Cattle egrets (Bubulcus ibis) are small white herons that are often seen accompanying cattle and other large grazing animals. They have developed a commensal relationship with cattle, meaning they benefit from feeding on the insects stirred up by the cattle as they graze, while the cattle are generally unaffected by the presence of the birds.
In this article we will explore the close ecological relationship between cattle egrets and cattle and how the two species interact and rely on each other. Some key questions we will answer include:
What is the cattle egret?
The cattle egret is a cosmopolitan species of heron that is found in tropical and subtropical habitats across the world. They are relatively small herons, standing around 20–30 cm tall and weighing between 100–300 g.
They have predominantly white plumage, with orange or yellow beaks, legs and feet. During breeding season they develop orange or red lore (the area between the eye and bill). Their diet consists mainly of insects and other small creatures disturbed by grazing cattle and other animals.
Cattle egrets are highly social and are usually found in groups. They breed in colonies that can contain hundreds to thousands of nests. Cattle egrets have proven to be highly adaptable and their populations have expanded enormously in some regions due to their association with human agriculture.
What is the historical relationship between cattle egrets and cattle?
The ecological relationship between cattle egrets and cattle has developed over the past few centuries. Originally cattle egrets were native to Africa, parts of Asia and southern Europe. However, the expansion of the cattle farming industry around the world has allowed cattle egrets to dramatically expand their range.
It’s believed the first cattle egrets arrived in North America in the early 1900s, having hitched rides on ships carrying cattle from Africa. The egrets discovered they could follow the cattle around and take advantage of the insects stirred up by the large grazing herds. The relationship proved to be highly beneficial, and allowed cattle egrets to colonize North America.
A similar expansion occurred in South America following the introduction of cattle egrets in the late 1800s. Today cattle egrets are found through most warmer regions of the world thanks to their successful ecological partnership with cattle.
How do cattle egrets benefit from cattle?
Cattle provide several important resources and opportunities that cattle egrets have adapted to exploit, including:
– Access to food: The rich supply of insects and other invertebrates stirred up by grazing cattle provides a reliable food source for egrets. Cattle disturb insects hidden in the grass and vegetation as they graze.
– Locating prey: Cattle egrets will often follow directly behind or between cattle so they can pick off insects immediately as they are flushed out by the cattle. The cattle essentially do the work of finding food for the egrets.
– Greater feeding efficiency: Studies have found that cattle egrets following cattle have higher feeding success rates and efficiency compared to egrets foraging on their own. The disturbance and flushing out of prey makes them easier for egrets to catch.
– Protection from predators: Being around livestock likely provides some protection from predators. Most predators will avoid approaching cattle due to the risk from large livestock that will defend themselves if threatened. Egrets may gain some protection just by associating closely with cattle.
So by developing a symbiotic relationship with cattle, cattle egrets gain access to a rich and reliable food source, as well as protection, allowing them to thrive. The commensal relationship is highly beneficial to the egrets.
How do cattle interact with and react to cattle egrets?
Cattle appear largely indifferent to the presence of cattle egrets and the egrets do not actively affect the cattle or their grazing. The cattle carry on feeding normally while the egrets follow closely behind.
Occasionally cows will be observed flicking their tails in irritation at egrets if they get too close. But generally the cattle tolerate the presence of the birds.
The egrets give cattle a wide berth if they lie down to rest. The small birds will wait patiently for the cattle to begin grazing again before approaching more closely to pick off disturbed insects.
Calves may be curious of the small white birds, sometimes approaching them closely. But again, minimal interaction occurs. The egrets will fly away if a cow gets too close.
The relationship operates smoothly with cattle providing food and shelter opportunities, while the egrets capitalize on these without actively affecting the cattle or their normal behavior.
Do cattle receive any benefits from the presence of cattle egrets?
There is little evidence that cattle directly benefit from having cattle egrets around. The egrets do not perform any helpful services for cattle such as removing parasites or ticks. They don’t actively reduce annoyance from insects enough to aid the cattle.
However, one study did note that calves that spent more time with cattle egrets had slightly higher weight gain compared to calves without egrets. This suggests the egrets might help reduce annoyance from flies enough to allow calves to feed more efficiently and gain weight faster.
But overall the relationship is a commensal one where the egrets benefit disproportionately. Cattle largely tolerate the egrets but don’t gain any major advantage from their presence. The egrets are opportunistically exploiting the resources cattle provide without giving anything directly back.
How have cattle egrets adapted to take advantage of cattle?
