Pre-migratory life history stages of juvenile Arctic birds: costs, constraints, and trade-offs

Many young birds on the Arctic tundra are confronted by a challenging task: they must molt their feathers and accumulate fat stores for the autumn migration before climatic conditions deteriorate. Our understanding of the costs and constraints associated with these stages is extremely limited. We investigated post-juvenal molt and premigratory fattening in free-ranging juvenile White-crowned Sparrows (Zonotrichia leucophrys gambelii) on the Arctic tundra. We found evidence for trade-offs between premigratory fat accumulation and molt: heavily molting birds had significantly less fat. Birds increased the rate of fat accumulation as the season progressed, but we found no evidence of a similar increase in rate of molt. Using a controlled captive study to isolate the energetic costs of body feather replacement, we found no difference in fat or size-corrected mass of birds actively growing body feathers as compared to controls. Molting birds, however, consumed 17% more food than controls, suggesting a significant cost of body feather growth. Our results provide evidence of significant costs, constraints, and trade-offs associated with post-juvenal molt and premigratory fat accumulation in young Arctic birds.     

Female oviposition-site preference and egg hatching success in the damselfly Calopteryx splendens xanthostoma

By removing males and controlling the rate of water flow past oviposition patches in an experimental area, we showed that female Calopteryx splendens xanthostoma preferred to oviposit in fast flowing water rather than slow flowing water. A series of manipulations revealed the fitness benefits to females for this preference: eggs placed in fast-flowing water developed significantly faster and showed significantly lower mortality than eggs placed in slow-flowing water. A major factor determining this difference was the deposition and growth of encrusting algae which prevented the successful hatching of eggs in slow-flowing water.     

Do novel genotypes drive the success of an invasive bark beetle-fungus complex? Implications for potential reinvasion

Novel genotypes often arise during biological invasions, but their role in invasion success has rarely been elucidated. Here we examined the population genetics and behavior of the fungus, Leptographium procerum, vectored by a highly invasive bark beetle, Dendroctonus valens, to determine whether genetic changes in the fungus contributed to the invasive success of the beetle-fungal complex in China. The fungus was introduced by the beetle from the United States to China, where we identified several novel genotypes using microsatellite markers. These novel genotypes were more pathogenic to Chinese host seedlings than were other genotypes and they also induced the release of higher amounts of 3-carene, the primary host attractant for the beetle vector, from inoculated seedlings. This evidence suggests a possible mechanism, based on the evolution of a novel genotype during the two or three decades since its introduction, for the success of the beetle-fungal complex in its introduced region.     

Endocrine influences on parental care during a short breeding season: testosterone and male parental care in Lapland longspurs (Calcarius lapponicus)

In males of socially monogamous birds, plasma testosterone (T) typically declines to low levels during the parental phase. Studies on multiple-brooded species indicate that high T may be incompatible with high-quality paternal care. The length of the breeding season may affect the costs and benefits of high T and its effect on paternal care. We studied the effect of experimentally elevated T on paternal care in a single-brooded species with a short breeding season, the Lapland longspur (Calcarius lapponicus). We monitored T levels and parental behavior in 16 males with subcutaneous T implants, 14 males with empty implants, and 14 unimplanted males. We videotaped nests when nestlings were 2–3 days old and again at 4–5 days. T males with 2- to 3-day-old young visited nests and fed young less often than control males, and the mates of the T males compensated with elevated visits and feedings. However, when nestlings were 4–5 days old, T males visited their nests at normal rates – though feeding movements remained below normal – and T females visited and fed at normal rates. Nestling mass and nest success were similar in both groups. Overall, high T suppresses paternal care in Lapland longspur males. The partial improvement of paternal care when nestlings are older, despite high T, may be related to the short 6-week breeding season of this arctic species, and the consequently reduced benefits of sexual behavior late in the breeding season. Received: 2 February 1998 / Accepted after revision: 2 November 1998     

Brood parasitism and cooperative nesting in the moorhen,Gallinula chloropus

Summary During a two year study of the moorhen, Gallinula chloropus, a quarter of the nests found had two or more females laying in them. This was the result of two features of the moorhens' breeding biology: (1) Cooperative nesting; two (or more) females were paired to the same male. Both laid in the same nest, the second laid a large clutch synchronously to the first, and both cooperated in parental care. (2) Intraspecific brood parasitism; some females (parasites or dumpers) laid a small number of eggs at random in the nests' of neighbours. Cooperatively nesting females were mother and daughter, while parasite and host were not. Parasitic females laid in their own nest as well, and generally did so after completing a dumping sequence. Parasitic eggs were laid at random during the host's laying and incubation period, and were about a quarter as likely to produce an independent chick as were non-parasitic eggs. Parasitic females would probably have reared more chicks by laying in their own nests straight away, instead of dumping any eggs at all, because there was a strong decline in reproductive success with season. Possible reasons why they did not do so are discussed. Parasitised pairs tended to rear fewer own chicks than non-parasitised pairs. There was no evidence that hosts selectively destroyed parasitic eggs. Four hypotheses that explain why hosts accepted dumped eggs are considered. (1) The host pair were unaware of the dump, (2) the host male had copulated with the dumping female, (3) the dumper was related to one of the host pair, and (4) there was a benefit to the host pair because they gained extra helpers. These hypotheses were not supported by the data, but further tests are needed. As the cost of desertion was greater than the cost of acceptance of parasitic eggs, hosts may have been forced to accept.     

Delayed breeding in the cooperatively breeding Florida scrub-jay (Aphelocoma coerulescens): inhibition or the absence of stimulation?

