Testosterone and group size in cliff swallows: testing the "challenge hypothesis" in a colonial bird

The "challenge hypothesis" states that increases in testosterone levels of male animals during the breeding season are directly related to the extent of intrasexual competition for resources or mates that they experience. Although often tested in territorial species, the challenge hypothesis has not been evaluated for colonial animals that live in groups of different sizes and that thus experience different intensities of intrasexual competition. We measured circulating testosterone levels of male and female cliff swallows (Petrochelidon pyrrhonota) in southwestern Nebraska, where these birds nest in colonies of widely different sizes. Males had significantly higher testosterone levels than females, as expected. For males especially, there was a seasonal rise in testosterone levels early in the nesting cycle, corresponding to the period when birds were establishing nest ownership and egg laying, and then a fall as they switched to parental duties. Testosterone levels varied significantly with colony size; for both sexes, birds in larger colonies had higher levels of testosterone than those in smaller colonies when controlling for date. Age and body mass were not related to testosterone levels. Higher levels of testosterone for birds of both sexes in larger colonies probably reflect greater competition for matings, often extra pair, in the more social nesting situations. The results support the predictions of the challenge hypothesis.     

Testis size increases with colony size in cliff swallows

By using a sample of over 800 male cliff swallows (Petrochelidonpyrrhonota) that died during a rare climatic event in our Nebraskastudy area in 1996, we investigated how testis size was relatedto body size, age, parasite load, a bird's past colony-sizehistory, and spleen size. Testis volume increased with bodysize. After correcting for body size, testis volume was lowestfor birds age 1 and 2 years but did not vary with age for males3 years old or more. Birds occupying parasite-free (fumigated)colonies had significantly larger testes than did birds at nonfumigatedsites. Testis volume increased significantly with the size ofthe breeding colonies a bird had used in the past. Testis volumehad no relationship with spleen volume after correcting forbody size. The results show within a species that larger testesare favored in more social environments, probably reflectinga response to increased rates of extrapair copulation (and thussperm competition) among cliff swallows in large colonies. Thepresence of ectoparasites, by inflating levels of plasma corticosterone,may in turn reduce testis mass. These data provide no supportfor the hypothesis that large testes, perhaps by producing moretestosterone, are immunosuppressive and thus costly for thatreason.     

Risk in mobbing for solitary and colonial swallows

Barn swallows (Hirundo rustica) often nest solitarily and are ecologically similar to colonially nesting cliff swallows (H. pyrrhonota). Northern rough-winged swallows (Stelgidopteryx serripennis) also nest solitarily and are ecologically similar to colonially nesting bank swallows (Riparia riparia). We investigated risk-taking in the mobbing of predators for these four species of North American swallows. We presented model owl and weasel predators and examined tendencies of individuals to dive at these predators during mobbing. Individuals of the two solitary species were significantly more likely to dive at predators than were individuals of the two colonial species. Since diving may be risky, we suggest that solitary species take greater per capita risks in mobbing than do colonial species. The reduced risk of falling victim during mobbing may be a benefit of group living that is perhaps independent of the effectiveness of mobbing.     

Rapid adjustments of migration and life history in hemisphere-switching cliff swallows

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Parasites favour intermediate nestling mass and brood size in cliff swallows

A challenge of life‐history theory is to explain why animal body size does not continue to increase, given various advantages of larger size. In birds, body size of nestlings and the number of nestlings produced (brood size) have occasionally been shown to be constrained by higher predation on larger nestlings and those from larger broods. Parasites also are known to have strong effects on life‐history traits in birds, but whether parasitism can be a driver for stabilizing selection on nestling body size or brood size is unknown. We studied patterns of first‐year survival in cliff swallows (Petrochelidon pyrrhonota) in western Nebraska in relation to brood size and nestling body mass in nests under natural conditions and in those in which hematophagous ectoparasites had been removed by fumigation. Birds from parasitized nests showed highest first‐year survival at the most common, intermediate brood‐size and nestling‐mass categories, but cliff swallows from nonparasitized nests had highest survival at the heaviest nestling masses and no relationship with brood size. A survival analysis suggested stabilizing selection on brood size and nestling mass in the presence (but not in the absence) of parasites. Parasites apparently favour intermediate offspring size and number in cliff swallows and produce the observed distributions of these traits, although the mechanisms are unclear. Our results emphasize the importance of parasites in life‐history evolution.     

