and threat levels.
Conservation and population impacts
Understanding sandhill crane vocalizations provides insights for conservation. Wetland drainage has reduced crane nesting habitat across North America. Their screeches draw attention to disturbances like aircraft overflights or human encroachment on key migration stopover sites where thousands of cranes concentrate.
Studies show migration delays if resting cranes are frequently flushed and have higher energy expenditures. Analyzing responses to different noise sources helps identify threats. The Mississippi sandhill crane subspecies, hunted to near-extinction, has learned to screech at any approaching humans due to a long history of persecution. Their aggressive calls now hinders release efforts in protected areas. Maintaining habitat quality to reduce interactions with humans can help ensure sandhill crane populations remain healthy into the future.
Conclusion
Sandhill cranes produce their distinctively rattling, screeching vocalizations through specialized anatomical adaptations. These calls play important roles in territory defense, alarming flock mates of threats, deterring nest predators, establishing social hierarchies, strengthening pair bonds, and signaling aggressive motivation during conflicts. Understanding why sandhill cranes screech provides insight into their behavior and ecology and can help inform conservation efforts for these charismatic wetland birds. Their loud calls are a central part of wetland soundscapes across North America.