To determine whether fundamental differences exist in the reproductive physiology of breeder and nonbreeder Florida scrub-jays (Aphelocoma coerulescens), we compared plasma levels of testosterone (T) and luteinizing hormone (LH) in males, and estradiol (E₂) and LH in females. Although male breeders had higher overall T and larger testes, nonbreeders’ T paralleled that of breeders, and their testes were more than an order of magnitude larger than regressed testes. Breeder and nonbreeder males had equivalent baseline LH, and equivalent changes in LH following a gonadotropin-releasing hormone (cGnRH-I) challenge. The T, LH and GnRH challenge data indicate that nonbreeder males have functional hypothalamo-pituitary-gonadal (HPG) axes. We found no hormonal evidence of inbreeding suppression in males: nonbreeders that did not live with their mothers and those that did had similar T. Male nonbreeders that were exposed to E₂-implanted females had higher T than did controls, suggesting that the lack of within-pair stimulation is a key factor in whether an individual delays breeding. Female nonbreeders had E₂ titres equal to or higher than breeders and neither basal LH nor LH following GnRH challenge differed by breeding status. Nonbreeders’ ovarian follicles were smaller than breeders’, but were larger than they would be during the non-breeding season. These data suggest that nonbreeders were primed for breeding and were simply waiting for an opportunity or a required stimulus. Female nonbreeders that lived in a territory with an unrelated male breeder had significantly higher E₂ than those that remained with their fathers. Similarly, nonbreeders that were captured away from their home territories had elevated E₂. However, nonbreeders that lived with their fathers had E₂ that was equivalent to breeding females, suggesting that inbreeding avoidance may not be the primary factor leading to delayed breeding in females. Received: 13 June 1995 /Accepted after revision: 27 April 1996     

Tests of association between the humoral immune response of red-winged blackbirds (<Emphasis Type="Italic">Agelaius phoeniceus</Emphasis>) and male plumage,testosterone, or reproductive success

We explored the potential relationships between male traits, reproductive success, testosterone levels in the breeding season, and humoral immunocompetence in male red-winged blackbirds (Agelaius phoeniceus). Males responded to a single injection of diphtheria-tetanus vaccine by producing antibodies to both antigens. The primary responses to each antigen were positively correlated with each other, but the relationship was clearly non-linear. No male responded strongly to diphtheria without also responding strongly to tetanus, but many males had strong responses to tetanus and weak responses to diphtheria. Response to tetanus was positively associated with hormone levels, whereas the relationship between testosterone and the response to diphtheria was weakly and nonsignificantly negative. We found no convincing relationships between immune responses and male morphological traits (epaulet size, body blackness, and body size), male success in male-male competition (territory size and tenure), or male reproductive success (number of mates, average clutch size, proportion of offspring sired, or proportion of young fledged). These results do not support testosterone acting as a constraint on immunity. Testosterone could be mediating a condition-dependent trade-off between sexual ornaments and the immune system, but the evidence for this was weak, as neither any male sexual trait nor any measure of performance was associated with immune responses. We could not eliminate the possibility that male sexual traits advertise other aspects of immunity that are not dependent upon condition or hormones.     

Social context modulates sickness behavior

Sickness behaviors constitute an array of symptoms exhibited by an animal during the course of an infection, including reduced activity, reduced food and water intake, and reduced social interactions. It is hypothesized that these symptoms enable reallocation of finite energy resources to fight infection. In this way, by focusing energy on healing, available resources are being removed from other activities, potentially reducing adaptive opportunities, such as mating. Hence, to achieve increased reproductive success, animals might be able to adjust the expression of sickness behaviors to their environmental circumstances. While abiotic conditions such as temperature and season can modulate sickness behaviors, no studies in passerines have linked modulation of sickness behaviors to social settings. Here, it is demonstrated that social surroundings affect the extent to which animals exhibit symptoms of sickness. After an immune challenge, zebra finches kept in isolation markedly reduced activity, but those kept in a colony setting did not. The same trend is verified when looking at the time they spent resting. Additionally, a proinflammatory cytokine (interleukin-6) was quantified in plasma samples and all animals that had been immune challenged showed increased levels of this marker, showing that the physiological response was similar. Hence, birds in a social context were able to overcome the behavioral, but not physiological, symptoms usually associated with an inflammatory response. These findings suggest a trade-off between allowing the body to respond to an infection and taking advantage of being in a social situation.     

Is avian humoral immunocompetence suppressed by testosterone?

A key issue in sexual selection theory is how a correlation between male secondary sexual characters and male genetic quality can be maintained. The immunocompetence-handicap hypothesis proposes that testosterone-dependent male characters remain honest signals because of the immunosuppressive effect of elevated steroid hormone levels. The hypothesis requires that physiological levels of testosterone depress immune system function. We quantified testosterone titers and humoral immunocompetence of captive male and female red-winged blackbirds (Agelaius phoeniceus) at three points in the annual cycle (autumn, prebreeding, and breeding). We also conducted an implant experiment on the males to assess the effects of prolonged, above-normal testosterone titers on humoral immune responses. Humoral immunocompetence was measured as secondary antibody production to a non-pathogenic protein antigen, keyhole limpet hemocyanin, using an enzyme-linked immunosorbent assay we developed for A. phoeniceus. Secondary antibody responses of individuals were highly repeatable between sampling periods. Neither physiological nor above-normal levels of plasma testosterone suppressed secondary antibody production. In paired tests of the same individuals between prebreeding and breeding, and between breeding and implant, plasma testosterone increased significantly but secondary antibody responses were unaffected. We are confident in these results because with 80% power, an 11–14% difference in antibody titers would have been detected. There was no relationship between plasma testosterone levels and humoral immunocompetence in free-ranging males tested at the peak of breeding. These results cast doubt on a key assumption of the immunocompetence-handicap hypothesis. Received: 13 April 1998 / Accepted after revision: 7 October 1998