The Parasite-Host Interrelation is interesting from an evolutionary point of view. According to Darwinism, every species will do the utmost to ensure the future generation. In the case of the parasite, the host cares for a chick not belonging to the bird usually at the expense of his own chick.

The Cuckoo is one of five known bird families known for parasitism.Parasite - lay is defined as laying eggs in the nest of another bird (a Host) that cares for the hatching chick
There are two kinds of Cuckoos nesting in Israel. The European Cuckoo (Cuculus canorus) is known as the common) Cuckoo; its chicks throw the host bird eggs or her chicks from the host’s nest. The host for the European Cuckoo in Israel is the Long-billed Pipit (Anthussimilis) and the Scrub Warbler (Scotocerca inquieta). The European Cuckoo's eggs are so similar to the host's eggs that it is hard to distinguish between them.

On the other hand, belonging to the same family is the Great Spotted Cuckoo (Clamator glandarius). Her chicks do not throw the host’s chicks from their nest but cause their death by competing for food, though not continuously ). In Israel, the host for the Great Spotted Cuckoo is the Hooded Crow (Corvus corone) and the Jackdaw (Corvus monedula)(Moti Chaster-Tel Aviv University). In Europe the host is mainly the Magpie (Pica pica).The Spotted Cuckoo is found in almost the entire of Israel from the northern valleys to the Negev. Recently, there has been an increase in the number of Hooded Crows and they have come closer to populated areas. There has been an increase in the number of Spotted Cuckoos who remain in Israel to lay their eggs. They come from South West Africa at the beginning of February and leave toward the end of July after their last hatching. In 2006 they left later, toward the end of August. The Great-spotted Cuckoo is recognized by their loud calls in open areas and in forests.

From an evolutionary point of view there are a number of reasons for the phenomenon of parasitism.

• A lack of nesting area, which leads to sharing care of chicks and subsequently to parasitism.

• There are cases of an adult bird's food being harmful to its own chickswhich encourages the feeding on another's bird food, until the chick is old enough to eat the same food as the adult bird.

• The spreading of eggs in several nests so as” not to put all the eggs in one basket”. Parasitism also allows the bird to lay more eggs in a season while not having to care for the eggs. The Spotted Cuckoo can lay up to 30 eggs in one season dispersed in many nests, while the Hooded Crow lays 4-5 eggs per season.

The Cuckoo usually lays 1-2 eggs in each nest but there are exceptional instances of laying more than two eggs in one nest. In 2003 I found one nest with 5 eggs of a Spotted Cuckoo and three eggs of a Crow. This nest produced three Cuckoos (not clear if the eggs were from one or more female Cuckoos).

There are many adaptations of the parasite t that improve its position in the parasite-host relationship. Usually, at the egg stage, the parasite's egg resembles that of the host in size, color, shape, and spots. In Israel, this is so for the European Cuckoo and her host, but not for the Great Spotted Cuckoo and the Hooded Crow. The eggs of the Hooded Crow are not similar to each other in the same region. The eggs can differ of one another even when layed by the same female Crow.

The differences are in the egg size and egg shape. Also, the last egg of the clutch is smaller than that of previous eggs. The eggs of the Great Spotted Cuckoo are smaller and more purplish than those of the Hooded Crow. The eggs of all Spotted Cuckoos in Israel are very similar.

In the chick stage, the Spotted Cuckoo throws chicks and eggs of the Crow from the nest, imitates the call of the Crow chick and has a similarly shaped pharynx.

Advantages of the Spotted Cuckoo chick versus the Crow chick are many: faster growth, the eyes open earlier (at 5 days of age as against 7 days for the Crow), the ability of the Cuckoo to fly at less than 3 weeks of age against at more than a month for the Crow.

The incubation period of the Cuckoo egg is shorter: 13 days as against 19 days for the Crow, insuring earlier hatching and thus securing more food for the critical early development period of the chick. This is assuming that when the Crow starts its brooding there already is a Cuckoo egg in the nest. The Cuckoo usually lays its egg in the nest after the Crow has layed its first egg and in any event before the Crow starts the brooding period, which will usually guarantee the Cuckoo chick hatching five to six days before the Crow chick.

