Beat Naef-Daenzer

The design of artificial nestboxes for the study of secondary hole-nesting birds: a review of methodological inconsistencies and potential biases

Abstract. The widespread use of artificial nestboxes has led to significant advances in our knowledge of the ecology, behaviour and physiology of cavity nesting birds, especially small passerines. Nestboxes have made it easier to perform routine monitoring and experimental manipulation of eggs or nestlings, and also repeatedly to capture, identify and manipulate the parents. However, when comparing results across study sites the use of nestboxes may also introduce a potentially significant confounding variable in the form of differences in nestbox design amongst studies, such as their physical dimensions, placement height, and the way in which they are constructed and maintained. However, the use of nestboxes may also introduce an unconsidered and potentially significant confounding variable due to differences in nestbox design amongst studies, such as their physical dimensions, placement height, and the way in which they are constructed and maintained. Here we review to what extent the characteristics of artificial nestboxes (e.g. size, shape, construction material, colour) are documented in the ‘methods’ sections of publications involving hole-nesting passerine birds using natural or excavated cavities or artificial nestboxes for reproduction and roosting. Despite explicit previous recommendations that authors describe in detail the characteristics of the nestboxes used, we found that the description of nestbox characteristics in most recent publications remains poor and insufficient. We therefore list the types of descriptive data that should be included in the methods sections of relevant manuscripts and justify this by discussing how variation in nestbox characteristics can affect or confound conclusions from nestbox studies. We also propose several recommendations to improve the reliability and usefulness of research based on long-term studies of any secondary hole-nesting species using artificial nestboxes for breeding or roosting.

Radio‐tracking reveals that lesser horseshoe bats (Rhinolophus hipposideros) forage in woodland

Over the past 50 years European populations of the lesser horseshoe bat Rhinolophus hipposideros have severely declined, probably because of the loss of foraging habitat. To date, studies of the foraging behaviour of this species have been limited as its low mass (4‐8 g) precluded the use of radio-telemetry because commercially available radio-transmitters exceeded 10% of its body mass. In this study, radiotransmitters weighing < 0.35 g were built. These increased the body mass of the animals from 4.5% to 8.1%, with no demonstrable adverse effect on their flight behaviour. The habitat selection of eight female lesser horseshoe bats was studied in Monmouthshire, U.K. The bats had foraging ranges between 12 and 53 ha (100% kernel). Although one bat foraged 4.2 km from the roost, for 50% of the time tracked bats were recorded within 600 m of the nursery roost. The estimated density within 200 m of the roost was 5.8 foraging bats/ha. This decreased to 0.01 bats/ha at 1200 m. Compositional analysis revealed that this species used woodlands, predominately broadleaf, more than any other habitat. In addition, the bats foraged in areas of high habitat diversity. Conservation management of this species should concentrate on such areas within 2.5 km of the nursery roost.

FITNESS CONSEQUENCES OF PRE‐ AND POST‐FLEDGING TIMING DECISIONS IN A DOUBLE‐BROODED PASSERINE

The fitness consequences of a delayed timing of breeding are expected to affect the temporal characteristics of the whole annual breeding system. One major problem in quantifying the fitness relevance of timing is that individual differences between pairs may cause the seasonal trend. Differentials in juvenile survival due to pre-fledging timing decisions often only appear after fledging of the chicks. Therefore, timing decisions in the post-fledging period, i.e., the duration of parental care, might additionally influence juvenile survival. We tested the effects of timing and parental competence on the post-fledging survival of second-brood juvenile Barn Swallows (Hirundo rustica L.) by swapping earlier and later hatching clutches and radio-tracking the juvenile subjects. The mark-recapture models controlled for the effects of duration of post-fledging care and food availability. There was an annually varying negative seasonal trend in offspring survival that was associated with environmental conditions. Directional selection for early breeding occurred in the two years with scarce autumnal food supply. Furthermore, we found strong selection for long post-fledging parental care. The duration of care neither declined seasonally, nor did longer care compensate for the seasonal decline of juvenile survival. Hence, the reproductive output three weeks after fledging was determined by two parental timing decisions: the timing of breeding and the timing of family breakup. We suggest that differential survival of second-brood fledglings in relation to these decisions is an important part of the selective mechanisms shaping the reproductive system of Barn Swallows.

