Tara R. Robinson

Interspecific associations between circulating antioxidant levels and life-history variation in birds

Antioxidants play an important role in protecting tissues against aging‐associated oxidative damage and are thus prime candidates for relating physiological mechanisms to variation in life histories. We measured total antioxidant capacity, antioxidant response to stress, and levels of uric acid, vitamin E, and four carotenoids in 95 avian species, mostly passerines from Michigan or Panama. We compared antioxidant measures to seven variables related to life histories (clutch size, survival rate, incubation period, nestling period, basal metabolic rate, body mass, and whether the species lived in a tropical or temperate climate). Life‐history‐related traits varied over at least three statistically independent axes. Higher antioxidant levels were generally characteristic of more rapid development, lower survival rate, smaller body size, larger clutch size, and higher mass‐adjusted metabolic rate, but the relationships of particular antioxidants with individual life‐history traits showed considerable complexity. Antioxidant–life history associations differed between tropical and temperate species and varied with respect to taxonomic sampling. Vitamin E showed few relationships with life‐history traits. Overall, our results partly support the hypothesis that antioxidant levels evolve to mirror free radical production. Clearly, however, the complex patterns of physiological diversification observed here result from the interplay of many factors, likely including not just investment in somatic maintenance but also phylogenetic constraint, diet, and other aspects of ecology.

The Status of the Panama Canal Watershed and Its Biodiversity at the Beginning of the 21st Century

P is a small Central American country, but it operates a big canal and the world keeps an eye on developments there. Problems with the canal or ecological disasters in its watershed would attract a lot of attention. As Theodore Roosevelt planned, the Panama Canal is a major shipping corridor, transporting 37 ships a day and providing substantial income to the Panamanian government. Yet as Roosevelt could not have recognized—despite his interest in conservation—the canal sits in the center of one of the world’s most biologically diverse areas (Myers et al. 2000): Southern Central America has more forest bird species than any other region in the world, except Amazonia and the northern and central Andes, each of which is vastly larger than southern Central America (Stotz et al. 1996); and Panama has as many plant species per 10,000 km2 as any region in the world, more than Amazonia or the Malay Peninsula (Barthlott et al. 1996). Roosevelt may have suspected, though, that forests are crucial for protecting the water supply of the Panama Canal and for maintaining the plant and animal communities. Fortunately, the year 2000 still found extensive forests around the canal, protected largely thanks to military and shipping interests, but it also found the watershed adjacent to a large and expanding capital city. Maintaining the ecosystem integrity of the canal will pose a major challenge for conservation in the 21st century. Is urban and economic development compatible with a hydrologically functioning canal and conservation of an extremely diverse flora and fauna?

OBSERVATIONS OF PREDATION EVENTS AT BIRD NESTS IN CENTRAL PANAMA

Abstract We describe 14 nest predation events witnessed in the Republic of Panama during studies of avian nesting success. Eight predations were by birds, including two species of toucan and one forest-falcon. Four predations were by two species of snake, one was by monkeys, and one near-predation was by army ants (Eciton burchelli). In three cases, a fraction of the nest contents were removed by the predator on one visit before it returned later to consume the remaining items. Our observations, although limited to diurnal encounters, indicate that Neotropical birds are susceptible to a diverse array of potential predators. Detailed investigations using cameras are needed to quantify the potential importance of each predator species and to identify the occurrence and importance of nocturnal predation.

Sex Hormones in the Song Wren: Variation with Time of Year, Molt, Gonadotropin Releasing Hormone, and Social Challenge

Abstract Little is known about the role that hormones play in controlling the life cycles and behaviors of resident, tropical-forest passerines. In this study, we document levels of the sex hormones dihydroepiandosterone (DHEA), estradiol, luteinizing hormone (LH), progesterone, and testosterone in male and female Song Wrens (Cyphorhinus phaeocephalus) during the breeding season. In males, we found no significant seasonal changes in sex hormones and no effect of molt or experimental social challenge on hormone levels. The lack of population-level variation in sex hormone levels throughout the reproductive season highlights the asynchronous nature of reproduction in this species. Stimulation of the hypothalamo-pituitary-gonadal axis through injection of gonadotropin-releasing hormone (GnRH) did not affect sex steroid levels in males but did cause a significant increase in LH levels. Low basal testosterone levels in males and the failure of males to respond to GnRH or social challenge with an increase in testosterone indicates either a limited role for systemic testosterone or a system highly sensitive to small changes in hormone titers. Levels of LH, progesterone, and testosterone were similar in females with and without active brood patches, though DHEA levels were higher in females with them. As in males, reproductive hormone levels in females did not change with body or wing molt. However, although wing molt was not significantly related to brood patch status, body molt was negatively associated with the presence of active brood patches in females. This result may suggest an inability of females to engage in body molt and breeding simultaneously.

A Natural Experiment: Heterospecific Cross-fostering of House Wrens (Troglodytes aedon) by Tree Swallows (Tachycineta bicolor)

Abstract We report a natural cross-fostering experiment of two House Wren (Troglodytes aedon) eggs and nestlings by Tree Swallows (Tachycineta bicolor). We document an increase in duration of the House Wren incubation period by 1–2 d, which closely corresponds to the mean incubation period of their hosts. We also documented nestling growth rates of both the host Tree Swallows and the alloparented House Wrens. The host parents apparently fed the House Wren nestlings; however, the wrens exhibited slow growth compared with conspecific nestlings in neighboring nests. The wren nestlings eventually died at 6 and 13 d of age. We hypothesize that the lower incubation temperatures, and later, interspecific sibling competition that the House Wren nestlings were exposed to in the Tree Swallow nest may have prolonged the incubation period and slowed the growth rates of the House Wren nestlings.

Avian Abundances on Yap, Federated States of Micronesia, after Typhoon Sudall

Abstract: Typhoons can add to chronic threats such as incremental habitat loss and pressure from feral predators to jeopardize the unique biodiversity of Pacific Ocean islands. Typhoon Sudall severely damaged Yap, Federated States of Micronesia, in April 2004. To establish an initial assessment of consequences of the typhoon for the avian community on this poorly studied island, we surveyed birds along line transects in mangroves, savannah, and forest on Yap during the last week of August 2004. We found all expected 22 resident species in abundances whose rank order was very similar to results from surveys in the early 1980s. Most-common species were White Tern, Plain White-eye, Micronesian Starling, Micronesian Myzomela, and Yap Monarch. All five species were common in all three major habitats, mangroves, upland forest, and savannah, although terns nested primarily in forests. Least commonly encountered resident species were Common Cicadabird, Micronesian Pigeon, and White-throated Ground-Dove. We found one to six individuals of each of those species. We also encountered 27 species of migratory birds, including the first record for Yap of Greater Scaup. Populations of birds appeared to be about 50% lower across all resident species than estimates from 1983 – 1984, suggesting the possibility that Typhoon Sudall caused widespread mortality. However, other possible explanations for reduced abundances exist, which we review briefly. A long-term monitoring plan is needed to track population dynamics of birds on Yap.