Nesting distributions of Galápagos boobies (Aves: Sulidae): an apparent case of amensalism

Abstract

Blue-footed boobies (Sula nebouxii) in the Galápagos Islands nest at coastal sites such as cliff edges if Nazca boobies (S. granti) are absent. However, if sympatric with nesting Nazca boobies, they nest nearby, but farther inland, in areas with little topographical relief. Nazca boobies nest at the coastal sites whether blue-footed boobies are present or not. The segregated nesting pattern of these two species offers a model system to investigate factors influencing community structure. We tested a non-interactive hypothesis, in which different fundamental niches generate the non-overlapping distributions, and an interactive hypothesis, in which the two fundamental niches overlap and an interaction between the two species causes the segregation. Data on three factors considered as likely parameters differentiating fundamental niches (nest microclimate, nature of the nesting substrate, and ease of taking flight from nest sites) failed to support the non-interactive hypothesis. These results suggest that the two species have indistinguishable fundamental niches with respect to these parameters, but different realized niches. Researchers studying resource partitioning by ecologically similar species often only consider competition (a "-/-" interaction) to explain situations like this, ignoring the more parsimonious amensal (0/-) possibility. Nesting segregation in this situation is apparently caused by attacks of non-breeding adult Nazca boobies on blue-footed booby nestlings, injuring nestlings and ultimately preventing them from fledging. The interaction does not result in any discernible costs or benefits (i.e., effects on fecundity or survival) for the adult Nazca boobies, so it is best described as an amensal interaction. This interaction provides a sufficient explanation of the observed nesting segregation, and precludes present competition for nesting space.

Chronic lack of breeding by Galápagos Blue-footed Boobies and associated population decline

“…The formerly large colony at Punta Cevallos (489 nests in 1994; Townsend et al 2002) was not monitored as part of this study, but was checked frequently as part of our ongoing research there; no more than three nests were ever present there during this study.…”

“…These two islands (Fig. 2) differ in presence of their main native predator (Galápagos Hawks Buteo galapagoensis; Anderson 1991, Anderson andHodum 1993) and nesting habitat occupied by Blue-footed Boobies (Townsend et al 2002), yet their respective Blue-footed Booby populations ceased effective breeding simultaneously, suggesting a regional, marinebased cause, such as diet. Along the continental margin, S. n. nebouxii eats primarily schooling, lipid-rich (Schew andRicklefs 1998, Müllers et al 2009) members of two families: Clupeidae (sardines and herrings) and Engraulidae (anchovies; Zavalaga et al 2007, Weimerskirch et al 2009).…”

REPRODUCTION AND IMMUNE HOMEOSTASIS IN A LONG-LIVED SEABIRD, THE NAZCA BOOBY (Sula granti)

“…Other members of the Sulidae also show relatively different body masses but similar structural sizes (Lewis et al 2005, Weimerskirch et al 2006. Wing-loading of females thus exceeds that of males in several booby species (Townsend et al 2002, Weimerskirch et al 2006) and may be related to sex-specific foraging and provisioning behaviors (Guerra and Drummond 1995, Tershy and Croll 2000, Lewis et al 2005, Lormee et al 2005, Weimerskirch et al 2006. Our time-budget data showed that Nazca Booby mothers spent more time at sea than fathers during much of their chicks' growth, which is consistent with earlier results indicating that mothers have a greater provisioning role than fathers: mothers make longer foraging trips during chick rearing and return with larger loads (Anderson and Ricklefs 1992).…”

“…This tradeoff confronted by parents, between food delivery (requiring absence at sea) and protection (requiring presence at the nest), puts a premium on foraging efficiency. Although the higher wingloading of mothers imposes costs during take-off (Townsend et al 2002) and probably during flight, it is associated with a higher efficiency (mass of food per time at sea) of food delivery to the nest (Anderson and Ricklefs 1992). The larger average prey size of mothers (Anderson 1989a) probably contributes to higher efficiency, and access to larger prey may be a consequence of larger body size if larger size permits deeper plungedives (Lewis et al 2005).…”

Mirror-Induced Behavior in the Magpie (Pica pica): Evidence of Self-Recognition

“…Unusual social interest of non-breeding adult Nazca boobies in conspecific and heterospecific nestlings [21,22] provides an opportunity to examine organizing effects of androgens on behavior. These Non-parental Adult Visitors (NAVs) search the breeding colony for unguarded nestlings, join them at the nest, and display parental/courtship behavior, aggression, and sexual behavior in various mixtures [21].…”