Cuckoos develop eggs and chicks that resemble those of their hosts, to avoid detection. When they have more than one host, this can lead to the creation of new species of cuckoos – especially if they are more “violent” towards the host.

Cuckoos develop eggs and chicks that resemble those of their hosts, to avoid detection. When they have more than one host, this can lead to the creation of new species of cuckoos – especially if they are more “violent” towards the host.
In Leah Goldberg's story "An Apartment for Rent," the rabbit is unwilling to live next door to the cuckoo. "All her children grew up in strange nests, all abandoned, all promiscuous," she scolds her.
This is not entirely fair: not all species of cuckoos lay eggs in the nests of birds of other species, and this custom is not unique to the cuckoo, although it is particularly associated with it. Quite a few other birds also pass the burden of caring for their offspring on to unwilling "foster parents", a phenomenon known as "Social parasitism".
In addition, it is also not true that the cuckoo's young are abandoned and prodigal. They usually receive caring and devoted care from the birds in whose nest the egg was laid. Those birds do not intend to care for a cuckoo chick, of course, but their instincts lead them to feed any chick found in their nest. In this way, they "waste" time and resources, which should have been devoted to their own offspring, on a foreign chick. And this is even more so in the best case, where the parents also get to raise their own chicks alongside the cuckoo chick. In certain species of cuckoos, the cuckoo chick hatches from the egg early, throwing out the host's eggs. This ensures that parental care will be devoted exclusively to it, and the host birds lose all their offspring.
Hosts who remove the cuckoo egg, or the chick that hatched from it, from the nest will gain a significant evolutionary advantage – especially if the cuckoo that parasitizes them is the type that kills the chicks that share the nest with it. Indeed, in the course of evolution, various species have developed anti-cuckoo traits, which mainly include the ability to recognize foreign eggs and chicks, and the tendency to throw them out. Birds that did this more effectively lost fewer chicks and did not waste resources on raising foreign chicks, and so these traits spread rapidly through the population.
In response, the cuckoos evolved a countermeasure: eggs and chicks that closely resembled those of the host. Thus began a process of co-evolution, or co-evolution, which is also a kind of arms race – a very common phenomenon in parasites and their hosts. As the hosts become better at identifying and eliminating foreign agents, the cuckoos’ eggs and chicks become more similar to those of the hosts.
The arms race sometimes leads to quite creative solutions. For example, the African drongo has Very different The color of the eggs and the pattern of the spots or spots on them. The eggs laid by a particular female are all similar to each other, but the eggs in a neighboring nest may be very different. The cuckoos that parasitize the drongo have developed similar eggs, with a variety of colors and patterns, but because the eggs in each nest look different, chances are that the egg that the cuckoo lays will look different from the other eggs in the same nest, and the parents will recognize and discard it.
A surrogate bird throws a cuckoo chick out of the nest
Specialized cuckoo
In quite a few cases, cuckoos of the same species lay eggs in the nests of several host species. This poses a challenge for the parasitism: the different host species have different eggs and chicks. How will they adapt their eggs and chicks to several species at the same time? One solution is specialization: a certain group of cuckoos will lay their eggs in the nests of host A, and over time develop eggs and chicks similar to its own; while another group of the same species will prefer host B, and its eggs and chicks will adopt the appearance of its eggs and chicks.
Such specialization can lead to the separation of different groups of cuckoos, thus creating two species from a single species. Bifurcation often occurs when a physical barrier, such as a mountain or lake, separates two populations of the same species, preventing them from interbreeding – but under certain conditions, as in the case of cuckoos, this May also happen without separation, with the two new species sharing the same habitat.
In a new study, Australian researchers examined the consequences of coevolution between cuckoos and their hosts, and in particular the creation of new species. Using statistical models, they showed that lineages of "violent" cuckoos, whose chicks throw the other eggs out of the nest, create new species at a faster rate than lineages of "tolerant" cuckoos, whose chicks grow up with their stepbrothers.
This is because the violent cuckoos exact a higher price from their surrogates. Failure to eliminate the cuckoo will result in the death of their chicks, so there is strong mucus pressure on them to develop the ability to recognize cuckoo eggs and chicks. This in turn leads to mucus pressure on the violent cuckoos: only those whose eggs and chicks resemble those of the host will be able to pass on their genes to future generations. And when cuckoos have two possible surrogates, this pressure pushes them to specialize in one of them, thus separating into two species.

On the way to parting
To show how this process occurs, the researchers focused on the pygmy cuckoo (Chrysococcyx minutillus), which lives in Australia and Papua New Guinea. The pygmy cuckoos of northeastern Australia lay their eggs in the nests of two different species, both of the genus Cuckoos (GerygoneThe researchers showed that there are two subspecies of cuckoo in this area, one of which regularly parasitizes the nests of the common cuckoo (Gerygone palpebrosa), and the other prefers the nests of large-billed ibis (Gerygone magnirostris). The chicks of the first subspecies have light skin and the base of their beaks is yellow, just like the chicks of the fairy siskin which they mimic. The chicks of the second subspecies, like the chicks of the great-billed siskin, have dark skin and the base of their beaks is white.
The researchers tested the DNA of cuckoo chicks and eggs, including eggs preserved in museums and collections along with the host's eggs, so they knew which species of cuckoo it belonged to. They found that there are genetic differences between the two subspecies – they appear to be on their way to splitting apart, creating two species where there is currently one.
One of the causes of biodiversity
Cuckoos usually lay their eggs in the nests of the same host species in whose nest they were raised, but before they do so, the females need to find a mate. For the separation of the two populations to occur, and two new species to be created, male cuckoos raised in the nests of the Lesser Spotted Cuckoo must mate only, or at least primarily, with females raised in the same nests – and so do the females and males raised in the nests of the Greater Spotted Cuckoo. How will they recognize each other? The researchers showed that even in the adult cuckoos, there was a difference in appearance between the two subspecies, even if it was much smaller than the difference between the chicks.
The researchers speculate that behavioral differences may also contribute to this separation. They examined the calls of cuckoos raised by different hosts, and found subtle differences between them. Cuckoos also tend to hang out in the same area as their hosts. Although both species of sibirians live in northeastern Australia, the large-billed sibirian prefers riverbanks, and the fairy sibirian lives on the edges of forests. If the cuckoos also stick to these areas, they will find mostly mates of the same subspecies there.
More of the topic in Hayadan: (Beresheet is the Hebrew name for the book of Genesis)