Bettina Mahler

HYBRIDIZATION AND POSTZYGOTIC ISOLATION PATTERNS IN PIGEONS AND DOVES

Abstract.— The study of the patterns of reproductive isolation in relation to species divergence is critical for the understanding of the process of speciation. Comparative analyses of this kind were previously conducted in Drosophila, Lepidoptera, frogs, ducks, and birds in general. In the present study, we used information from the literature to analyze hybrid inviability in relation to species divergence in pigeons and doves. Four main patterns arose from this analysis: (1) as in the other groups studied, F1 hybrid inviability gradually increases as species diverge, the time needed to reach total inviability being higher in birds than in the other groups; (2) as expected, the presence of geographic overlap does not influence the evolution of postzygotic isolation; (3) the percentage of unhatched eggs does not differ between hybrids of the first generation and the backcrosses, but it increases in the second hybrid generation; and (4) pigeons and doves follow Haldanes rule, as found in the other groups studied so far. The similarity between the results of this and previous studies contributes to the growing evidence suggesting that the patterns of the evolution of postzygotic isolation, and the process of speciation in general, are shared among animal groups.

Different recognition cues reveal the decision rules used for egg rejection by hosts of a variably mimetic avian brood parasite

Brood parasitism imposes several fitness costs on the host species. To reduce these costs, hosts of avian brood parasites have evolved various defenses, of which egg rejection is the most prevalent. In the face of variable host-parasite mimicry and the costs of egg discrimination itself, many hosts reject only some foreign eggs. Here, we experimentally varied the recognition cues to study the underlying cognitive mechanisms used by the Chalk-browed Mockingbird (Mimus saturninus) to reject the white immaculate eggs laid by the parasitic Shiny Cowbird (Molothrus bonariensis). Immaculate eggs are the only parasite eggs rejected by this host, as it accepts all polymorphic, spotted eggs laid by cowbirds. Using a within-breeding pair experimental design, we tested for the salience of spotting, UV reflectance, and brightness in eliciting rejection. We found that the presence of spotting significantly decreased the probability of rejection while increments in brightness significantly increased rejection frequencies. The cognitive rules underlying mockingbird rejection behavior can be explained by a decision-making model which predicts changes in the levels of rejection in direct relation to the number of relevant attributes shared between host and parasite eggs.

Partial host fidelity in nest selection by the shiny cowbird (Molothrus bonariensis), a highly generalist avian brood parasite

Obligate avian brood parasites can be host specialists or host generalists. In turn, individual females within generalist brood parasites may themselves be host specialists or generalists. The shiny cowbird Molothrus bonariensis is an extreme generalist, but little is known about individual female host fidelity. We examined variation in mitochondrial control region sequences from cowbird chicks found in nests of four common Argentinean hosts. Haplotype frequency distributions differed among cowbird chicks from nests of these hosts, primarily because eggs laid in nests of house wrens Troglodytes aedon differed genetically from those laid in nests of the other three hosts (chalk‐browed mockingbird Mimus saturninus, brown‐and‐yellow marshbird Pseudoleistes virescens, and rufous‐collared sparrow Zonotrichia capensis). These differences in a maternally inherited marker indicate the presence of a nonrandom laying behaviour in the females of this otherwise generalist brood parasite, which may be guided by choice for nest type, as house wrens nest in cavities whereas the other three species are open cup nesters.

Eggshell spotting in brood parasitic shiny cowbirds (Molothrus bonariensis) is not linked to the female sex chromosome

For avian brood parasites in which individual females are host-specialists, the arms race between hosts and parasites has favored egg color polymorphism in the parasite, with female lineages laying mimetic eggs that resemble those of the host species they parasitize. Female sex-linked inheritance of egg color fosters evolutionary stability of egg polymorphism if female lineages show both consistent eggshell color and host use. This co-evolutionary relationship is unlikely to occur if individual brood parasites use different hosts or if egg color is not maternally inherited. The shiny cowbird (Molothrus bonariensis) is an extreme generalist brood parasite that shows a very high degree of egg polymorphism. We tested whether egg spotting in this species has female sex-linked inheritance. If genetic factors controlling the expression of egg spotting were present on the female-specific W chromosome, we expected co-segregation between spotting patterns and mtDNA haplotypes, as both W and mtDNA are maternally inherited. In contrast to the known maternal inheritance of spotting patterns in great tits, we found no associations between eggshell spotting and mtDNA haplotypes, which suggests that eggshell spotting is not maternally inherited in this cowbird species.

