Introduced cane toads Bufo marinus are active nest predators and competitors of rainbow bee-eaters Merops ornatus: observational and experimental evidence

Abstract

During a three-year study, introduced cane toads ( Bufo marinus) ruined one-third of nest attempts of ground-nesting rainbow bee-eaters ( Merops ornatus) by usurping their nest burrows and preying upon their eggs and young nestlings. Birds that had lost their nest to cane toads were less likely to be recaptured in subsequent years. Cane toads are having a significant negative effect at the population level: at present, rainbow bee-eaters produce 0.8 fledglings per nest. However, in the absence of cane toads each nest would produce 1.2 fledglings. Rainbow bee-eaters had little defence against the cane toads. The diurnal birds were not able to mob the nocturnal toads, nor were they able to eject cane toads from the nest. Deep nests and nests built on steep slopes were still preyed upon by the toads. Cane toads that were removed from the nest had the ability and motivation to return to the nest when displaced up to 1200 m away, suggesting that the nest is a valuable resource to toads. Cane toads were significantly more likely to: (a) prey upon nests containing hatchlings rather than eggs; (b) enter and occupy artificial nest tunnels (1.2 m deep) containing a small food item (20 g of raw chicken) rather than artificial nests that were empty. These results indicate that cane toads actively prey upon nests using olfactory cues. The predatory and homing abilities of cane toads are reviewed, as well as their need for diurnal shelter sites. The possibility that cane toads are having a negative impact upon other native, ground-dwelling vertebrate fauna via their role as opportunistic predators requires urgent investigation.