Evolution of nest-building behavior in Agapornis parrots

Auk, The, Apr 1998 by Eberhard, Jessica R

JESSICA R. EBERHARD1

ABSTRACT.-Five species in the African lovebird genus Agapornis are the only parrots, other than Monk Parakeets (Myiopsitta monachus), that construct nests. Four species (A. personata, A. fischeri, A. lilianae, and A. nigrigenis) build domed nests within cavities, and a fifth (A. roseicollis) builds a cup-shaped nest within a cavity. The other members of the genus have nesting behavior that is more typical of other parrots: A. cana and A. taranta nest in cavities that are lined with nesting material, and A. pullaria excavates burrows in arboreal ant or termite nests. To reconstruct the evolution of nest-building behavior in Agaporis, I sequenced a 622-bp portion of the cytochrome-b gene (mtDNA) and used the sequence data to build a phylogenetic tree. The phylogeny shows that the divergence between the nestbuilding species and cana, taranta, and pullaria occurred early in the evolution of the genus. The nest builders form a monophyletic clade, and the small amount of sequence divergence between personata,fischeri, lianae, and nigrigenis indicates that they probably should be considered subspecies of a single species. A reconstruction of the evolution of nest-building behavior on the phylogeny indicates that the construction of a domed nest is derived from the habit of lining the nest, because the nesting material is used to build progressively more complex nest structures. Within Agapornis, nest building is associated with colonial breeding. The construction of a nest within a cavity may allow breeding pairs to modify and use cavities that otherwise might be unsuitable. This would, in turn, give pairs added flexibility in nest-site choice, thereby facilitating colonial breeding. Received 5 May 1997, accepted 11 November 1997.

CAVITY NESTING has evolved multiple times among birds (Collias and Collias 1984). Because cavity nests are safe and well protected, elaborate nest-building behavior is not predicted to evolve in cavity-nesting lineages (Collias and Collias 1984). Parrots present some exceptions to this pattern: tree hollows probably are the primitive nest type of parrots (Eberhard 1997), yet two genera construct complex nests. Monk Parakeets (Myiopsitta monachus), which are native to South America, build domed stick nests, and some members of the African genus Agapornis construct domed nests within cavities (Forshaw 1989). A survey of the nesting behavior of extant parrots suggests two hypotheses for the evolution of nest-building behavior in the Psittaciformes: (1) nest building evolved from the habit of lining the cavity with nest material; and (2) nest building evolved via nest adoption.

The genus Agapornis is an interesting group for studying the evolution of nest-building behavior in parrots because its nine species include examples of most types of nesting behavior observed across the parrot family. Members of the genus Agapornis are small, short-tailed parrots that are native to African forests and savannas. Two species (A. cana and A. taranta) nest in tree holes, and one (A. pullaria) uses cavities excavated in arboreal ant or termite nests (Forshaw 1989). In all three of these species, the nest cavity is lined with material (e.g. seed husks, small pieces of bark, grass or leaves) that the female carries tucked in her body feathers (Forshaw 1989). The nesting habits of a fourth species, A. swinderniana, are poorly known, but it may nest in termitaria as well (Forshaw 1989). Agapornis roseicollis females carry nesting material (strips of bark, leaves, or grass) tucked in their rump feathers and build cup-shaped nests within cavities. Females of the remaining four species (A. fischeri, A. personata, A. lilianae, and A. nigrigenis) carry nesting material (long stalks and strips of bark) in their beaks and build bulky, domed nests within cavities (Forshaw 1989). The nesting material is woven together, and the resulting structure retains its shape even if removed from the cavity (Dilger 1960, Vriends 1978).

The first comprehensive attempt to understand the evolution of the genus Agapornis was made by Moreau (1948), who summarized what was known about the distribution, ecology, morphology, and behavior of lovebirds. Using all of this information, he proposed a classification of the genus. Moreau's (1948) grouping of Agapornis is based on seven morphological and behavioral characters (see Table 1). The designation of some as "primitive" and others as "advanced" results from the selection of Loriculus as the most closely related genus, because members of the latter genus also carry nesting material in their feathers and are sexually dimorphic. A. roseicollis appears to be intermediate with respect to the "primitive" (Group A) and "advanced" (Group B) species, because it shares three characters with each group. Placing it as a phylogenetic intermediate would indicate that nest-construction behavior evolved from the habit of lining the nest, reflecting a gradual elaboration of nest building. However, Moreau (1948:236) suggests that A. roseicollis and the Group B species are different lineages: ". . . it is curious that of all the Group B birds, A. nigrigen is, the member that is geographically nearest to A. roseicollis, should be most unlike it. This suggests that, although A. roseicollis is intermediate in characters, Groun B evolved indevendently."