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The intraspecific variability of nest construction behavior in birds underlies environmental—as well as builder-dependent drivers. To draw correct conclusions about the bird’s nest construction investment, an accurate assessment of the nest size or weight is crucial. The weight of a nest depends on the material collected by the builder but may still increase over time with the presence of chicks, for instance if fecal material accumulates. Here, we provide evidence from a nesting box population of European Pied Flycatchers (Ficedula hypoleuca) that nest weight was positively correlated to hatchling number when weighed post-fledging. We use a novel approach of sorting, washing, and sieving the nesting material to reduce the effects of the chick-rearing phase on nest weight. Finally, we compare nest height and nest bottom thickness as indirect measurements of nest size to nest weight. Both nest height and nest bottom thickness became significantly smaller throughout the course of breeding and chick-rearing, which is why these parameters should be measured right after nest completion. While neither pre-incubation nest height nor nest weight (post-sieving) was related to number of eggs or number of hatchlings, we found a significant positive correlation between nest weight before sieving and number of hatchlings, showing the impact of chick presence on nest weight. Based on our data, we want to highlight the importance of an accurate and well-timed assessment of nest size and weight to facilitate an accurate characterization of the investment of birds into nest construction.

The breeding success of a nest-box breeding population of Pied Flycatcher Ficedula hypoleuca in subalpine birch forest in Swedish Lapland was studied over 55 years, with emphasis on the impact of small mustelids and ambient temperature during egg laying, incubation, and brood care. The proportion of nests predated was used as an index for mustelid abundance. Breeding failures were either total, due to breeding interruptions, or partial due to partial hatching failure or nestling death. Overall, mustelid abundance explained the rate of breeding interruptions at any breeding phase, but not when seasons with exceptional (>10%) predation rates were excluded. Variation in the rate of partial failure was unrelated to mustelid abundance. Climatic conditions affected clutch size, rate of breeding interruption, and partial loss of clutches or broods, with particularly pronounced effects during the incubation period. The mustelid abundance and weather conditions also influenced rates of nest-box occupation in subsequent breeding seasons, and the next-box population dynamics covaried with surrounding populations.

RETURN RATES of 4,178 female and 1,565 male breeding Pied Flycatchers ringed in 1965-2017 were studied near the species’ upper elevation and climatic limit in northern Sweden. Female return rate was 7.5 % in the season subsequent to the first breeding season. Having returned once, 38 % continued to return the next three seasons. Corresponding return rates of males were 27 % and 39 %. Female return rate decreased with more than 30 % during the study period whereas that of males did not decrease. This difference was probably due to increased mortality during the non-breeding season that selectively struck females after the 1970s. Local factors affected return rates in both sexes. Return rate was positively correlated with breeding success in females but negatively in males, whereas it was correlated with nest predation in the opposite way. Predation by mustelids accounted for a significant part of female return rate. Females that had returned once were continuously faithful to the former breeding site. Males showed faithfulness only after having returned twice.

Many bird populations in temperate regions have advanced their timing of breeding in response to a warming climate in recent decades. However, long-term trends in temperature differ geographically and between seasons, and so do responses of local breeding populations. Data on breeding bird phenology from subarctic and arctic passerine populations are scarce, and relatively little data has been recorded in open-nesting species. We investigated the timing of breeding and its relationship to spring temperature of 14 mainly open-nesting passerine species in subarctic Swedish Lapland over a period of 32 years (1984-2015). We estimated timing of breeding from the progress of post-juvenile moult in mist-netted birds, a new method exploring the fact that the progress of post-juvenile moult correlates with age. Although there was a numerical tendency for earlier breeding in most species (on average -0.09 days/year), changes were statistically significant in only three species (by -0.16 to -0.23 days/year). These figures are relatively low compared with what has been found in other long-term studies but are similar to a few other studies in subarctic areas. Generally, annual hatching dates were negatively correlated with mean temperature in May. This correlation was stronger in long-distance than in short-distance migrants. Although annual temperatures at high northern latitudes have increased over recent decades, there was no long-term increase in mean temperature in May over the study period at this subarctic site. This is probably the main reason why there were only small long-term changes in hatching dates.

Diet shifts are common in mammals and birds, but little is known about how such shifts along the food web affect contaminant exposure. Voles are staple food for many mammalian and avian predators. There is therefore a risk of transfer of contaminants accumulated in voles within the food chain. Osmium is one of the rarest earth elements with osmium tetroxide (OsO4 ) as the most toxic vapor-phase airborne contaminant. Anthropogenic OsO4 accumulates in fruticose lichens that are important winter food of bank voles (Myodes glareolus). Here, we test if a) anthropogenic osmium accumulates in bank voles in winter, and b) accumulation rates and concentrations are lower in autumn when the species is mainly herbivorous. Our study, performed in a boreal forest impacted by anthropogenic osmium, supported the hypotheses for all studied tissues (kidney, liver, lung, muscle and spleen) in 50 studied bank voles. In autumn, osmium concentrations in bank voles were even partly similar to those in the graminivorous field vole (Microtus agrestis: n =3D 14). In autumn but not in late winter/early spring, osmium concentrations were generally negatively correlated with body weight and root length of the first mandible molar, i.e. proxies of bank vole age. Identified negative correlations between organ-to-body weight ratios and osmium concentrations in late winter/early spring indicate intoxication. Our results suggest unequal accumulation risk for predators feeding on different cohorts of bank voles. 

