Peter H. Becker

Reproductive improvement and senescence in a long-lived bird

Heterogeneity within a population is a pervasive challenge for studies of individual life-histories. Population-level patterns in age-specific reproductive success can be broken down into relative contributions from selective disappearance, selective appearance of individuals into the study population, and average change in performance for survivors (average ontogenetic development). In this article, we provide an exact decomposition. We apply our formula to data on the reproductive performance of a well characterized population of common terns (Sterna hirundo). We show that improvements with age over most of adult life and senescence at old ages are primarily due to a genuine change in the mean among surviving individuals rather than selective disappearance or selective appearance of individuals. Average ontogenetic development accounts for approximately 87% of the overall age-specific population change.

A New Application for Transponders in Population Ecology of the Common Tern

We injected transponders subcutaneously to mark single Common Tern (Sterna hirundo) adults and all chicks of a colony at Wilhelmshaven with the aim of establishing a completely marked colony. We present details on the equipment and methods and report preliminary results. Microtagged terns can be identified for life, not only at their nest when breeding, but also at resting places by fixed antennas at distances of < 11 cm. Thus, nonbreeders can be identified as well. We also weighed terns remotely to obtain information on their body condition. Body mass data as well as identification codes were electronically stored. Preliminary data indicated that adult survival was ≥ 87% and subadult survival until age two was ≥ 20%.

Sex, age, experience and condition as factors affecting arrival date in prospecting common terns, Sterna hirundo

Abstract Several studies have shown that seabird colonies consist to a large extent of young nonbreeders (prospectors). These individuals appear at the colony later in the season than established breeders. The reasons for this late arrival have remained unclear in most cases, mainly because of technical difficulties in collecting sufficient data from nonbreeding individuals. We used a novel transponder system to identify remotely the members of a common tern colony, including nonbreeders, during eight breeding seasons and we combined the system with automatic balances. Ninety-two per cent of prospectors returned for the first time when 2 years old and 88.9% of recruits to the breeding population had spent at least one previous season at the colony as prospectors. In both sexes, most individuals prospected for one season, but more males than females prospected for more than one season, although a higher proportion of females started breeding without a previous prospecting phase. Terns arrived earlier in the season the older they were and the more experience of the colony they had, but experience proved to be more important than age. Prospectors gained about 3 weeks with a previous prospecting season whereas an additional year of age allowed birds to arrive only about 6 days earlier. Prospectors returning later in the season arrived with lower body masses. Males on average arrived earlier at the colony than females. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.

How Life History Influences Population Dynamics in Fluctuating Environments

A major question in ecology is how age-specific variation in demographic parameters influences population dynamics. Based on long-term studies of growing populations of birds and mammals, we analyze population dynamics by using fluctuations in the total reproductive value of the population. This enables us to account for random fluctuations in age distribution. The influence of demographic and environmental stochasticity on the population dynamics of a species decreased with generation time. Variation in age-specific contributions to total reproductive value and to stochastic components of population dynamics was correlated with the position of the species along the slow-fast continuum of life-history variation. Younger age classes relative to the generation time accounted for larger contributions to the total reproductive value and to demographic stochasticity in “slow” than in “fast” species, in which many age classes contributed more equally. In contrast, fluctuations in population growth rate attributable to stochastic environmental variation involved a larger proportion of all age classes independent of life history. Thus, changes in population growth rates can be surprisingly well explained by basic species-specific life-history characteristics.

Telomere length reflects phenotypic quality and costs of reproduction in a long-lived seabird

Telomere length is associated with cellular senescence, lifestyle and ageing. Short telomeres indicate poor health in humans and reduced life expectancy in several bird species, but little is known about telomeres in relation to phenotypic quality in wild animals. We investigated telomere lengths in erythrocytes of known-age common terns (Sterna hirundo), a migratory seabird, in relation to arrival date and reproductive performance. Cross-sectional data revealed that, independent of age, individuals with short telomeres performed better: they arrived and reproduced earlier in the season and had more chicks in the nest. The latter effect was stronger the older the brood and stronger in males, which do most of the chick provisioning. Longitudinal data confirmed this pattern: compared with birds that lost their brood, birds that raised chicks beyond the 10th nestling day experienced higher telomere attrition from one year to the next. However, more detailed analysis revealed that the least and most successful individuals lost the fewest base pairs compared with birds with intermediate success. Our results suggest that reproductive success is achieved at the expense of telomeres, but that individual heterogeneity in susceptibility to such detrimental effects is important, as indicated by low telomere loss in the most successful birds.

