Robert A. Mauck

PHILOPATRY, NATAL DISPERSAL, AND INBREEDING AVOIDANCE IN AN ISLAND POPULATION OF SAVANNAH SPARROWS

Over a 9-yr period, we studied dispersal of young banded Savannah Sparrows (Passerculus sandwichensis) from their natal nest to the site where they first bred 1 yr later in a population on an isolated archipelago in the Bay of Fundy, Canada. On a broad geographic scale, young birds were highly philopatric, returning from wintering grounds several thousand kilometers away to nest on the same island and often in the same field where they had hatched the year before. In some cases, birds chose nest sites within a few meters of their natal nest. The median dispersal distance between a bird’s natal nest and its first nest as an adult was 228 m, a distance equivalent to about six times the diameter of an average territory. Nearly three-quarters of the breeding birds in the study population had been banded as nestlings or fledglings within a 10-ha area on one island, which suggests that most birds in the population originated within the 127-ha archipelago. Within the archipelago, males and females dispersed simi...

AGE-SPECIFIC REPRODUCTIVE SUCCESS: EVIDENCE FOR THE SELECTION HYPOTHESIS

Abstract Age-specific reproductive success has been demonstrated in many species. Three hypotheses have been raised to explain this general phenomenon: the experience hypothesis based on age-specific reproductive experience, the effort hypothesis based on age-specific reproductive effort, and the selection hypothesis based on progressive disappearance of phenotypes due to variation in individual productivity and survival. We used data from a long-term study of Leachs storm-petrels (Oceanodroma leucorhoa) to present a single test of mutually exclusive predictions about the relationship between early breeding success and longevity. There should be no correlation between early breeding success and longevity under the experience hypothesis, a negative correlation under the effort hypothesis, and a positive correlation under the selection hypothesis. We found a significant (P < 0.0001) positive relationship between success in the first two breeding attempts and longevity in this population of long-lived seabirds, strongly suggesting that low-productivity parents were also less likely to survive early breeding. These data provide some of the strongest support to date for the selection hypothesis.

Adult Survival and Imperfect Assessment of Parentage: Effects on Male Parenting Decisions

Applications of molecular methods to assess parentage have revealed that the distribution of reproductive success among individuals often differs, sometimes dramatically, from expectation based on observation of behavioral association. Much theory exists on whether and when males should reduce parental care in response to level of paternity. Life‐history theory predicts that trade‐offs in reproductive effort should be influenced by adult survival. We used a dynamic programming approach to address how level of paternity, ability to assess paternity, and adult survival rate interact to affect male tolerance of reduced parentage in a given brood. Adult survival has the greatest influence on male decisions such that, for any given cost of reproduction and value of male care, tolerance of extrapair fertilizations (EPFs) decreases as adult survival increases. An unexpected result of these models is that an optimal response also depends on a males ability to predict probability of parentage (i.e., uncertainty). These models better characterize the nature of paternity uncertainty and its effect on EPF tolerance than have previous models and add to our understanding of the complex relationship between uncertainty, mating strategies, and adult survival.

MONOGAMY IN LEACH'S STORM-PETREL: DNA-FINGERPRINTING EVIDENCE

We used multilocus minisatellite DNA fingerprinting to estimate the frequency of extrapair fertilizations in a population of Leachs Storm-Petrel (Oceanodroma leucorhoa) on Kent Island, New Brunswick, Canada. Leachs Storm-Petrel is a member of Procellariiformes, an order of long-lived pelagic birds characterized by long-term pairbonds, single-egg clutches, and extended periods of parental care. We found no evidence of extrapair fertilizations in 48 families (42 full families and 6 partial families consisting of the putative father and the single offspring). Thus, our results indicate that the breeding system (genetic monogamy) matches the mating system (social monogamy) in our study population, a condition that no longer can be assumed in socially monogamous bird species. Genetic monogamy in Leachs Storm-Petrels may be maintained by last-sperm precedence and frequent copulation by mates during the females fertile period. Such tactics employed by a male may yield a high prob- ability of fertilizing the single egg laid by his mate. Received 15 June 1994, accepted 31 August