Chapter 3 – Host Use by Generalist and Specialist Brood-Parasitic Cowbirds at Population and Individual Levels

Abstract We analyzed host use at population and individual levels in the host-generalist shiny cowbird ( Molothrus bonariensis ) and in the host-specialist screaming cowbird ( M. rufoaxillaris ). At the population level, shiny cowbirds were less generalists than previously assumed and host use varied considerably among regions. There was, however, no clear preference for host species, since we found no associations between frequency of parasitism and hosts body mass, type of nest, or genetic distance between host and parasite. Regarding screaming cowbirds, frequency and intensity of parasitism varied among host populations. Parasitic eggs and chicks were equally successful in the primary and secondary hosts, but they experienced high mortality rates when transferred to other suitable, but unused hosts. Screaming and shiny cowbirds overlapped little in host use and had no apparent effect on each others success when reared together in a common host. At the individual level, there was an association between mtDNA haplotypes of shiny and screaming cowbird chicks and the hosts in whose nests they were found, indicating that nest choice by parasitic females is not random. We discuss how host imprinting by females can lead to the formation of host-specific races and how the occurrence of recognition errors and social learning can lead to the colonization of new hosts.

Reproductive Success and Nestling Growth of the Baywing Parasitized by Screaming and Shiny Cowbirds

Abstract We studied the breeding biology of the Baywing (Agelaioides badius), a shared host of Screaming (Molothrus rufoaxillaris) and Shiny (M. bonariensis) cowbirds. We monitored 193 nests from December 2002 to March 2007 in the Province of Buenos Aires, Argentina. Baywings used a wide variety of nesting sites, mainly old nests of furnarids. Their breeding season lasted from late November to February and was closely matched by that of Screaming Cowbirds. The breeding season for Shiny Cowbirds started in late September but overlapped that of Baywings. Frequency and intensity of Screaming Cowbird parasitism were 93% and 5 eggs per parasitized nest, while for Shiny Cowbirds they were 16% and 1.4 eggs. Host clutch size was 4.0 ± 0.1 eggs and did not vary with time of breeding. Weight at hatching and age of maximum growth were similar for host and Screaming Cowbird nestlings. Shiny Cowbird nestlings had higher weight at hatching and lower age of maximum growth than the other two species. Screaming and Shiny cowbird nestlings had higher growth rates and asymptotic weights than host nestlings. Sex-specific growth curves of Screaming Cowbirds indicated males had higher growth rate and asymptotic weight than females. Only 19% of the nests produced fledglings. Host egg survival, hatching success, and nestling survival were 0.92, 0.88, and 0.94, respectively. Excluding nest failures, hosts fledged 0.78 chicks per egg laid. Baywings were smaller than Screaming and Shiny cowbirds, and experienced a high frequency and intensity of parasitism. However, the effect of parasitism on host hatching success and chick survival was low and comparable to that observed in larger hosts.

Chapter 2 The Evolution of Song in the Phylloscopus Leaf Warblers (Aves: Sylviidae): A Tale of Sexual Selection, Habitat Adaptation, and Morphological Constraints

Differences in song between species of birds are often the most reliable criteria by which to identify them. Thus, the study of the evolution of bird song provides biologists with a powerful insight into the nature of speciation processes. Both adaptive and nonadaptive explanations have been proposed to account for the evolution of song differences in birds. In this study, we put several of these hypotheses to a test in the genus Phylloscopus, a species-rich group of Old World leaf warblers in which song is used by males for mate attraction and territory defense. We found that song characteristics contained significant amounts of phylogenetic information, although they were more labile than morphological traits. Song frequency characteristics were more phylogenetically preserved than temporal or song structure traits. Changes in body size between species were correlated with changes in maximum and minimum frequencies and frequency bandwidth: small-bodied species had higher frequencies and wider bandwidths than large-bodied species. Beak shape was not found to limit overall frequency patterns. We used an ecomorphological correlate of habitat use, the tarsus/beak ratio, to test for song adaptation to specific habitat sound transmission characteristics. We found that species with larger tarsus/beak ratio, typically inhabiting broadleaf habitats and understory vegetation, had lower emphasized frequencies in their songs, as expected if songs are adapted to maximize sound transmission. However, this relationship did not held when controlling for common descent. Song complexity, a wide-ranging measurement encompassing temporal and structural sound complexity estimates, was best explained by breeding latitude. We interpret this relationship as a correlate of increased sexual selection by mate choice toward more Northern latitudes. We failed to find a significant role of character displacement in the evolution of song in this group: sympatric pairs of species did not show larger differences in song than allopatric pairs of species. To sum up, superimposing upon large haphazard selection of song themes, selective pressures for higher song elaboration in areas of high sexual selection, and correlative change in song brought about by natural selection of body size, would explain some of the diversity of songs that are found within the genus Phylloscopus.

