Egg-Eating by Galápagos Mockingbirds

Abstract

The Galapagos mockingbirds (Mimidae) belong to the endemic genus Nesomimus, and it is generally accepted that there are four distinct species: N. trifasciatus (on Champion and Gardner-by-Floreana), N. melanotis (on San Crist6bal [= Chatham]), N. macdonaldi (on Hood [= Espafiola]), and N. parvulus -seven subspecies on different islands not inhabited by any of the other species (Swarth, Occas. Papers Calif. Acad. Sci. 18:1, 1931). It is therefore of some interest that Hatch (Condor 67:354, 1965) reports that only one species, N. macdonaldi, feeds on seabird eggs. N. trifasciatus and N. parvulus he found did not, and he does not mention N. melanotis, and says that is an interesting example of a behavior pattern differing strikingly between closely related species. On Hood Island N. macdonaldi, which has by far the largest bill of any of the species, is frequently seen attacking the unincubated eggs of the Galapagos Albatross (Diomedea irrorata), Blue-footed Booby (Sula nebouxii), Masked Booby (S. dactylatra), and Swallow-tailed Gull (Creagrus furcatus). L6veque (Alauda 32:5, 1964) reports these mockingbirds as eating the eggs of the American Oystercatcher (Haematopus palliatus), and on several instances I have seen them take the eggs of the Galapagos Dove (Zenaida galapagoensis). However, in the normal course of events it is unlikely that many eggs of any of these species are available to the mockingbirds except when extreme adverse conditions, such as food shortage or human interference, drive the birds from the nests. Also it must be only infrequently that the birds manage to crack an intact egg of the larger seabirds, although they would soon devour a hatching or cracked egg. Alan Root tells me that in 1966 the mockingbirds were pecking at bleeding wounds on the feet of the albatrosses. The mockingbird on Tower Island (N. parvulus bauri) was noted by Hatch (op. cit.) to be indifferent to the eggs of the abundant seabirds and even to freshly broken eggs. However, during field work on Tower Island in 1966-67, it was seen that this species would eagerly eat eggs of the Red-footed Booby (S. sula) and frigatebirds (Fregata minor) broken by other frigates or Lava Gulls (Larus fuliginosus) and would even peck furiously at unattended frigratebird eggs and try to eject them from the nests when the adults were scared away by human intruders in the colony (fig. 1). There is on Tower Island an extremely large colony of the Wedge-rumped Storm-Petrel (Oceanodroma tethys) which lays eggs in the large but well-protected open spaces under the lava flows. Large numbers of broken eggs are found near the nest sites, and Nelson (Ibis 108:430, 1966) thought that these had not been eaten by mockingbirds (he infers by the use of the abbreviation Nesomimus spp. that there is more than one species present, but there is no evidence for this) but were more likely a result of interspecific competition for nest sites. Close inspection of the colony, however, has now shown that the majority of these eggs have been ejected by competition (but intraspecific) and then eaten by mockingbirds. Great care had to be taken while examining petrel nests as the ?~