Cattle egrets exhibit several behavioral and ecological adaptations that allow them to exploit their relationship with cattle effectively, including:
– Social foraging – By feeding in groups around livestock, egrets can maximize the number of insects flushed out.
– Following behavior – Egrets closely follow cattle while grazing to instantly pick off disturbed prey.
– Diet shifting – Egrets shift their diet to mainly insects when with cattle rather than the fish and crabs they eat in wetlands.
– Migration and range expansion – Egrets have expanded their range dramatically by following cattle to new continents and regions.
– Reduced anti-predator behavior – Egrets lower their guard around cattle and spend more time foraging instead of being vigilant for predators.
– Heightened breeding displays – The reliable food source allows for increased breeding displays and reproductive success.
– Habituation – Egrets easily habituate to the presence of cattle and learn to interpret behaviors that predict cattle activity and food.
These adaptations allow cattle egrets to gain the most benefit possible from the inadvertent food and opportunities cattle provide through their grazing behavior.
Feeding Behavior of Cattle Egrets Around Cattle
The feeding behavior of cattle egrets around livestock shows just how adapted they are to exploiting this resource. Their foraging strategy and technique allows them to maximize feeding opportunities. Some key aspects of their feeding behavior include:
Following closely behind cattle
Cattle egrets will walk or fly closely behind cattle, often less than one meter away. This allows them to instantly snap up prey disturbed by cattle as they graze. By following closely, they don’t miss opportunities.
Positioning between cattle legs
Egrets will position themselves right amongst the feet and legs of moving cattle. This puts them in prime position to catch prey immediately as hooves knock insects from vegetation.
Landing on cattle’s backs
It’s common to see an egret casually riding on the back of a grazing cow. Again this positions them over insect rich patches that can be targeted.
Flushing insects by foot-trembling
Egrets have been observed using a foot trembling technique to stir up hidden insects, taking advantage of the disturbance first created by cattle.
Feeding in groups
Groups of up to around 20 egrets will closely follow cattle together. This allows them to collectively take advantage of insects. Studies show they have higher feeding success when foraging socially like this.
Habituation
Through close association with cattle, egrets have become highly habituated to their presence. They are comfortable feeding very close to cattle and learn to understand and take advantage of cattle behaviors.
Diet switching
Cattle egrets switch their main diet to insects and other terrestrial invertebrates when following cattle herds. Their diet converges on what cattle activities make most available.
Examples of Cattle Egret and Cattle Interactions
Some specific examples that demonstrate the close relationship between cattle egrets and cattle include:
Perched waiting for grazing
Cattle egrets will perch on the backs of cattle waiting patiently for them to begin grazing again after resting. This shows their ability to closely interpret cattle behaviors.
Swarming disturbed habitat
When cattle are herded into a new pasture or disturbed area, egrets will swiftly swarm the area to take advantage of the flush of insects.
Distinct paths around resting cattle
Egrets will make distinct paths around resting cattle to avoid disturbing them and wait close by for when they become active.
Roosting on cattle at night
Egrets have been observed perching on cattle at night while roosting, showing their extreme comfort around livestock.
Following movements to fresh patches
Cattle egrets closely follow the movements of cattle between grazing patches to stay with the supply of food.
Young egrets “imprinting”
Young egrets appear to form associations with cattle from a very early age, instinctively following them, showing the relationship is likely hard-wired genetically after many generations.
These examples demonstrate the opportunistic and adaptive nature of the cattle egret as well as the largely indifferent reaction from cattle to their presence.
Differences in Cattle Egret Behavior Around Cattle Compared to Natural Foraging
When foraging around cattle, cattle egrets exhibit some notable differences in their behavior compared to when feeding naturally in wetland habitats:
Diet
As mentioned their diet shifts predominantly to terrestrial insects rather than small aquatic animals like crabs and fish.
Foraging method
They adopt a following strategy around cattle vs a patient waiting strategy when alone.
Habitat
Egrets spend far more time in grassland/pasture habitats when utilizing cattle rather than just wetlands.
Sociality
They feed in larger and denser aggregations around cattle. Social foraging enhances feeding efficiency.
Wariness
Their vigilance and anti-predator behavior appears reduced around cattle where they perhaps feel more secure. More time can be spent feeding.
Activity
Cattle egrets spend more time actively foraging when with cattle vs roosting or resting when alone.
Energy obtained
Analysis shows cattle egrets gain higher quality energy by feeding on insects vs crustaceans and fish. The insect diet supports breeding.
So cattle provide superior feeding opportunities that the egrets are adapted to fully exploit.
