Donald C. Dearborn

Motivations for Conserving Urban Biodiversity

In a time of increasing urbanization, the fundamental value of conserving urban biodiversity remains controversial. How much of a fixed budget should be spent on conservation in urban versus nonurban landscapes? The answer should depend on the goals that drive our conservation actions, yet proponents of urban conservation often fail to specify the motivation for protecting urban biodiversity. This is an important shortcoming on several fronts, including a missed opportunity to make a stronger appeal to those who believe conservation biology should focus exclusively on more natural, wilder landscapes. We argue that urban areas do offer an important venue for conservation biology, but that we must become better at choosing and articulating our goals. We explored seven possible motivations for urban biodiversity conservation: preserving local biodiversity, creating stepping stones to nonurban habitat, understanding and facilitating responses to environmental change, conducting environmental education, providing ecosystem services, fulfilling ethical responsibilities, and improving human well-being. To attain all these goals, challenges must be faced that are common to the urban environment, such as localized pollution, disruption of ecosystem structure, and limited availability of land. There are, however, also challenges specific only to particular goals, meaning that different goals will require different approaches and actions. This highlights the importance of specifying the motivations behind urban biodiversity conservation. If the goals are unknown, progress cannot be assessed.

Begging behavior and food acquisition by brown-headed cowbird nestlings

Abstract Understanding the selective forces that limit the exaggeration of begging signals is a critical issue in understanding the evolution of begging behavior. I studied the begging behavior of nestlings of the brown-headed cowbird (Molothrus ater), a brood parasite. In the nests of indigo buntings (Passerina cyanea), brown-headed cowbird nestlings received approximately twice as much food per hour than their host nestmates. I tested three hypotheses for the mechanism by which cowbirds acquired more food than their bunting nestmates: the size advantage hypothesis, the signal exaggeration hypothesis, and the novel begging behavior hypothesis. I found support for the hypotheses that cowbirds acquire more food as a result of their larger body size, and due to the exaggeration of begging signals that are not dependent on body size. I did not find support for the role of novel begging behaviors in cowbird food acquisition. These results suggest that food acquisition by host chicks in unparasitized nests could be increased by the exaggeration of begging signals. Recent work suggests that such exaggeration may be limited by the risk of nest predation, but further studies are needed.

Factors affecting predation at songbird nests in old fields

We determined the effects of microhabitat, year, weather, time of season, stage of the nesting cycle, and brood parasitism on nest predation from a 7-year dataset on field sparrows (Spizella pusilla) and indigo buntings (Passerina cyanea) in central Missouri, USA. Year, site, and the interaction of species and 2-week interval of the season were important factors explaining nest predation. The only microhabitat variable that consistently explained predation was nest height: nests over 3 m high almost always fledged. Validation of the model parameters on an independent set of nests resulted in proper categorization (e.g., lost or not lost to predation) of 61.5% of nests. In models testing weather and temporal effects, year was related to daily survival for indigo buntings, and 2-week intervals of the season explained daily survival for both species. Nest predation was higher overall in the nestling stage than in the incubation stage for indigo buntings, and indigo buntings parasitized by brown-headed cowbirds (Molothrus ater) experienced higher predation than nonparasitized buntings. Temporal patterns within the breeding season were consistent between years, and between-year variance appeared to be important, whereas microhabitat was generally unimportant. Research on the mechanisms underlying temporal variability in nest mortality due to predation may identify management options to reduce nest predation.

Great frigatebirds, Fregata minor, choose mates that are genetically similar

Inbreeding occurs infrequently in most natural populations, with the level of relatedness between mates potentially being influenced by behavioural mechanisms of mate choice and spatial genetic structure of the population. We studied mate relatedness in a Hawaiian population of great frigatebirds with the expectation, based on frigatebird ecology, that mated pairs were unlikely to be more genetically similar than a random draw of adults from the breeding population. We used a paired analysis to compare the DNA fingerprint similarity of a female and her mate versus the same female and a random male that was breeding elsewhere on the same island. Overall, band-sharing scores between mates were higher than band-sharing scores between nonmates, and the mean coefficient of relatedness between mates was 0.082. Relatedness between mates was not a consequence of strong natal philopatry coupled with random mate choice: historical data revealed several recent between-island shifts in the location of the breeding colony, and the degree of genetic similarity between nonmates was not well predicted by the physical distance between their nest sites on Tern Island. Instead, females may deliberately choose related individuals as mates. Tests for a relationship between the genetic similarity of mates and the size and ectoparasite load of their 1-week-old chick were equivocal. The question of how and why females may be choosing genetically similar mates is unresolved, but it appears to be a consequence of active choice rather than spatial genetic structure or limited availability of unrelated mates.

