An experimental test of defenses in a recent host

Theoretical studies predict that hosts of avian brood parasites should
evolve defenses against parasitism in a matter of decades. However,
opportunities to test these predictions are limited because brood
parasites rarely switch to naïve hosts. Here, we capitalize on a recent
host switch by the brood-parasitic Pacific Koel (Eudynamys orientalis) in
eastern Australia, to investigate how quickly the Red Wattlebird
(Anthochaera carunculate), a recent host that has been annexed by the koel
within the last 90 years, can learn to recognize and mob adult cuckoos and
evolve the ability to eject parasite eggs. Pacific Koel nestlings kill all
host young, so there should be strong selection for hosts to evolve
defenses. However, low parasitism rates and high egg recognition costs
might slow the spread of egg ejection in our study populations, while
adult parasite recognition should be able to spread more rapidly, as this
defense has been shown to be a learned trait rather than a genetically
inherited defense. We tested Red Wattlebirds at two sites where parasitism
rate differed. As predicted, we found that the Red Wattlebird showed
little or no ability to eject foreign model eggs at either site, whereas
two historical hosts showed high levels of egg ejection at both sites.
However, Red Wattlebirds responded significantly more aggressively to a
koel mount than to mounts of a harmless control and nest predator at the
site with the higher parasitism rate and gave significantly more alarm
calls overall towards the koel mount. Our results support previous
evidence that recognition and mobbing of a brood parasite are learned
traits and may be especially beneficial to naïve hosts that have not had
enough time or a high enough selection pressure to evolve egg rejection.