Case Officer: April J Burt
Suspect: Tropic bird 42
Date of alleged crime: 4th March 2013
Evidence: This 2 day old chick was found dead in the nest, the parent bird’s beak was full of down. Chick looked as if it had been pecked. Chick had been seen 2 hours previously alive and well.
Motive: The phenomenon known as infanticide is not rare in birds, it is perhaps easiest to understand when it involves killing the offspring of others. This type of infanticide is well documented and is related in most cases to the scarcity of food (Ashbrook et al, 2010). A parent may also kill its own young and there are several motives for this filial infanticide. Firstly a parent may kill a chick if this act leads to a higher survival of brood mates (Hardy, 1979); in terms of infanticide this seems reasonable but will not work in this case because Tropicbird chicks do not have brood mates. Secondly a parent may kill a chick to enhance its chance of future reproductive success via filial cannibalism; this has been documented in several bird species (Gilbert et al, 2005; Picman & Bellas, 1987; Rodenburg et al, 2008). Again this is not the case for our Tropicbird chick; it had not been eaten. A third more recently discovered motive exists, that of secondary sex-ratio adjustments via sex specific infanticide. Choosing to kill your young based on its sex, this sounds familiar – humans have been doing it for centuries! A study by Heinsohn et al (2011) on the Eclectus parrot discovered that this type of infanticide could also be seen in birds. But does this sound possible for our Tropicbird? It is unlikely for many reasons. A forth motive for infanticide that again is known in both birds and humans is if the paternity is either different or questionable; a study on Pheasant-tailed Jacanas by Chen et al (2008) witnessed this type of infanticide, however as the breeding biology of tropic birds differs exceedingly, this also will not answer as a motive. Finally there is another motive, which may well suit our case; the parent discerns a defect in the chick and will consequently eliminate the chick thus denying investment of time and energy to what is genetically inferior. This is known as the progeny choice hypothesis; parents are seen as creating an enlarged array of offspring from which a genetically superior subset is chosen for full investment (Forbes & Mock, 1997). This is not to be overlooked but without a thorough post mortem of the chick to determine any defects it may have had, we cannot be certain. There is one other explanation all of my own; that this unlucky chick was pecked by accident. It is possible that the parent was aiming for a Skink that had come into the nest or maybe the parent had bad eyesight and mistook the chick for an intruder.
Verdict: It cannot be proved beyond a reasonable doubt that Tropicbird 42 is guilty of infanticide and therefore until further evidence is collected the verdict stands as not guilty.
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Chen, T. C., Lin, Y. S., Deng, P. L., & Ding, T. S. (2008). Male pheasant‐tailed jacanas commit infanticides to avoid cuckoldry when paternity of eggs is doubtful. Journal of Natural History, 42(47-48), 2991-3000.
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Gilbert, W. M., Nolan, P. M., Stoehr, A. M., & Hill, G. E. (2005). Filial cannibalism at a house finch nest. The Wilson Bulletin, 117(4), 413-415.
Heinsohn, R., Langmore, N. E., Cockburn, A., & Kokko, H. (2011). Adaptive secondary sex ratio adjustments via sex-specific infanticide in a bird. Current Biology.
Picman, J., & Belles-Isles, J. C. (1987). Intraspecific egg destruction in marsh wrens: a study of mechanisms preventing filial ovicide. Animal behaviour, 35(1), 236-246.
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