Inter-island movements and population differentiation in a pelagic seabird

We used mark–resight data and amplified fragment length polymorphism (AFLP) markers to assess movements and gene flow between Central Pacific breeding colonies of the great frigatebird, Fregata minor. Of 715 adult frigatebirds marked on Tern Island and Johnston Atoll, 21.3% were resighted at other frigatebird colonies at least 582 km away. Mark–resight data indicated regular movement of males and females between Tern Island and Johnston Atoll (873 km apart), and less frequent movements to other islands; no birds marked on Tern or Johnston were seen on Christmas Island, but one was seen in the Philippines, 7627 km from where it was marked. Despite the regular occurrence of interisland movements, Bayesian analyses of AFLP data showed significant genetic differentiation between Tern Island and Johnston Atoll, and more pronounced differentiation between these two islands and the more distant Christmas Island. The AFLP profiles of three birds breeding on Tern Island fell within the profile‐cluster typical for Christmas Island birds, both in a nonmetric multidimensional scaling analysis and in a population assignment test, suggesting dispersal events from Christmas Island to Tern Island. Several factors could explain the persistence of genetic structure despite frequent movements between colonies: many movements occurred during the nonbreeding season, many breeding‐season movements did not involve mate‐acquisition behaviours and individuals that do disperse may be selected against, as suggested by morphometric differences between colonies. The persistence of genetic structure among breeding colonies despite significant interisland movements suggests limits to the effectiveness of migration as a homogenizing force in this broadly distributed, extremely mobile species.

Do mosquitoes filter the access of Plasmodium cytochrome b lineages to an avian host

Many parasites show fidelity to a set of hosts in ecological time but not evolutionary time and the determinants of this pattern are poorly understood. Malarial parasites use vertebrate hosts for the asexual stage of their life cycle but use Dipteran hosts for the sexual stage. Despite the potential evolutionary importance of Dipteran hosts, little is known of their role in determining a parasites access to vertebrate hosts. Here, we use an avian malarial system in Panama to explore whether mosquitoes act as an access filter that limits the range of vertebrate hosts used by particular parasite lineages. We amplified and sequenced Plasmodium mitochondrial DNA (mtDNA) from Turdus grayi (clay‐coloured robin) and from mosquitoes at the same study site. We trapped and identified to species 123 141 female mosquitoes and completed polymerase chain reaction (PCR) screening for Plasmodium parasites in 435 pools of 20 mosquitoes per pool (8700 individuals total) spanning the 11 most common mosquito species. Our primers amplified nine Plasmodium lineages, whose sequences differed by 1.72%–10.0%. Phylogenetic analyses revealed partial clustering of lineages that co‐occurred in mosquito hosts. However PAN3 and PAN6, the two primary parasite lineages of T. grayi, exhibited sequence divergence of 8.59% and did not cluster in the phylogeny. We detected these two lineages exclusively in mosquitoes from different genera — PAN3 was found only in Culex (Melanoconion) ocossa, and PAN6 was found only in Aedeomyia squamipennis. Furthermore, each of these two parasite lineages co‐occurred in mosquitoes with other Plasmodium lineages that were not found in the vertebrate host T. grayi. Together, this evidence suggests that parasite–mosquito associations do not restrict the access of parasites to birds but instead may actually facilitate the switching of vertebrate hosts that occurs over evolutionary time.

Effects of Cowbird parasitism on parental provisioning and nestling food acquisition and growth

Brood parasitism by Brown-headed Cowbirds (Molothrus ater) is known to affect the fitness of many hosts by causing a reduction in the number of chicks that fledge from parasitized nests. However, little is known ahout less immediate effects on host fitness. We studied nestling growth and food acquisition and parental provisioning in parasitizcd and unparasitized nests of the Indigo Bunting (Passerina cyanea). Indigo Bunting nestlings in parasitized nests exhibited reduced rates of mass gain, but not tarsus growth, relative to bunting chicks in unparasitized nests. Bunting nestlings in parasitized nests received less food than did buntings in unparasitized nests. Buntings in parasitized nests spent more time begging than did those in unparasitized nests, but energy expended in this behavior may not have detracted greatly from the amount of energy available for growth. Adults at parasitized nests exhibited a higher provisioning rate than those at unparasitized nests. Increascd provisioning by adult buntings at parasitized nests did not come at the expense of time spent brooding nestlings, but increased provisioning has the potential to affect the survival and future reproductive success of host adults. Because cowbird parasitism appears to impose substantial costs on Indigo Bunting nestlings and adults, concern over the conservation implications of parasitism should not be limited to species that suffer total reproductive failure when parasitized.

