Living next to the Lane Cove national park has always been a luxury. I wake up every morning to a beautiful view of lush bushland and listen to the various melodic sounds of native birds. However, a few months ago my serene backyard had become the home to a very unwelcomed pair of birds. Loud repetitive screeching calls were continually made throughout the day, driving me and my neighbours a little crazy. In the midst of my annoyance, one day I decided to investigate. After getting a good look it was not only obvious these birds were of two different species, but strangely the smaller bird, a crow, was feeding the much larger younger bird. It then became apparent the continuous screeching was a result of the crow’s inability to satisfy the enormous feeding requirements of this giant youngster.
After a little research I discovered this young bird was indeed a type of cuckoo, belonging to a group of birds known as brood parasites. Brood parasites are birds that trick other birds into raising their young. Unfortunately, this crow in my backyard appeared to be a prime example and victim of such trickery.
Coincidentally, I attended a seminar by Dr Ros Gloag of Sydney University, who discussed her work on exactly this topic: brood parasitism in birds.
At the beginning of the seminar, the two most forefront questions on my mind where;
- How do parasite birds trick others into bearing the costs of raising their offspring?
- Why don’t victims (hosts) simply just reject foreign eggs/chicks?
Gloag explained that almost always, the answers involve mimicry.
As a means of defence, host birds will often attack and mob parasitic birds in an effort to prevent them from laying foreign eggs in their nests . In response, some parasitic birds have evolved to prey only on host species that visually mistake them for predatory hawks [2, 8]. Hosts are therefore too scared to attack hawk mimics, allowing foreign eggs to be laid in their nests .
A more common strategy involves parasitic birds producing eggs that look almost identical to host eggs [3,7]. Host birds that may otherwise remove or damage foreign eggs, instead mistake them for their own leaving them in the nest (See figure 1).
Once these imposter birds hatch you would expect adult birds to distinguish between their own and foreign chicks, especially if they are strikingly different in size or colour. It appears this is often not the case, instead open mouths and hungry begging calls are the signals parents attend to the most . Research suggests some parasitic species evolve chick begging calls which sound very similar to host chick begging calls [1,3,4]. For example, De Marisco and colleagues (2012) revealed screaming cowbird chicks emitted calls that were structurally similar to the calls of baywing host chicks. Suggesting host birds are tricked by copied calls and respond by feeding these imposters [1,2].
Shiny cowbirds do not use mimicry
The significance of Gloags research lies in her discovery of a successful brood parasitic bird that does not use mimicry at all [3,4]. This species, the shiny cowbird of South America, does not display predator, egg or vocal mimicry [3,4]. In fact it exhibits no mimicry of any type. This puzzled Gloag, who was lead to believe, as suggested by research, that trickery and mimicry go hand in hand when it comes to brood parasitic birds .
Gloag subsequently performed several experiments using video and sound recordings. Gloags (2012, 2013) research revealed shiny cow birds are instead successful for three different reasons:
- Birds are successful egg layers even whilst being severely mobbed by host birds (see video 1) .
- Host birds do not remove eggs that look different . Unfortunately for hosts, cowbird parasitism is extremely common. As a consequence, after cowbirds lay their eggs they attempt to destroy any previously laid eggs, from both other cowbirds and host birds. Gloag suggested hosts intentionally leave foreign eggs in the nest so the next cowbird attempting to lay, by chance, may destroy only other cowbird eggs and leave host eggs (see video 2) [3,5,7].
- Even though begging calls of chicks sound different, they are emitted at such a fast rate they sound similar to whole group of individual host chicks begging for food. Gloag suggests this may elicit an extreme response in parental feeding, as intense calling simulates the sounds of many of the hosts own hungry chicks begging for food (figure 2) .
Ultimately, the message here is; even though most brood parasitic birds use mimicry as trickery, like with anything….there are always exceptions to the rule.
 Davies, N.B., Kilner, R.M., Noble, D.G., (1998). Nestling cuckoos, Cuculus canorus, exploit hosts with begging calls that mimic a brood. Proceedings of the Royal Society B: Biological Sciences 265, 673-678.
 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, 20130076.
 Gloag, R. (2014). Trickery without mimicry in brood parasitic birds. School of Biological Science Seminar Series. University of Sydney, 4/4/2014
 Gloag, R., & Kacelnik, A. (2013). Host manipulation via begging call structure in the brood-parasitic shiny cowbird. Animal Behaviour, 86(1), 101-109.
 Gloag, R., Fiorini, V. D., Reboreda, J. C., & Kacelnik, A. (2012). Brood parasite eggs enhance egg survivorship in a multiply parasitized host. Proceedings of the Royal Society B: Biological Sciences, 279(1734), 1831-1839.
 Gloag, R., Fiorini, V. D., Reboreda, J. C., & Kacelnik, A. (2013). The wages of violence: mobbing by mockingbirds as a frontline defence against brood-parasitic cowbirds. Animal Behaviour, 86(5), 1023-1029.
 Sato, N. J., Mikamf, O. K., & Ueda, K. (2010). The egg dilution effect hypothesis: a condition under which parasitic nestling ejection behaviour will evolve. Ornithological Science, 9(2), 115-121.
 Welbergen, J.A., Davies, N.B., (2011). A parasite in wolf’s clothing: hawk mimicry reduces mobbing of cuckoos by hosts. Behavioral Ecology 22, 574-579.
Dr Ros Gloag website:
Video 1. Cowbird laying an egg in a hosts nest whilst under attack.
Video 2. Cowbird laying egg is hosts nest and destroying previously laid eggs
BBC video Natural World, narrated by Sir David Attenbourgh – Cuckoo
Want to learn more about the shiny cowbird?
Want to learn more about the common cuckoo?