Rats are highly opportunistic and successful predators, and they definitively kill birds. This predatory interaction is a significant ecological concern in diverse environments, ranging from urban parks to remote island ecosystems. The relationship is driven by the rats’ flexible diet and their ability to exploit vulnerable avian life stages, particularly eggs and young. This threat influences the survival and biodiversity of entire bird populations.
Which Rats Target Which Birds
The primary culprits in avian predation are the two most widespread invasive rodent species: the black rat (Rattus rattus), also called the ship or roof rat, and the brown rat (Rattus norvegicus), known as the Norway or wharf rat. The black rat is an agile climber, posing a threat to nesting birds in trees and shrubs. The brown rat is larger and more terrestrial, frequently targeting ground-level nests. Both species are generalist omnivores, readily incorporating eggs, nestlings, and even adult birds into their diet.
Vulnerability is highest for specific groups of birds, including ground-nesting species such as mallards and robins, and cavity nesters like blue tits and great tits, especially in urban environments. On islands, the impact is severe on seabirds, like petrels and albatrosses, and endemic land birds. These island populations evolved without terrestrial mammalian predators, lacking the defensive behaviors needed to protect their nests, making them highly susceptible to rat invasions.
Methods of Predation
The most common predatory method rats employ is nest raiding, which exploits the most defenseless life stages. Rats primarily target eggs and nestlings, which provide a concentrated, easily accessible source of protein and fat. Eggs are consumed by piercing the shell and consuming the contents. Smaller eggs are generally more vulnerable, though rats can consume a wide range of sizes.
Rats use their natural traits to facilitate hunting, including nocturnal activity, which allows them to hunt when birds are resting and visibility is low. The black rat’s climbing ability allows it to access nests meters off the ground, threatening tree-nesting birds that rely on height for safety. Predation on adult birds is less common but occurs when adults are incapacitated, sleeping, or defending a ground nest.
Nest failures due to rat predation can drive population decline. Studies show rats are responsible for up to 60% of nesting failures in some urban areas. On islands, the black rat is known to destroy native bird nests, causing high rates of total nest depredation.
Ecological Consequences
The presence of invasive rats creates significant ecological disruption, especially within isolated ecosystems. Rats are a major factor in the decline and extinction of endemic bird species, particularly those confined to islands. Their high reproductive rate, combined with a lack of natural controls on islands, allows them to sustain high predator densities that stress bird populations.
The concept of “predator naiveté” is a specific vulnerability in island birds, who have not evolved responses to terrestrial mammalian predators and do not recognize the danger posed by rats. This lack of defensive behavior means that seabird colonies, which often nest densely on the ground or in burrows, can be devastated by invasion. For example, the introduction of black rats to Lord Howe Island contributed to the extinction of five endemic bird species.
Beyond direct predation, the decline in bird numbers can cascade through the ecosystem, disrupting processes like seed dispersal and insect control. The impact of rats is most acute on smaller islands where rat densities are highest and bird populations are smaller and more localized. Successful rat eradication programs on islands have led to a recovery in bird populations, demonstrating the negative effect of rat presence.
Reducing Predation Risk
For individuals in human-dominated environments, mitigating rat predation risk involves managing attractants and access points. Securing bird feeders is a primary step, including using rodent-proof feed bins for seed storage and cleaning up spilled feed immediately. Temporarily stopping feeding altogether for a few weeks can encourage rats to seek alternative food sources away from the area.
Physical barriers are effective at protecting active nests and feeding stations. Installing metal baffles or predator guards on birdhouse poles or trees prevents rats from climbing to reach the contents. For aviaries and coops, using hardware cloth with a small mesh size, such as a quarter-inch, creates a security cage that rats cannot chew through or squeeze past.
On a larger scale, conservation managers implement sophisticated strategies, including island-wide rodent eradication programs, often using specialized poisonous bait. These programs are complex and costly, but they have proven successful in restoring native bird populations in remote areas. For urban settings, integrated pest management focuses on sealing building entry points, removing debris, and securing waste to eliminate shelter and food that attract rats.