The cuckoo has a terrible reputation as a parent. However, only 40% of all cuckoo species are guilty of this lack of responsibility. Their strategy of non-attachment parenting, if you will, is termed brood parasitism in the animal world and plenty of things do it.
(Warning: some of these are right out of a horror movie)
Caterpillars do it.
Phengaris rebeli (previously, Maculinea rebeli) or the Mountain Alcon Blue butterfly mimicking a queen ant’s stridulations (or calls) to receive care (in the form of royal jelly) from worker ants of the genus Myrmica. Once in the nest it feeds on some ant eggs but everyone is too blinded by its elevated status of queen to do more than bring it more royal jelly for dessert.
Honeyguides do it.
Insects do it.
Even some fish do it.
Cichlid mouthbreeding fish from Lake Tanganyika are parasitised by the cuckoo catfish (Synodontis multipunctata), and the young catfish hatches first and feeds on the cichlid eggs.
Battle of the Birds
Among birds, when a host actually detects an unknown and unwanted egg in its nest, it evicts the outsider using a method known as grasp-ejection. Simply put, the host can either grab the egg and evict it, or puncture it with their beak to then evict pieces of the shell and egg.
In the constant drama that is evolution, some brood parasites have evolved strategies to counter this eviction strategy – they make their eggs just too big to get a good grip on. Or they beef up the thickness of the shell, so that it cannot be punctured.
Maria Mársico and colleagues have studied the unique relationship between baywings and their parasites, the screaming and shiny cowbirds (two species!), in the El Destino Reserve in Argentina over 10 years. The cowbirds, which are obligate brood parasites, lay eggs that are too large for the bills of the baywing to do them any harm. Yet researchers discovered several eggs outside the nest. Intact!
These baywings were aware of the parasitic egg, and either rejected the egg before laying their clutch, or removed all the eggs (including their own) from the nest and laid a second clutch.
How were the baywings foiling the cowbirds? Would you believe it, they literally kick them out!
Here a screaming cowbird (Molothrus rufuaxillaris) visits the empty nest of a baywing (Agelaiodes badius). The baywing, realising it has been duped, proceeds to kick-eject the egg using its feet.
Above, a baywing kick-ejects an egg along with the rest of its own clutch.
Researchers documented a unique running-on-the-spot mechanism that resulted in high eviction rates for eggs. One can almost see the evolutionary pressure on the baywings to adapt, given that each host finds anywhere from one to 14 parasitic eggs in its nest (the average is three).
Almost 67% of all nests monitored in the study were parasitised during the pre-laying season, when nests are empty. Fortunately for the baywings, they were able to use their feet to eject all but 2% of the nests.
Of those nests that were parasitised after a clutch had been laid by a baywing, 31% were completely rejected and replacement clutches laid in their stead. Kicking eggs is such a good strategy, in fact, that it reduced the rate of parasitism by 75%, bringing it down from roughly 4 parasitic eggs per nest, to just 1 egg per nest.
In the end, the system seems fair to both the adult cowbirds and the baywings, with each expending energy to secure the future of their young, as we all do I suppose. Unfortunately, an awful lot of baby birds of both species are sacrificed along the way.
De Marsico M.C., Gloag R., Ursino C.A. & Reboreda J.C. (2013). A novel method of rejection of brood parasitic eggs reduces parasitism intensity in a cowbird host, Biology Letters, 9 (3) 20130076-20130076. DOI: 10.1098/rsbl.2013.0076
Spottiswoode C.N. & Koorevaar J. (2012). A stab in the dark: chick killing by brood parasitic honeyguides, Biology Letters, 8 (2) 241-244. DOI: 10.1098/rsbl.2011.0739
More on brood parasitism
A good run through of avian mating systems.
Several videos by Spottiswoode and Koorevaar (2011) on brood parasitic honeyguides.