Nocturnal exploratory flights, departure time, and direction in a migratory songbird

Stopover studies have concentrated so far mostly on mechanisms regulating the temporal organisation on the day-to-day level. Taking advantage of the small and isolated island of Helgoland in the North Sea, we investigated the stopover and departure behaviour of a nocturnal migrant by using radio telemetry. Special attention was paid particularly to nocturnal behaviour, their departure times within the night, and departure directions. Here, we show that Northern Wheatears, Oenanthe oenanthe, performed regularly nocturnal exploratory flights on nights before and on departure night, which might be a common behaviour of nocturnal migrants to evaluate meteorological conditions aloft prior to departure. We proposed that migrants being prepared for an endurance flight would depart early in the night within a short time window, whereas individuals departing with low fuel load would be less prone to take off early. Our data, however, could not support this hypothesis. In respect of the migratory direction, there was a significant correlation between departure direction and departure fuel load. Northern Wheatears with high departure fuel loads headed more towards the north than lean migrants, which departed mostly towards the nearest coastline, i.e. east to south. Thus, birds with high fuel loads showed their seasonally appropriate migratory direction irrespective of the ecological barrier ahead, whereas lean birds avoided this direction. To our knowledge, this is the first study that investigates the relationship of fuel load and departure direction in a free-flying songbird.ZusammenfassungIn Studien zum Rastverhalten von Zugvögeln wurden bis jetzt hauptsächlich die Mechanismen untersucht, die die zeitliche Organisation des Rastverhaltens auf der Ebene von Tagen steuern. Wir haben das Rast- und Abzugsverhalten eines Nachtziehers mit Hilfe von Radiotelemetrie untersucht und uns dabei die isolierte Lage der kleinen Nordseeinsel Helgoland zu Nutze gemacht. Besondere Aufmerksamkeit galt dem nächtlichen Verhalten, der Abzugszeit in der Nacht, und der Abzugsrichtung. Hier zeigen wir, dass Steinschmätzer Oenanthe oenanthe regelmäßig nächtliche Erkundungsflüge in Nächten vor und während der Abzugsnacht durchführten, was ein typisches Verhalten von Nachtziehern sein könnte, um die Windbedingungen in verschiedenen Höhen zu testen. Wir nahmen an, dass Zugvögel, die ausreichende Reserven für einen Langstreckenflug hatten, früh in der Nacht während eines relativ engen Zeitfensters abziehen würden, wohingegen Individuen mit geringeren Energiereserven wahrscheinlich früh aber auch spät in der Nacht abziehen. Unsere Ergebnisse konnten diese Hypothese allerdings nicht bestätigen. Zwischen der Abzugsrichtung und den Energievorräten beim Abzug bestand ein signifikanter Zusammenhang. Steinschmätzer mit hohen Energiereserven flogen in nördlichere Richtungen als magere Vögel, die zum größten Teil in Richtung der nächstgelegenen Küste, d.h. nach Osten bis Süden, abzogen. Also zeigten Vögel mit großen Energiereserven ihre jahreszeitlich angemessene Abzugsrichtung unabhängig von der vor ihnen liegenden ökologischen Barriere, während magere Vögel diese Richtung vermieden. Unseres Wissens ist dies die erste Studie, die den Zusammenhang zwischen Energievorräten und Abzugsrichtung an Singvögeln im Freiland untersucht.

Survival benefits of post-fledging care: experimental approach to a critical part of avian reproductive strategies.