Dispersal decisions : common terns, Sterna hirundo, choose between colonies during prospecting

In long-lived bird species, a deferred onset of reproduction is assumed to be linked with a so-called prospecting phase when young individuals compare potential breeding sites before they decide to settle in the home area or to emigrate. However, this has rarely been documented with empirical data because of technical difficulties in collecting sufficient data from nonbreeding individuals at several sites. In the long-lived common tern, we used a novel transponder system to identify remotely all natal prospectors visiting two colonies of different size throughout the breeding season in 2001. Males attended the colonies about twice as frequently as females. Independently of sex, the proportion of birds recorded at both colonies was much higher among individuals born at the smaller colony. In the first half of their attendance time, individuals moved twice as often between colonies as in the second half. In both sexes, prospectors clearly favouring the larger colony were much more likely to breed there in the following season than prospectors with no clear colony choice, whereas no individual favouring the smaller colony during prospecting bred subsequently at the larger colony. Among birds born at the smaller colony, a higher proportion of female than of male prospectors attended mainly the larger colony, whereas among prospectors born in the larger colony, we did not find any sexual difference in attendance patterns and most birds favoured their home colony. The study suggests that prospectors select future breeding sites and that differences in philopatry between sexes are influenced by environmental quality.

Temporal and spatial pattern of common tern (Sterna hirundo) foraging in the Wadden Sea

In 1989, 313 foraging flights of common terns in the Wadden Sea were radio-tracked. The feeding trips lasted on average 115 min covering about 30 km per flight. Completely tracked flights had a mean radius of 6.3 km. The terns preferred distinct foraging areas in the Wadden Sea. These were visited at site-specific phases of the tidal cycle resulting in a temporal and spatial pattern of foraging, caused by the site-specific and tide-related fluctuations of food availability.

Seabirds as monitor organisms of contaminants along the German North Sea coast

In the 1980s, significant regional, interspecific and annual differences in contamination with toxic chemicals were found in eggs of breeding birds along the German North Sea coast. In some regions residue levels approached a range endangering breeding success, especially in terns. In 1987, a three-year monitoring-program for common ternSterna hirundo and oystercatcherHaematopus ostralegus was started, in order to recognize such contamination trends. Eggs from 9–14 areas along the coast are now being analysed for chlororganic residues and mercury. First results on geographical and year-to-year variation are presented here, and the advantages of seabird eggs as spatial and temporal monitors of marine pollution are discussed. The suggestion is made to include a shore-breeding bird species in the European monitoring programs.

Improvement in chick provisioning with parental experience in a seabird

Age-related differences in the reproductive success of birds may be the result of experience-dependent changes in foraging and parental skills. We tested this hypothesis in a long-lived seabird, the common tern, Sterna hirundo. Transponder-marked birds were observed while feeding their young (1) as first-time breeders (recruits) and (2) as experienced breeders in 2 consecutive years in a breeding colony at Port Wilhelmshaven, Lower Saxony, Germany. The influence of individual breeding experience on feeding rate, feeding success and food composition was investigated, including potential year effects, using multivariate statistics. In addition, we compared the breeding success of experienced and inexperienced breeders. Feeding rate was not linked to experience, whereas experienced breeders had a higher feeding success than recruits. Both experienced breeders and recruits fed their young with the same prey species, but in recruits the proportion of prey items with low energy content was higher. The latter was linked to a lower breeding success of recruits. Our study is, to our knowledge, the first to document an individual improvement in breeding performance as a reason for age-dependent improvement in reproductive success in birds after recruitment.

Archiving primary data: solutions for long-term studies

The recent trend for journals to require open access to primary data included in publications has been embraced by many biologists, but has caused apprehension amongst researchers engaged in long-term ecological and evolutionary studies. A worldwide survey of 73 principal investigators (Pls) with long-term studies revealed positive attitudes towards sharing data with the agreement or involvement of the PI, and 93% of PIs have historically shared data. Only 8% were in favor of uncontrolled, open access to primary data while 63% expressed serious concern. We present here their viewpoint on an issue that can have non-trivial scientific consequences. We discuss potential costs of public data archiving and provide possible solutions to meet the needs of journals and researchers.