Sperm traits in relation to male quality in colonial spawning bluegill

Sperm traits (morphology, motility and concentration within ejaculates) and various correlates of male quality (age, body condition, spawning location and timing) were studied in bluegill Lepomis macrochirus , breeding in both the interior and periphery of six colonies in Lake Opinicon, Ontario, Canada. Sperm traits varied significantly more among than within males suggesting that some aspect of male phenotype might influence sperm morphology and behaviour. No measures of male body condition or size were correlated with any sperm or ejaculate traits, when controlling statistically for confounding variables. Sperm swimming speed increased significantly with male age and varied significantly among spawning bouts (controlling for sperm tail length) suggesting that some unknown aspects of male quality might influence the fertilization capacity of spermatozoa. Sperm concentration in ejaculates was significantly higher in males nesting in the interior rather than the periphery of a colony suggesting that those males might also have higher fertilization capacity correlated with their superior dominance status or the lower risk of sperm competition. Thus, older males nesting in the interior of a colony during the first spawning bout of the season are expected to be the best sperm competitors in this population, but the physiological reasons for this increased fertilization capacity remain unknown.     

Nest predation in hole-nesting birds in relation to habitat edge: an experiment

We experimentally tested the hypothesis that nest predation rate in hole-nesting birds, especially the common goldeneye Bucephala clangula , was related to the edge between aquatic and terrestrial habitats We also studied whether nest predation rate was related to habitat patch (lake) size In three study areas nest-boxes were erected in pairs one nest-box was erected at the shoreline and the other one inside the forest at a varying distance (range from 14 to 140 m) from the shoreline One chicken egg was placed in each nest-box (dummy nest) Overall predation rate on dummy nests varied from 34 6% to 52 6% depending on the study area The pine marten Martes manes was the primary nest predator m all study areas Daily predation rates did not differ between natural and dummy nests Predation rate on dummy nests was not related to the distance of the nest-box from the shoreline in any study area Predation rate was not related to lake size in two study areas but in one area predation rate on dummy nests was lower around largest lakes However, the result may be accidental and not because of lake size per se because the proportion of forest was exceptionally low in that part of the study area     

Pattern of settlement and natural chimerism in the colonial urochordate Botryllus schlosseri

Colonies of the cosmopolitan urochordate Botryllus schlosseri that share one or both alleles at a single allorecognition locus (Fu/HC) and come into tissue contacts, may fuse and form a mixed entity, a chimera. Botryllus populations worldwide exhibit unprecedented extensive polymorphism at this locus, a result that restricts fusions to kin encounters. This study aims to compare spatiotemporal configurations in source and introduced B. schlosseri populations, residing on natural and man-made substrata, respectively. By using four microsatellite loci, we tested genetic consanguinity of colonies settled naturally along spatial vectors on both, natural (native populations) and man-made (introduced) substrates. Four populations were studied. Results revealed that B. schlosseri colonies, on both substrate types, assemble in groups of relatives that share similar microsatellite profiles. We suggest that this pattern of settlement promotes the formation of chimeras, which evoke conflicting interactions: cooperation between different somatic cell lines that constitute the colonial soma and competition between germ cells that inhabit the chimera gonads. Under natural conditions, the chimera may allow genetic flexibility that depends on joint genomic fitness of its partners. This is probably one of the life history characteristics that led to the worldwide distribution success of this species.     

No evidence for adjustment of sex allocation in relation to paternal ornamentation and paternity in barn swallows

Sex allocation theory predicts that parents should adjust investment in sons and daughters according to relative fitness of differently sexed offspring. In species with female preference for highly ornamented males, one advantage potentially accruing to parents from investing more in sons of the most ornamented males is that male offspring will inherit characters ensuring sexual attractiveness or high-quality genes, if ornaments honestly reveal male genetic quality. Furthermore, in species where extra-pair fertilizations occur, offspring sired by an extra-pair male are expected to more frequently be male than those of the legitimate male if the latter is of lower quality than the extra-pair male. We investigated adjustment of sex ratio of offspring in relation to ornamentation of the extra-pair and the social mate of females by direct manipulation of tails of male barn swallows Hirundo rustica . Molecular sexing of the offspring was performed using the W chromosome-linked avian chromo-helicase-DNA-binding protein (CHD) gene while paternity assessment was conducted by typing of hypervariable microsatellite loci. Extra-pair offspring sex ratio was not affected by ornamentation of their biological fathers relative to the experimental ornamentation of the parental male. Experimental ornamentation of the parental males did not affect the sex ratio of nestlings in their broods. Female barn swallows might be unable to bias offspring sex ratio at hatching according to the quality of the biological father. Alternatively, fitness benefits in terms of sexual attractiveness of sons might be balanced by the cost of compensating for little parental care provided by highly ornamented parental males, if sons are more costly to rear than daughters, or the advantage of producing more daughters, if males with large ornaments contribute differentially more to the viability of daughters than sons.     

