Evolutionary ecology of Rainbow Bee-eaters (Merops ornatus)

Abstract

This thesis describes the evolutionary ecology of rainbow bee-eaters (Merops ornatus), a migratory, cooperatively breeding, and colonially nesting bird. The thesis focuses upon (i) their breeding biology, (ii) the effects of introduced species upon nesting, (iii) the ornamental and morphological correlates of reproductive success that have arisen through sexual selection, and (iv) their cooperative and colonial nesting behaviour. The reproductive output from 433 nests was monitored over four breeding seasons (1997/98 to 2000/01) in the open sclerophyll forests and heathlands of Cooloola National Park, southeast Queensland, Australia. Morphological measurements and blood samples were obtained from 414 adult birds and 435 chicks. The sex of each individual was determined using a PCR-based technique. Rainbow bee-eaters nest either in simple pairs, or with the assistance of a single - usually male - helper. Alpha-males are larger than females in all body size and most ornamental traits. All adults possess elongated central tail feathers. The lengths of male tail ornaments are distinctly bimodal, though the categories of tail ornament length do not correlate with age or breeding status, and thus remain enigmatic. Alpha-male testosterone peaks prior to mating, while the low testosterone of helper-males suggests they are hormonally incapable of reproduction. Helper-males advertise their lower status with reduced ornamentation. Bee-eaters nest in self-excavated tunnels, ensuring predation by native predators (lace monitors Varanus varius and yellow-footed antechinus Antechinus jlavipes) is rare, affecting only 10% of nests. The duration of the incubation and nestling period is variable and protracted. Eggs are laid and hatch asynchronously, usually at intervals of 0-3 days, creating an appreciable size hierarchy within the brood, which facilitates the death of 22% of chicks via starvation. The long nesting period of bee-eaters means that the nests are vulnerable to predation by two introduced species: cane toads (Bufo marinus) and dingoes (Canus lupus dingo). Cane toads ruin one-third of all nest attempts by usurping

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the bird's nests and preying upon their eggs and small chicks. The toads can return to nests from which they were evicted after being released up to 1200 m away. Further, they are more likely to find artificial nests containing a small food reward than an empty nest, suggesting the toads use olfactory cues to find food. These data demonstrate that cane toads are active and efficient predators. Dingoes 'farm' bee-eater chicks by preying most often upon nests containing very large offspring in larger colonies of bee-eaters, even though they are aware of the presence of nests containing only eggs. Rainbow bee-eaters possess multiple plumage ornaments. In females, the different ornaments correlate with different aspects of reproductive potential (territory size, nesting date and clutch size), supporting the 'multiple message hypothesis' for multiple ornamentation. Pairs form assortatively according to a number of traits. Females paired to males with long tail ornaments nest earlier in the season, which is likely to improve reproductive success. The tail ornaments are costly aerodynamic handicaps that reduce the body condition of males and inhibit the production of red blood cells required for the high flight demands of nesting. Rainbow bee-eaters are able to ameliorate these costs by shedding their tail ornaments after most females have been fertilised. High quality birds are able to maintain their tail ornaments further into the breeding season. Females incubate for longer durations than alpha-males or helpers and as a result lose mass rapidly while nesting. In contrast, males gain weight in preparation for their return migration. Females lighten their incubation workload in the presence of an auxiliary. C<;>operative trios enjoy significantly higher reproductive success (mean= 2.96 fledglings per nest) than pairs (2.01 fledglings per nest) because (i) helpers increase the overall provisioning rate, and (ii) cooperatively breeding adults are higher quality birds than their pair-breeding counterparts. Colonies of up to 50 nests form wherever large patches of bare ground containing firm sand can be found. Group mobbing by bee-eaters in colonies can thwart the predatory attempts of lace monitors. Therefore, large colonies represent 'safe havens' from the bee-eaters' principal native predator. High quality birds are able to nest in colonies, whereas low quality birds are either despotically excluded or choose to nest in small colonies or solitarily, where

there is a far greater risk of losing their nest to predation. Reproductive success increases with colony size because of (i) the lower predation rates by native species, and (ii) the high proportion of high quality birds. However, reproductive success is low in very large colonies (50 nests) because high feeding rates deplete the local food supply leading to chick starvation. Show more