TECHNICAL ADVANCES: New strategies for telomere‐based age estimation

Telomere dynamics link molecular and cellular mechanisms with organismal processes and therefore may explain variation in a number of important life‐history traits. Telomere length has been used to estimate age in free‐living populations of animals. Such estimation is a potentially powerful tool in the context of population dynamics and management, as well as the study of life‐history trade‐offs. The number of studies utilizing telomere restriction fragment assays in the fields of ecology and evolution is steadily growing. However, the field lacks methodological and analytical standardization resulting in considerable variation in telomere length and therefore in the usefulness of these techniques. Here, we illustrate new laboratory and analytical methods to reliably measure telomere length from blood erythrocytes and accurately assess the relationship between telomeres and age. We demonstrate the importance of analysing those telomeres most relevant to age‐related studies: the shortest telomeres. We present a reliable method to quickly identify an analysis window (the telomere optimal estimate, TOE) which approaches the optimal window for age estimation. Because the TOE focuses on the shortest telomeres — those telomeres which signal cellular senescence and ageing — TOE can also be used to compare telomeres in age‐matched individuals. We also compare constant‐ and pulsed‐field gel electrophoresis to show how each can influence telomere measurement. The use of TOE should provide powerful telomere‐based age estimation and enable organismal biologists to readily uncover individual and longitudinal differences with regard to telomere dynamics.

The effect of group membership on hiding behaviour in the northern rock barnacle, Semibalanus balanoides

Animals living in groups are thought to gain fitness through decreased predation risk, while often paying a cost in terms of increased competition in foraging. Thus, the balance struck between predator avoidance and foraging should be affected by group membership. For animals that avoid predation by withdrawing into a refuge (i.e. ‘hiding’), such as the northern rock barnacle, Semibalanus balanoides, that balance should be particularly important since predator avoidance excludes foraging altogether. We tested the hypothesis that barnacles living in groups should spend less time hiding when faced with a perceived threat than should solitary barnacles. We presented group-living and solitary barnacles with a simulated threat and measured hiding time with the prediction that barnacles in groups would return to foraging more quickly than solitary barnacles. Hiding time for group-living barnacles was significantly less than for solitary barnacles. We then manipulated barnacle group size with the prediction that an individual barnacles behaviour would change based on group membership alone. We tested individual barnacles three times in an A–B–A design in which barnacles were tested in one of two sequences, solitary–group–solitary, or group–solitary–group. We found that group membership had a significant effect on barnacle foraging behaviour in that individuals emerged from hiding sooner when tested in a group than when tested alone. We conclude that, as predicted by optimality theory, group membership strongly affects foraging decisions by this refuge-using animal. We argue that the proximate mechanism for such behaviour may involve a simple binary reaction to living in a group.

Changes in plasma hormone levels correlate with fledging in nestling Leach’s storm-petrels

Leachs storm-petrels accumulate large amounts of body mass throughout the nestling period. Approximately 4days before fledging, nestlings weigh 50-100% more than adults. In order to shed this excess mass, nestlings engage in behavioral anorexia, and leave the burrow when they are light enough to fly. During this prefledging period, we measured several plasma hormones (corticosterone, thyroxine, and testosterone) to determine whether the behavioral changes associated with fledging are correlated with endocrine changes. In several species, including petrels, corticosterone levels are known to increase near fledging. Reduced food consumption has been shown to elevate corticosterone levels and decrease thyroxine levels in nestling birds. In nestling storm-petrels, levels of both corticosterone and thyroxine increased. Storm-petrels were found to secrete measurable levels of testosterone, but levels did not change during the prefledging period. Increased corticosterone levels might be part of an endocrine signal that initiates changes in feeding behavior, or may result from reduced food intake. Elevated thyroxine levels may be related to metabolic changes involved in mass loss. Future experimental work is needed to ascertain whether the described endocrine changes are responsible for, or result from, prefledging changes in petrel feeding behavior.

Gene duplication and divergence produce divergent MHC genotypes without disassortative mating.

