A nesting association between invasive and native bird species can counteract predators to aid the spread of an invasive species across non-urban habitats, where the invasives may become crop pests

After reading lots of research about parrots, most people are well aware that their cleverness rivals that of the great apes — and even human children (read more here and here). But a recent study of naturalized parakeets living in central Spain adds a new dimension to these birdsbehavioral flexibility. This study found that free-roaming monk parakeets, Myiopsitta monachus, are building their large stick nest apartments either near to or inside the nests of a native European species, the white stork, Ciconia ciconia, to benefit from these much larger birds aggressive nest defence against hawks and other aerial predators (ref).

It is true that monk parakeets are no strangers to sharing a nest with a stork species — monk parakeets in their native range in Brazil sometimes build their nests in close association with native storks, particularly the jabirus, Jabiru mycteria (ref) — but white storks are native to Europe, not Brazil.

These small parrots build large apartment complexes to live in

Monk parakeets are small lime-green parrots with grey underparts, a yellow beak and a long, pointy tail. They originated in the temperate lowlands of Argentina, Brazil, and Uruguay, and were legally trapped by the millions to satisfy the global pet trade. These parrots are particularly comfortable in open woodlands and adapt well to living in close proximity to people, so it didn’t take long before free-roaming individuals that had either escaped or been intentionally released were spotted in urban parks in many cities throughout the United States, western Europe, Asia, Africa and even on some oceanic islands (ref).

Monk parakeets have several qualities that make them well-suited for city life. First, they are unique amongst parrots because they build their own nests from sticks instead of competing with other birds for nest cavities. A group of parakeets builds a large stick nest comprised of individual ‘apartments’ with separate entrances where the adults raise their chicks. These nests, which are expanded each year, can eventually rival the size of a car. Second, they may have helpers at their nests, usually offspring from previous breeding cycles that help their parents raise their younger brothers and sisters. Third, because the ancestors of many captive parakeets were trapped in the foothills of the Andes mountains, they adapt easily to cold temperatures.

These parakeets are very common in their native lands, where their populations are expanding, thanks to the presence of large eucalyptus forests cultivated for paper pulp. Lacking the plant and animal diversity that is typical in the tropics, these artificial forests are home to limited numbers of other species, which provides a safe haven for nesting parakeets. Additionally, their expanding populations have transformed these parakeets into major crop pests.

Large cities provide homes for parakeets

Cities are often the initial point of introduction for many alien species because they have microclimatic conditions, a variety of resources provided by people, or the lack of competitors or predators. For these reasons, cities are hotspots where invasive species can establish themselves before they then spread into nearby rural landscapes. But rural areas can often halt the spread of invasive species either through competition with local native species or through predation.

But monk parakeets appear to be working through these challenges. Already, monk parakeet populations are growing exponentially in several cities in a number of Mediterranean countries. Further, because of their close association with the massive stick nests constructed by white storks (Figure 1), monk parakeets have been successfully moving out of cities and into rural areas where it is feared they may become crop pests.

But is this association with native white storks the reason that monk parakeets are becoming so successful at invading foreign landscapes? If so, what is it about this relationship that works so well for the parakeets? For example, because monk parakeets and white storks both build large nests of sticks, perhaps the parakeets are taking advantage of the storks choice of nest materials as a resource- and time-saving scheme? Or maybe the parakeets instead are seeking to benefit from the storks’ aggressive defence against birds of prey?

Parakeets nest with storks more frequently in rural areas

Madrid is home to the largest naturalized population of monk parakeets in Spain, and its urban-dwelling parakeets have recently begun moving into the nearby rural countryside. Thus, a team of researchers identified whether monk parakeets are more likely to associate with white storks in rural or urban habitats in Madrid. Based on what they were seeing, the researchers predicted that parakeet-stork nest associations should be more frequent in rural areas.

To test their hypothesis, the researchers studied a large area that included the city of Madrid and the surrounding rural areas along the Manzanares and Jarama rivers (Figure 2) where there are ongoing intensive agriculture and gravel extraction activities. In these areas, birds of prey nest primarily in riparian forests whereas white storks nest in forests as well as on electricity pylons and on the roofs of buildings. Based on observations, the researchers considered five local raptor species to be likely parakeet predators, but found no evidence that other bird species, such as corvids, prey upon monk parakeets or their nests.

During the breeding seasons (April–August) of 2014 and 2015, more than 900 monk parakeet nests were GPS mapped in the Madrid metropolitan area, most of them in urban areas (purple circles; Figure 2). Additionally, the nests of storks (yellow circles; Figure 2), raptors (red circles; Figure 2) and nest associations between storks and parakeets (blue circles; Figure 2) were also GPS mapped, and the type of substrate upon which the nests were located (tree, pylon or roof) were also noted. For storks and raptors, each nest is home to a single breeding pair, whereas parakeet nests house between one and thirty-five breeding pairs, each pair living in its own apartment.

The data revealed that parakeets nesting in rural areas were rare and almost always associated with nesting storks — an association that was almost never seen amongst urban parakeets. The researchers proposed this nesting association was intended to reduce the risk of predation.

The researchers then predicted that rural nesting parakeets should breed preferentially in stork nests constructed on electricity pylons. This is because electricity pylons are not accessible to mammalian or reptilian predators, so storks nesting on pylons could focus their anti-predator efforts on defending their nests from raptors. Thus, if rural parakeets preferentially nested with storks on electricity pylons, they too would enjoy a low nest predation risk, and if they nested in areas where there were few aerial predators, they would also reduce their overall predation risk whilst foraging nearby.

Indeed, the data showed that rural parakeets are more likely to nest with storks that were located farther from nesting raptors, but were located close to other groups of nesting parakeets (Figure 3).

The researchers then asked whether the parakeets were more interested in benefitting from the resources in the stork nests rather than from the presence of the storks themselves. For example, if the parakeets were taking advantage of the stork nests for their abundance of sticks and other nest materials, the parakeets should remain on their shared nest after the storks abandoned it. But on the other hand, if the parakeets were relying on the storks to keep them safe from raptor attacks, they should abandon their shared nests shortly after their storks moved on.

In short, the parakeets nested with storks to reduce their risk of raptor attacks: the data showed that rural parakeet colonies abandoned their nests shortly after their storks did.

Parakeets nesting with storks showed different responses to raptor intrusions

The researchers recorded 47 instances in which three different raptor species (the black kite, the booted eagle, and the common buzzard) closely approached parakeet nests, mostly in rural habitats. The number of parakeets present during these intrusions ranged between 1 and 50. The researchers noted that urban parakeets always flushed from their nests in response to raptor intrusions compared to rural parakeets, which only flushed in 31.5% of the events (Figure 4). Yet, amongst rural parakeets that were not nesting with storks, they too always flushed when a raptor flew too close, whereas rural parakeets nesting in association with storks only flushed in 4.8% of these instances (Figure 4).

This study documents how a protective-nesting association between invasive and native species can counteract the effects of predators, thereby supporting the spread of an invasive species across non-urban habitats, where they may potentially become crop pests. In the case of monk parakeets, we already know their populations are growing exponentially in a number of cities in several Mediterranean countries, where they coexist with white storks. This means that any responsible parakeet management plan should consider the risk of parakeet spread into rural areas and should look to native predators, such native birds of prey, as potential controllers of this spread.


Dailos Hernández-Brito, Guillermo Blanco, José L. Tella and Martina Carrete (2020). A protective nesting association with native species counteracts biotic resistance for the spread of an invasive parakeet from urban into rural habitats, Frontiers in Zoology | doi:10.1186/s12983-020-00360-2