Putting all of my eggs in your basket

You won’t find brood parasites like the cuckoo wasting time building nests or caring for their young, because unlike many of us that pay extortionate childcare costs, the cuckoo gets their childminding done for free.

Female brood parasites are expertly specialised in what they do. They’re extremely secretive and will go to great lengths to avoid detection by their hosts; hosts carry out regular checks of the environment to monitor parasitisation risk, and the parasites cannot risk hosts becoming more vigilant. They may travel under cover of night to case out their unsuspecting hosts, and have such specialised spatial memory that they are able to expertly recall their locations. They produce eggs which greatly resemble that of their hosts, and synchronise their laying behaviours in order to increase the chance of egg acceptance.

One recent study has found that the common cuckoo may exploit the defensive behaviours of the reed warbler following parasitisation, with females producing ‘chuckles’ that strikingly resemble Accipiter hawks – predators of the reed warbler. On the surface this may seem counterintuitive, why would the cuckoo go to such lengths to avoid detection to then draw the attention of the bird that it has just parasitised? It is thought that when the reed warbler hears the chuckles of the female cuckoo, it starts to prioritises self protection over clutch protection. This trickery distracts the warbler, making them less likely to notice that they have been parasitised and twice as likely to accept a foreign egg rather than ejecting it from the nest. Whilst the call is an imperfect mimic, it is similar enough to ensure that the reed warbler doesn’t take any chances, and as a result, successful parasitisation rockets from 40% to 80%.

However, some hosts such as the yellow warbler have escalated their conflicts to new heights. Upon discovering a parasitic egg of the parasitic brown-headed cowbird, they will entomb the egg within a new layer of nest, condemning the cowbird to death. Unfortunately, the yellow warbler must sacrifice their own young to do so, burying their own young alongside the foreigner. Oftentimes the cowbird will then return and lay again on the new nest layer, and so the cycle continues, resulting in tall egg graveyards up to five stories in height. I know what you’re thinking – why bother? Why not chuck the thieving little thing out of the nest and straight down to hell like the reed warbler? Well, the yellow warbler has such a small beak that they can’t physically remove the foreign egg, and research shows that brood parasites like the brown-headed cowbird have evolved to produce extremely thick egg shells which prevent the yellow warbler from destroying them. Many yellow warblers cut their losses, vacate their nests and spend a few days building another before laying a new brood, but some are persistent and will build these graveyards until the cows come home (pun intended).


Guigueno, M. and Sealy, S. (2010). Clutch abandonment by parasitised yellow warblers: egg burial or nest desertion? The Condor, 112(2).

Scardamaglia, R., Fiorini, V., Kacelnik, A. and Reboreda, J. (2016). Planning host exploitation through prospecting visits by parasitic cowbirds. Behavioral Ecology and Sociobiology, 71(1).

York, J. and Davies, N. (2017). Female cuckoo calls misdirect host defences towards the wrong enemy. Nature Ecology & Evolution.

Photo credit: Nature Picture Library/Alamy

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