The role of despotism and heritability in determining settlement patterns in the colonial lesser kestrel

Avian colony size variation is an evolutionary puzzle in terms of unequal fitness payoffs. We used a long-term marked lesser kestrel (Falco naumanni) population, where individual fitness increases with colony size, to test whether subordinates are evicted despotically from the largest colonies. Yearlings were smaller and lighter, were more attacked than expected, and lost most disputes over nest holes with older birds. Agonistic interactions increased with colony size; consequently, most first breeders recruited in colonies smaller than those at which they first tried to settle. As expected when subordination is a transient state, birds dispersed to a larger colony as they got older even after breeding successfully. The population consequences of these behavioral processes were that the relative frequency of yearlings and first breeders decreased with colony size. At the same time, breeding colony size was repeatable within individuals, so we estimated the amount of heritable variation in this trait. Estimates of heritability derived from parent-offspring and full-sib analyses were consistently high (h2=0.53) when individuals reached asymptotic morphological values and presumably overcame subordinate transient states. Age-related dominance asymmetries masked resemblance among relatives in colony size, but both phenomena seem to coexist in this population and explain a considerable proportion of colony size variation.     

Nucleosome positioning in relation to nucleosome spacing and DNA sequence-specific binding of a protein

Nucleosome positioning is an important mechanism for the regulation of eukaryotic gene expression. Folding of the chromatin fiber can influence nucleosome positioning, whereas similar electrostatic mechanisms govern the nucleosome repeat length and chromatin fiber folding in vitro. The position of the nucleosomes is directed either by the DNA sequence or by the boundaries created due to the binding of certain trans-acting factors to their target sites in the DNA. Increasing ionic strength results in an increase in nucleosome spacing on the chromatin assembled by the S-190 extract of Drosophila embryos. In this study, a mutant lac repressor protein R3 was used to find the mechanisms of nucleosome positioning on a plasmid with three R3-binding sites. With increasing ionic strength in the presence of R3, the number of positioned nucleosomes in the chromatin decreased, whereas the internucleosomal spacings of the positioned nucleosomes in a single register did not change. The number of the positioned nucleosomes in the chromatin assembled in vitro over different plasmid DNAs with 1-3 lac operators changed with the relative position and number of the R3-binding sites. We found that in the presence of R3, nucleosomes were positioned in the salt gradient method of the chromatin assembly, even in the absence of a nucleosome-positioning sequence. Our results show that nucleosome-positioning mechanisms are dominant, as the nucleosomes can be positioned even in the absence of regular spacing mechanisms. The protein-generated boundaries are more effective when more than one binding site is present with a minimum distance of approximately 165 bp, greater than the nucleosome core DNA length, between them.     

Timing of copulations and the pattern of paternity in relation to laying order in tree swallows Tachycineta bicolor

It is well established that extra-pair young are present to varying degrees in nests of most avian species. However, the timing of extra-pair copulations during a female's fertile cycle is not well known, nor is how these copulations translate into the distribution of extra-pair young within the laying sequence. We determined the rates of successful within-pair copulations where at least one cloacal contact was achieved, and the number of extra-pair copulations/attempts in 17 pairs of tree swallows Tachycineta bicolor during the mornings encompassing the female's fertile period. We also examined nestling paternity with respect to laying order in eighteen mixed-paternity tree swallow broods. The rate of successful within-pair copulations peaked three days prior to a female laying her first egg, and decreased steadily throughout the laying period. We therefore predicted that extra-pair young would be more likely to hatch out of later-laid than earlier-laid eggs within a female's brood. We found no evidence of an increased frequency of extra-pair young with laying order. Instead, extra-pair young appear to be randomly distributed within the laying sequence of tree swallows.