TELOMERE SHORTENING IN A LONG-LIVED MARINE BIRD: CROSS-SECTIONAL ANALYSIS AND TEST OF AN AGING TOOL

Abstract A correlation between length of telomere restriction fragments (TRFs) and age has recently been demonstrated in several bird species. Comparisons of different-aged individuals within a population have shown that TRFs typically shorten with age and that this shortening continues throughout the life span of the species. In addition, it has been shown that telomere rate-of-change (TROC) correlates tightly with life span across several bird species. Previous studies of long-lived birds, however, have shown exceptions to these trends, demonstrating no declines in TRF length in adults in some cases and increases in TRF length with age in other cases. Here, we report known ages of individuals from a colony of Great Frigatebirds (Fregata minor) based on recaptures of leg-banded birds, including two individuals that were at least 44 years of age, the oldest Great Frigatebirds ever reported. Using a previously developed molecular technique, we report a predictable, nonlinear decline of TRF length with age in this population. Telomere restriction fragments decline more rapidly early in life but continue to shorten throughout the life span examined. The rate of decline in TRF for this species does not fit the tight correlation previously reported between TROC and life span in other species. Finally, we tested the ability to estimate age and age structure of breeding females on the basis of the calibration of TRF length and individuals of known age. Because of the slow telomere-attrition rate and the variability observed in TRF lengths at given ages, estimations of age of individuals and of estimated age structure of breeding birds in this population are not particularly reliable. Encogimiento del Telómero en un Ave Marina Longeva: Análisis de Secciones Cruzadas y Evaluación de una Herramienta de Envejecimiento

Sequence-based evidence for major histocompatibility complex-disassortative mating in a colonial seabird

The major histocompatibility complex (MHC) is a polymorphic gene family associated with immune defence, and it can play a role in mate choice. Under the genetic compatibility hypothesis, females choose mates that differ genetically from their own MHC genotypes, avoiding inbreeding and/or enhancing the immunocompetence of their offspring. We tested this hypothesis of disassortative mating based on MHC genotypes in a population of great frigatebirds (Fregata minor) by sequencing the second exon of MHC class II B. Extensive haploid cloning yielded two to four alleles per individual, suggesting the amplification of two genes. MHC similarity between mates was not significantly different between pairs that did (n = 4) or did not (n = 42) exhibit extra-pair paternity. Comparing all 46 mated pairs to a distribution based on randomized re-pairings, we observed the following (i): no evidence for mate choice based on maximal or intermediate levels of MHC allele sharing (ii), significantly disassortative mating based on similarity of MHC amino acid sequences, and (iii) no evidence for mate choice based on microsatellite alleles, as measured by either allele sharing or similarity in allele size. This suggests that females choose mates that differ genetically from themselves at MHC loci, but not as an inbreeding-avoidance mechanism.

Feather coloration in museum specimens is related to feather corticosterone

Colorful ornaments in birds are often sexually selected signals of quality, and variation in ornament expression may be mediated by physiological stress through the secretion of corticosterone. However, testing for links between ornamentation and corticosterone often requires sampling live animals, and such physiological measures may not be matched in the time span in which they were sampled (e.g., very dynamic plasma corticosterone vs. plumage coloration, which is relatively static). Here, we use museum specimens to test for a link between the color of a sexual ornament and feather corticosterone at the time of ornament formation. In red-winged blackbirds, Agelaius phoeniceus, carotenoid-based epaulets appear to be important in male–male social interactions, territory maintenance, and female choice. We measured reflectance spectra of adult male epaulets and plucked adjacent feathers for corticosterone analysis via radioimmunoassay. We controlled for differences in the number of mates, specimen age, and geography by selecting only males with one mate and only birds collected in Florida during a 3-year period. Epaulet hue and red chroma did not vary with feather corticosterone, but males whose epaulets scored high for mean brightness and red brightness had significantly lower corticosterone than males with low brightness scores. This correlation with brightness but not hue or chroma is consistent with an effect of corticosterone (CORT) on feather microstructure, with elevated CORT leading to lower reflectance of white light from the keratin matrix surrounding the carotenoid pigments.