1. Caring for offspring beyond leaving the nest is an important but under-studied part of avian life histories. Theory predicts that prolonged post-fledging parental care should yield fitness benefits such as increased fledgling survival. Post-fledging care is also costly in terms of time and energy available for subsequent reproduction, moult or migration. So far, direct measurements of the fitness effects of the duration of post-fledging parental care are lacking. 2. In a partial cross-fostering experiment, barn swallow (Hirundo rustica) chicks were exchanged among broods close to fledging. Thereby, we separated the effects of post-fledging care from those of pre-fledging origin on juvenile survival. 3. Prolonging post-fledging care substantially increased juvenile survival up to 3 weeks post-fledging. Juvenile mortality was maximal in the days following the termination of parental care, and prolonging care delayed and reduced this peak mortality. Survival of fledglings experiencing 6 days of care was Phi = 0.227, whereas fledglings experiencing 14 days of care showed a survival of Phi = 0.571. 4. Offspring from pairs providing short care showed lower post-fledging survival than did offspring from pairs providing long care, irrespective of the actual duration of care experienced. This gives evidence for an additional survival effect of pre-fledging factors associated with the parental duration of care. 5. The results suggest that differential survival in relation to post-fledging parental care is a major fitness component. This relationship has profound effects on the reproductive trade-offs underlying the evolution of avian life histories.

Stopover optimization in a long-distance migrant: the role of fuel load and nocturnal take-off time in Alaskan northern wheatears (Oenanthe oenanthe).

IntroductionIn long-distance migrants, a considerably higher proportion of time and energy is allocated to stopovers rather than to flights. Stopover duration and departure decisions affect consequently subsequent flight stages and overall speed of migration. In Arctic nocturnal songbird migrants the trade-off between a relatively long migration distance and short nights available for travelling may impose a significant time pressure on migrants. Therefore, we hypothesize that Alaskan northern wheatears (Oenanthe oenanthe) use a time-minimizing migration strategy to reach their African wintering area 15,000 km away.ResultsWe estimated the factors influencing the birds’ daily departure probability from an Arctic stopover before crossing the Bering Strait by using a Cormack-Jolly-Seber model. To identify in which direction and when migration was resumed departing birds were radio-tracked. Here we show that Alaskan northern wheatears did not behave as strict time minimizers, because their departure fuel load was unrelated to fuel deposition rate. All birds departed with more fuel load than necessary for the sea crossing. Departure probability increased with stopover duration, evening fuel load and decreasing temperature. Birds took-off towards southwest and hence, followed in general the constant magnetic and geographic course but not the alternative great circle route. Nocturnal departure times were concentrated immediately after sunset.ConclusionAlthough birds did not behave like time-minimizers in respect of the optimal migration strategies their surplus of fuel load clearly contradicted an energy saving strategy in terms of the minimization of overall energy cost of transport. The observed low variation in nocturnal take-off time in relation to local night length compared to similar studies in the temperate zone revealed that migrants have an innate ability to respond to changes in the external cue of night length. Likely, birds maximized their potential nightly flight range by taking off early in the night which in turn maximizes their overall migration speed. Hence, nocturnal departure time may be a crucial parameter shaping the speed of migration indicating the significance of its integration in future migration models.

Equal nonbreeding period survival in adults and juveniles of a long‐distant migrant bird

In migrant birds, survival estimates for the different life-history stages between fledging and first breeding are scarce. First-year survival is shown to be strongly reduced compared with annual survival of adult birds. However, it remains unclear whether the main bottleneck in juvenile long-distant migrants occurs in the postfledging period within the breeding ranges or en route. Quantifying survival rates during different life-history stages and during different periods of the migration cycle is crucial to understand forces driving the evolution of optimal life histories in migrant birds. Here, we estimate survival rates of adult and juvenile barn swallows (Hirundo rusticaL.) in the breeding and nonbreeding areas using a population model integrating survival estimates in the breeding ranges based on a large radio-telemetry data set and published estimates of demographic parameters from large-scale population-monitoring projects across Switzerland. Input parameters included the country-wide population trend, annual productivity estimates of the double-brooded species, and year-to-year survival corrected for breeding dispersal. Juvenile survival in the 3-week postfledging period was low (S = 0.32; SE = 0.05), whereas in the rest of the annual cycle survival estimates of adults and juveniles were similarly high (S > 0.957). Thus, the postfledging period was the main survival bottleneck, revealing the striking result that nonbreeding period mortality (including migration) is not higher for juveniles than for adult birds. Therefore, focusing future research on sources of variation in postfledging mortality can provide new insights into determinants of population dynamics and life-history evolution of migrant birds.