Genes of the major histocompatibility complex (MHC) exhibit heterozygote advantage in immune defence, which in turn can select for MHC‐disassortative mate choice. However, many species lack this expected pattern of MHC‐disassortative mating. A possible explanation lies in evolutionary processes following gene duplication: if two duplicated MHC genes become functionally diverged from each other, offspring will inherit diverse multilocus genotypes even under random mating. We used locus‐specific primers for high‐throughput sequencing of two expressed MHC Class II B genes in Leachs storm‐petrels, Oceanodroma leucorhoa, and found that exon 2 alleles fall into two gene‐specific monophyletic clades. We tested for disassortative vs. random mating at these two functionally diverged Class II B genes, using multiple metrics and different subsets of exon 2 sequence data. With good statistical power, we consistently found random assortment of mates at MHC. Despite random mating, birds had MHC genotypes with functionally diverged alleles, averaging 13 amino acid differences in pairwise comparisons of exon 2 alleles within individuals. To test whether this high MHC diversity in individuals is driven by evolutionary divergence of the two duplicated genes, we built a phylogenetic permutation model. The model showed that genotypic diversity was strongly impacted by sequence divergence between the most common allele of each gene, with a smaller additional impact of monophyly of the two genes. Divergence of allele sequences between genes may have reduced the benefits of actively seeking MHC‐dissimilar mates, in which case the evolutionary history of duplicated genes is shaping the adaptive landscape of sexual selection.

Island life shapes the physiology and life history of eastern bluebirds (Sialia sialis).

Island organisms face a range of extrinsic threats to their characteristically small populations. Certain biological differences between island and continental organisms have the potential to exacerbate these threats. Understanding how island birds differ from their continental relatives may provide insight into population viability and serve as a predictive tool for conservation efforts. We compared an eastern bluebird population in Ohio with a threatened population in Bermuda in terms of the birds’ development, morphology, immunology, and reproduction. These comparisons revealed that island nestlings had shorter wings and island adults had longer wings than their continental analogs. Island nestlings also had shorter tarsi than continental nestlings at day 8 posthatch, but this difference was absent at day 15 and in adults. Adults weighed less in Bermuda than in Ohio, and both nestlings and adults in Bermuda exhibited higher levels of two immunological indexes (concentrations of an acute-phase protein and titers of nonspecific antibodies). Clutch sizes and hatch rates did not differ between the island and continental populations; however, as the breeding season progressed, brood sizes declined in Bermuda, whereas no such decline occurred in Ohio. Despite these differences and differences in nestling development, island and continental parents fed their nestlings at equal rates. Overall, our results suggest that the Bermuda phenotype may be adjusted to certain aspects of the island environment but not to others. Efforts to conserve the bluebirds of Bermuda may be improved by focusing on the intraseasonal patterns in nestling mortality and, more generally, the survival probabilities of different age classes.

Finding the best predictor of reproductive performance of Leach's Storm-Petrels

ABSTRACT Physiological and environmental factors shape foraging strategies and energy balance. For species that breed seasonally, physiological changes in an individual can have short-term effects, but also can persist as carry-over effects from one season to the next, such as from the overwintering season to the breeding season. We tested the hypothesis that reproductive performance could be predicted by diet and energy balance during the breeding and nonbreeding seasons in a long-lived seabird, the Leachs Storm-Petrel (Oceanodroma leucorhoa). Specifically, we predicted that better reproductive performance would be correlated with four factors: (1) a high-lipid diet, as indexed by a high C:N ratio in stable isotope analyses; (2) a diet rich in antioxidants, as indexed by high plasma antioxidant capacity; (3) foraging at a high trophic level, as indexed by high values of δ15N in stable isotope analyses, which is positively related to lipids; and (4) a positive long-term energy balance, revealed by low levels of corticosterone in feathers. Because of our interest in short-term effects vs. carry-over effects, stable isotope values were measured from two different tissue sources: erythrocytes, to test for short-term effects, and winter-grown feathers, to test for carry-over effects. We monitored reproductive performance through egg volume, chick growth, parental provisioning, and fledging success. Parents with more breeding experience were more likely to have a successful nest in 2010, but not in 2009. Individuals exhibited consistent egg volume and nonbreeding season feather δ15N values across the 2 years of our study, but, overall, neither diet nor feather corticosterone predicted reproductive performance. Nonetheless, our simple, noninvasive measures of breeding performance could be applied to other species to study life-history strategies and energy balance.