Most large rivers in northern Sweden are regulated to produce hydropower, with subsequent effects on flow dynamics and aquatic insect communities. Several studies have shown that aquatic and terrestrial systems are intimately connected via the export of emergent aquatic insects, but few have assessed how human modifications of aquatic habitats may influence this connection. We compared breeding success of the insectivorous Pied Flycatcher Ficedula hypoleuca in near-riparian upland forests along two regulated and two free-flowing large rivers in northern Sweden over 3years. The regulated rivers showed lower aquatic insect export to the surroundings, as a consequence of regulation-induced loss of suitable aquatic insect habitats. Survival of Pied Flycatcher nestlings was 1015% greater along the free-flowing rivers. Females breeding near the free-flowing rivers also started egg-laying earlier and with greater synchrony than those at the regulated rivers, and showed a smaller decrease in weight during breeding than did females along the regulated rivers. However, there were no differences in occupation rate, clutch size or number of successfully hatched juveniles between regulated and free-flowing rivers. As regulated rivers showed lower abundance of flying aquatic insects, which may also reduce the abundance of terrestrial invertebrate prey, regulation-induced changes in the export of emergent aquatic insects may explain both directly and indirectly the observed reduction in Pied Flycatcher breeding success along regulated rivers. Large-scale river regulation may therefore impair the breeding success of insectivorous birds through impacts on prey availability.

A nest-box breeding population of Pied Flycatcher Ficedula hypoleuca in subalpine birch forest at Ammarnäs in northern Sweden was studied in 1965–2010. The population showed on average a significantly decreasing trend during the initial 25 years, after which it remained on a static level. The yearly variation of the population size was significantly correlated with breeding result in preceding years, which in turn was affected by e.g. nest predation, adverse climatic factors, clutch size, and incidence of defective egg shell formation. It is also suggested that the population decline during the initial 10–15 study years followed from over-establishment of the breeding habitat. The average breeding result was 2.3 fledglings per pair, which is far below the number needed for the population to be self-reproducing. The occurrence of unusually high rates of nest predation and defective egg shells seems to be passing phenomena, predicted to approach zero after about 30 and 60 years, respectively, after the start of the study. But even with these factors eliminated and with other conditions remaining unchanged, the Ammarnäs population will continue to be a sink population.

The environment around metal industries, such as smelters, is often highly contaminated due to continuous deposition of metals. We studied nest box breeding populations of pied flycatchers (Ficedula hypoleuca) in a well-studied pollution gradient from a sulfide ore smelter in Northern Sweden, after reduced aerial metal emissions (by 93-99%) from the smelter. The deposition of arsenic, cadmium, copper and zinc (based on moss samples) reflected the reduced emissions fairly well. However, nestling pied flycatchers had similar concentrations of these elements and mercury in tissues (bone, liver and blood) and feces in the 2000s, as in the 1980s, when the emissions were substantially higher. The exposure to high metal concentrations in the close vicinity of the smelter resulted in inhibited ALAD activities, depressed hemoglobin and hematocrit levels and increased mortality of nestlings. Our results indicate that in the highly contaminated environment around the smelter, nestlings reflected the slowly cycling soil pool, rather than the atmospheric deposition, and the concentration in soils plays an important role for the response of pied flycatchers to reduced atmospheric deposition.

Osmium tetroxide (OsO4) is one of the most toxic air contaminants but its environmental effects are poorly understood. Here, for the first time, we present evidence of osmium uptake in a common herbivore (bank vole, Myodes glareolus) in boreal forests of northern Sweden. Voles (n = 22) and fruticose arboreal pendular lichens, the potential main winter food source of the vole, were collected along a spatial gradient to the west of a steelwork in Tornio, Finland at the Finnish-Swedish border. Os-187/Os-188 isotope ratios increased and osmium concentrations decreased in lichens and voles along the gradient. Osmium concentrations in lichens were 10,000-fold higher than those in voles. Closest to the steelwork, concentrations were highest in kidneys rather than skin/fur that are directly exposed to airborne OsO4. The kidney-to-body weight ratio was higher at the two localities close to the steelwork. Even though based on a small sample size, our results for the first time demonstrate that osmium is taken up, partitioned, and accumulated in mammal tissue, and indicate that high kidney-to-body weight ratios might be induced by anthropogenic osmium.

Mining activities affect the surrounding environment by increasing exposure to metals. In this study, metal accumulation and its effects on reproduction and health of pied flycatcher (Ficedula hypoleuca) nestlings were monitored before and up to five years after a lead mine and enrichment plant closed down. The lead concentration in moss, nestling blood, liver and feces all indicated decreased lead exposure by at least 31% after closure, although only blood lead decreased significantly. Although the birds responded fairly well to the changed atmospheric deposition (based on moss samples), concentrations were still higher compared with birds in a reference area, and breeding was affected at the mine (smaller clutches and higher mortality). Surviving nestlings suffered from lower hemoglobin levels, mean cell hemoglobin concentrations and inhibited delta-aminolevulinic acid dehydratase activity. Lead poisoning contributed to poor health and adverse reproductive effects, but other factors (e.g. increased parasitic load) probably also affected the birds.