Hooded Grebe Podiceps gallardoi population decreased by eighty per cent in the last twenty-five years

We estimated the present population size of the Hooded Grebe Podiceps gallardoi and compared it with the population in 1985. During the breeding seasons 2009/2010 and 2010/2011 we monitored 251 lakes on the main plateaus of Santa Cruz province, southern Patagonia, Argentina, where the entire population of Hooded Grebes spends the breeding season. During 2009/2010 we monitored 131 lakes and observed 521 Hooded Grebes in 16 lakes, while during 2010/2011 we monitored 186 lakes and observed 535 individuals in 14 lakes. At both breeding seasons five lakes contained near 85% of the population. We only observed 6 breeding colonies, two in 2009/2010 and four in 2010/2011, totalling 242 nesting attempts. Nearly 90% of the nests failed during egg stage, mainly due to strong winds and depredation by American mink Neovison vison . Only 28 nests hatched chicks, but none of the young survived to independence due to low temperatures. We compared the results of our 2010/2011 survey with one conducted during 1984/1985 and detected that the population diminished by approximately 80%. This decline was observed on all but one plateau (“Buenos Aires”) where most breeding attempts occurred. Our results show that at present Hooded Grebes are highly dependent on a few suitable lakes where they congregate to breed. We analyse possible causes for the decline of the population and propose conservation actions to protect this species.

Impact of Shiny Cowbird and botfly parasitism on the reproductive success of the globally endangered Yellow Cardinal Gubernatrix cristata

The Yellow Cardinal Gubernatrix cristata is a globally endangered passerine endemic to southern South America. Although the conservation status of this species is alarming, there is no information about factors that affect its reproductive success. We studied the breeding biology of a Yellow Cardinal population in Corrientes province in north-east Argentina and report reproductive parameters, such as timing of breeding, clutch size, hatching success and chick survival and analyse factors influencing nest survival. The breeding season lasted from October to December with most nesting attempts occurring during the second half of November. Re-nesting after nest failure occurred in approximately 35% of pairs but we did not observe re-nesting after successful attempts. Modal clutch size was three eggs and successful nests fledged on average 1.6 chicks. We observed brood parasitism by Shiny Cowbirds Molothrus bonariensis in 33% of the nests. The main cost of brood parasitism was the puncture of cardinal eggs by cowbird females, which very often resulted in nest abandonment. Another factor reducing reproductive success was parasitism by botflies Philornis sp., which occurred in 22% of the nests and reduced chick survival. We determined daily nest survival rate with MARK software and found that during the egg stage, it decreased with time of breeding and brood parasitism by Shiny Cowbirds. Our results indicate that Shiny Cowbird and botfly parasitism represent a threat for Yellow Cardinals. Conservation actions to protect the remaining populations of Yellow Cardinals should consider the impact of Shiny Cowbird parasitism.

Seasonal Variation in Reproductive Output of a Neotropical Temperate Suboscine, the Firewood-Gatherer (Anumbius annumbi)

ABSTRACT. The risk of breeding failure often varies strongly over the breeding season. If birds optimize production of offspring accordingly, investment in reproduction should be lowest when food for young is scarce or nest predation risk is highest. Evidence for this hypothesis is mixed and stems mainly from studies on northern temperate birds, and it is unclear whether these patterns apply to southern temperate birds with “slow” life histories that resemble those of tropical birds. We studied seasonal variation in reproductive investment and nesting success in a Neotropical temperate suboscine, the Firewood-gatherer (Anumbius annumbi). Pairs produced an average of 2.8 clutches over the very long (August–April) breeding season. Clutch size (mean = 5.1 eggs, range: 3–8) was very variable between and within pairs and showed an initial increase followed by a decline, the typical pattern of variation for multibrooded temperate species. The main determinant of breeding success was predation. Although the probability of nest failure increased over the season from 70% to >90%, clutch size was not correlated with failure risk. Egg volume did not show consistent seasonal patterns. Our results suggest that reproductive investment is not optimally adjusted to the seasonal variation in nest predation risk, and more research is needed to assess the influence of food availability.