Despite all this, there are many cases in which a pair of Crows succeeds in raising its own chicks alongside those of the Cuckoo until they all leave the nest. I observed a pair of Crows who managed to raise four of its own chicks in addition to two belonging to the Cuckoo. This, however, is rare.

The Cuckoo’s egg shell is thicker than that of the Crow with greater resistance to cracking. The Cuckoo lays her egg in just a few seconds thus avoiding observation by the Crow. In a tactic sometimes used by Cuckoos, the male Cuckoo flies close to the nest of a Crow. When the Crow leaves the nest to chase the male Cuckoo away, the female quickly lays her egg at edge of the nest. As the egg rolls down into the nest it often breaks one of the Crow’s eggs reducing the number of Crow’s eggs that can produce a chick. Sometimes as a Cuckoo lays an egg she also removes one of the Crow’s eggs, so that the Crow finds the number of eggs unchanged.

The Cuckoo learns the area well so as to lay her egg at the proper time. During this period, the Cuckoos live in pairs or in small groups numbering up to eight (four pairs). The female does not necessarily copulate with same male during the mating season: a female has been observed to mate with two different males in less than an hour.

During the mating ritual, both the male and female will release loud calls. At a given stage, the male will land to find a worm for the female. The female, waiting on a branch, will make weak calls as if calling for food. The male, after finding a worm, will fly back and offer it to the female while mating.
Through observations I have made during the last 18 years, I have not seen one case in which a Hooded Crow rejected the egg of a Spotted Cuckoo.

It is possible to imagine the host-parasite relation as an “arms race”, whereas every “weapon” employed by A brings about a counter defense by B.

Mimicry is a continuous evolutionary process by the parasite improves mimicry to avoid identification and rejection by of the foreign egg. On the other hand, rejection of the parasite’s eggs is advantageous to the host. The constant identity-rejection x mimicry brings about constant selection for better detection by the host on the one hand, versus better mimicry by the parasite.

There is no explanation why the Hooded Crow continues to care for the Spotted Cuckoo chick even after the chick has grown and left the nest. Many times this is at the expense of the Crow’s own chicks who are still in the nest due to slower development. At this stage there can be no mistaken identitification, and no danger of nest destruction by the Cuckoo. Nevertheless, the Crow will continue to care for the Cuckoo chick for a few weeks after its leaving the nest and even prevent adult Spotted Cuckoos from coming near to the growing chick.

An unanswered question is how does the genetic stamp of the developing causes the growing Cuckoo at one point to know its identity? At that point it leaves its adoptive parents, joins the adult Cuckoos, changes its eating habits, and migrates back to the country of its biological parents.

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Amit Kubade		04-Aug-2012 03:50
Interesting! Thank you.

Avi L.		13-Jun-2012 17:10
I have been wondering about a couple of birds being fed by crows even though they were apparently fully grown. I live in a sparsely wooded part of Acco, and after a few times watching them "dance" around I figured it must be cuckoos exploiting the crow's tricked instinct. I googled a few words and sure enough I found your article. Thank you for confirming what was not totally clear

Guest		07-May-2012 23:59
nick davies came to my uni a few months ago and gave a talk on parasitic cuckoos, they are very clever birds.

Guest		04-May-2012 13:28
thank you for a wondeful article. A, from pardess-Hanna, Israel

goutam		18-Jul-2011 23:40
why cukoo cares crow's eggs

tanhweeboon		23-Jul-2010 07:29
Nice article.

Garry Herzog		29-Apr-2009 18:05
Thank you for this site and your wonderful work. The article was thoughtful, thorough, and fascinating. Great job. The photos speak for themselves, they're wonderful. Congratulations on combining great images with a clear understanding of the biology at work. Best wishes, Garry Herzog

nissim krispil		04-Apr-2009 09:51
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pippa		22-Sep-2008 16:50
i like this but the typing was a bit careless it is cool and helpful and it help with my a levels and had helped me alot. thank you pippa

saswati		26-Jul-2008 07:13
interesting!!!... good collection of information

holly		27-Nov-2007 19:49
wow how amazing and interesting is this piece of text i like it alot coz it tels u alot about cuckoos and if you type about another bird iam sure that a text as interesting as this will come up agaian hopefully if i neesd to know more about birds than i might come opn this again so i have had a goodtime and a longtime reading this text and iam sure lots more of people will read this text. thankyou

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