Habitat use and foraging ecology of the nightjar (Caprimulgus europaeus) in the Swiss Alps: towards a conservation scheme

The European nightjar is one of the most endangered bird species in Switzerland. As its ecology in the country is poorly understood, we collected data on resource exploitation in the upper Rhone valley (Alps). The diet of two adult birds and one nestling consisted primarily of moths (81 and 93% of biomass), which were also the most abundant prey sampled at the study site. Three radiotracked nightjars selectively exploited oak scrubland compared to vineyards and pine forests; vineyard monocultures harbour presumably insufficient moth populations, whereas dense pine stands probably do not provide the flying and foraging requirements of nightjars. The survival of nightjars in Valais probably depends both on the existence of sufficient populations of moths and on the availability of semi-open natural habitats, such as oak scrubland, which seem to offer the best suitable foraging and nesting places.

Temperature characteristics of winter roost-sites for birds and mammals: tree cavities and anthropogenic alternatives.

The microclimate of potential roost-sites is likely to be a crucial determinant in the optimal roost-site selection of endotherms, in particular during the winter season of temperate zones. Available roost-sites for birds and mammals in European high trunk orchards are mainly tree cavities, wood stacks and artificial nest boxes. However, little is known about the microclimatic patterns inside cavities and thermal advantages of using these winter roost-sites. Here, we simultaneously investigate the thermal patterns of winter roost-sites in relation to winter ambient temperature and their insulation capacity. While tree cavities and wood stacks strongly buffered the daily cycle of temperature changes, nest boxes showed low buffering capacity. The buffering effect of tree cavities was stronger at extreme ambient temperatures compared to temperatures around zero. Heat sources inside roosts amplified Δ T (i.e., the difference between inside and outside temperatures), particularly in the closed roosts of nest boxes and tree cavities, and less in the open wood stacks with stronger circulation of air. Positive Δ T due to the installation of a heat source increased in cold ambient temperatures. These results suggest that orchard habitats in winter show a spatiotemporal mosaic of sites providing different thermal benefits varying over time and in relation to ambient temperatures. At cold temperatures tree cavities provide significantly higher thermal benefits than nest boxes or wood stacks. Thus, in winter ecology of hole-using endotherms, the availability of tree cavities may be an important characteristic of winter habitat quality.

Effects of radio-tag characteristics and sample size on estimates of apparent survival

BackgroundRadio-tracking is increasingly used to assess key characteristics of population dynamics. Since in many species re-encounter rates are frequently below 1.0 and vary with time and/or life-history stages, known-fate approaches to analyses may not apply. Cormack-Jolly-Seber (CJS) models estimate apparent survival on the basis of individual encounter histories. These models allow for complex re-encounter models and constitute an ideal tool to estimate apparent survival where re-encounter rates vary. However, the implications of radio-tag characteristics and sample size on the precision of survival estimates and the potential to determine temporal variation in survival have rarely been investigated. Here we analyze radio-tracking data from juvenile barn swallows (Hirundo rustica) that were instrumented with four types of transmitters similar in mass but differing in radiated power (n = 560, 132 broods).ResultsFor all transmitter types re-encounter probabilities varied between 0.2 and 0.75, depending on radiated power and bird age. Higher-power transmitters significantly improved the re-encounter rates and thus, the precision of survival estimates. Apparent survival varied with age, with a minimum around the break-up of families. Later on survival increased again and approached the rate of adults. Analyses of random sub-samples revealed that sample size strongly affected the variance in survival estimates, and thus the power in discerning temporal or between-group variation in survival. Small samples substantially underestimated the survival to independence. In small subsamples the standard errors of estimates were particularly large in later re-encounters. Consequently, model selection results of different survival models on the basis of small random sub-samples were highly inconsistent.ConclusionsInvestigating population processes requires modeling of time- and cohort-dependent survival rates, often for short time periods. We show that CJS estimates of apparent survival from small samples revealed rough patterns in barn swallow survival with samples of c. 50 individuals. However, small samples underestimated the number of survivors reaching independence. Inference on underlying mechanistic models based on Akaike’s Information Criterion (AIC) model selection was unreliable with sample sizes below 200 individuals. As samples are often limited for practical reasons, maximizing re-encounter rates by optimal choice of telemetry hardware and field logistics is a way to increase the precision of survival estimates.