Advantages and Disadvantages of the Cattle Egret-Cattle Relationship
This close ecological relationship provides both advantages and potential disadvantages to cattle egrets and cattle:
For cattle egrets:
Advantages:
– Reliable high quality food source
– Increased feeding efficiency
– Access to new habitats and range expansion
– Protection from predators
– Social foraging benefits
Disadvantages:
– Dependency on cattle presence – egrets decline when cattle are removed
– Vulnerability if cattle practices change e.g. insecticide use
– Competition between egrets when dependent on limited resource
For cattle:
Advantages:
– Possible minor benefit of insect reduction
– Slight increase in calf weight gain
Disadvantages:
– Potential to spread disease between egrets and cattle
– Egrets could disturb grazing behaviors
– Increased predation risk from egrets attracting predators
So for cattle egrets, the relationship provides huge advantages that have allowed their populations to thrive. For cattle, any effects are minimal with limited advantages or disadvantages.
Understanding the Symbiotic Relationship Through Remove-Replace Experiments
Ecologists have gained greater insight into the cattle egret – cattle relationship from “remove-replace” experiments. These involve removing egrets from cattle and observing the effects, and replacing egrets and monitoring changes.
Cattle egret removal effects:
– Egrets showed elevated stress behaviors when prevented from accompanying cattle, such as increased aggression and restlessness.
– When removed, egrets would quickly fly back to rejoin cattle herds once released, highlighting dependence.
– Growth rates of egrets reduced when they were denied access to cattle over a 2 month period.
Cattle egret replacement effects:
– When egrets were reintroduced after a 3 week absence, they immediately resumed close following and foraging behaviors with cattle.
– Feeding rates, social foraging group sizes and agitation behaviors returned to normal levels within days of reintroduction.
– Cattle showed no changes in behaviors or stress when egrets were removed or replaced.
This experiments reveal the egrets strong reliance on cattle and how their behaviors are adapted to exploit this food source. Cattle remain largely unaffected by egret presence or absence.
Coevolutionary History Between Cattle Egrets and Cattle
The unique relationship between cattle egrets and cattle likely emerged through a process of co-evolution over the past 300 years.
Some hypothesized steps in this coevolution include:
Early association
– First contact between cattle egrets and cattle occurred around 300 years ago in Africa when cattle farming began displacing native ungulate herds.
Opportunistic exploitation
– Egrets started exploiting the novel food source and feeding opportunities provided inadvertently by cattle.
Commensal adaptation
– Behaviors like following cattle and dietary switching evolved over successive generations to maximize exploitation of this niche.
Habituation
– Through repeated exposure, egrets formed habituation and trust of cattle, allowing even closer association.
Range expansion
– Egrets expanded dramatically out of Africa by traveling with cattle transports to colonies all cattle-inhabited continents.
Population growth
– Access to the cattle niche allowed cattle egrets to greatly increase their global population size.
So through many small adaptations over time, cattle egrets have evolved to fill the cattle niche – an new opportunity that they continue to exploit and thrive within today.
Threats to the Cattle Egret-Cattle Relationship
Some practices potentially threaten the successful cattle egret-cattle relationship including:
Insecticide use
Many ranchers and farmers apply insecticide to cattle to control insects like ticks or flies. This insecticide can harm egrets feeding on the insects.
Habitat loss
Destruction of wetlands and conversion to farmland destroys nesting habitat for egrets while also displacing ungulate populations.
Displacement of wild ungulates
Expanding cattle ranching and farms in Africa has reduced native animal populations like wildebeest that egrets previously followed.
Persecution
Some ranchers still perceive egrets as pest birds and destroy nesting colonies or attempt to discourage their presence. Education on the commensal relationship is needed.
Climate change
Shift in rainfall patterns could make both cattle grazing and egret breeding less viable in certain areas.
So maintaining habitat connectivity, reducing insecticides, and protecting nesting sites is important to ensure the continued success of this unique ecological relationship.
Conclusion
The commensal relationship between cattle egrets and livestock provides an excellent example of an adaptive interspecies interaction. Cattle egrets have evolved a range of behavioral and ecological adaptations to take advantage of this man-made ecological niche.
Their populations have grown exponentially thanks to the new food source and feeding opportunities cattle provide through their grazing activities. However, cattle remain largely indifferent to the presence of the birds and gain minimal benefit.
Care needs to be taken to ensure this ecologically important relationship continues to thrive into the future. The cattle egret-cattle partnership demonstrates the remarkable ability of species to opportunistically adapt and fill new niches created by human activities.
