78 BBCWildlife August 2020
parasite is common, and therefore
some reaction towards the great
spotted cuckoos should be expected.
The fact the crows are, generally,
secondary hosts that suffer relatively low
parasitism rates, and that the costs in terms
of brood loss are not so dramatic, have
been, so far, considered sufficient reasons to
explain why defences have not yet evolved.
However, an intriguing phenomenon, which
may provide an alternative explanation for
the peaceful convivence between host and
parasite, came to light a few years ago...Accidental hero
Examining the breeding performance
of crow nests, monitored over 16 years,
revealed that parasitised and non-parasitised
nests produced, on average, the same
number of crow chicks every year.
On the one hand, nests with cuckoos
present were more likely than non-parasitised
ones to fledge at least one crow chick, while
non-parasitised nests suffered complete
brood loss (usually attributed to depredation)
at higher rates. On the other hand, however,
nests that were successful produced
significantly fewer crow chicks when they
were parasitised, because, as expected, the
cuckoo chick usually outcompeted some crow
nest-mates for food.
Why on earth parasitised nests were more
likely to be successful was a mystery that
needed to be solved. A simple explanation
could be that great spotted cuckoos were
able to target the nests of high-quality crow
parents that were more skilled in defendingofthecuckoochickitselfwascausingthe
highersuccessofnests...buthow?
Asanyonewho’sworkedwithgreat
spottedcuckooswillknowalltoowell,
youngchicksexpela conspicuousquantity
ofa dark,fetid,liquidsecretionfromthe
cloacal(a commoncavityforfaeces,urine
andreproduction),asa self-defencestrategy,
assoonastheyaremanipulated.Could
thissubstancerepelpredators,savingcrow
chicksfromattackintheprocess?
Wecollectedsamplesofthesecretionand
analysedtheircomposition.Theyappeared
tobea causticmixtureofacid,indoles,
phenolsandsulphurcompoundsalready
knowntorepelmammalsandbirds.We
thenperformedanexperimentwherewe
presentedbait(sometreatedwiththecuckoo
secretion,othersuntreated)tomammalian
carnivoresandraptors– theywereclearly
repelledbythesubstance,stronglyavoiding
thefoodthatwasspreadwithit,and
thereforesupportingthetheory.
Altogether,thesefindingsrevealan
incrediblescenariowherethedefinitiontheir nest against predators, so their young
would be more likely to survive.
Unconvinced by this, we performed
an authorised translocation experiment
where we swapped cuckoo hatchlings from
parasitised nests into non-parasitised ones.
We then compared the reproductive success
of these nests with a set of unmanipulated
ones (half parasitised, half non-parasitised).
We found that the nests where we had
introduced cuckoo chicks were less likely to
fail than the ones from which we removed
the chicks – mirroring the patterns observed
in the unmanipulated control nests. These
results demonstrated that the presenceTop: a magpie
returns to the nest
and some hungry
youngsters. Above:
juvenile great
spotted cuckoos
have chestnut
primary feathers.In years of very low
predator density, great
spotted cuckoos are
true parasites for crows.
Great spotted cuckoos
shirk their parenting
duties by tricking
other species into
raising their young.All images: Luis Miguel Ruiz Gordón; flying: Fabrizio Moglia/Getty