High variation reduces the value of feather stable isotope ratios in identifying new wintering areas for aquatic warblers Acrocephalus paludicola in West Africa

Stable isotope analysis of feathers can be useful in the study of seasonal interactions and migratory connectivity in birds. For the Palaearctic–African migration system, however, the lack of isotope data from feathers of known origin in Africa renders the geographic assignment of birds captured on European breeding grounds to potential wintering areas problematic. Rectrices of the threatened aquatic warbler Acrocephalus paludicola grown in Africa were sampled across six European countries to assess whether birds in different breeding populations shared similar isotopic signatures and so were likely to have wintered in the same region in Africa. Freshly grown feathers of aquatic warblers collected at the only known wintering site in Senegal showed high variation in carbon, nitrogen, and hydrogen isotope ratios. Due to similarly high variation in isotope ratios of African‐grown feathers within all breeding populations, it was not possible to determine whether different populations wintered in different regions. However, isotope signatures of 20% of birds captured on European breeding grounds fell outside the range of those captured in Senegal, suggesting a wider wintering distribution than is currently known. We therefore assessed whether the origin of these feathers could be estimated by trying to establish isotopic gradients across sub‐Saharan West Africa. Feathers of three ecologically similar surrogate species were sampled from wetlands across a 3000 km east‐west and a 2000 km north–south transect. Within‐site variation in feather isotope ratios was frequently larger than the difference predicted by gradients across West Africa. Thus, predicting the origin of individual feathers using single‐isotope gradients was not reliable. The large within‐site variability of feather isotope ratios of a habitat specialist species like the aquatic warbler indicates that using feather isotope ratios will require large sample sizes from many locations, and may thus not be an efficient tool in identifying wintering areas of Palaearctic–African migrants.     

Philopatry of winter moult area in migratory Great Reed Warblers Acrocephalus arundinaceus demonstrated by stable isotope profiles

Stable carbon- (δ¹³C), nitrogen- (δ¹⁵N) and hydrogen (δD) isotope profiles in feathers of migratory Great Reed Warblers Acrocephalus arundinaceus recaptured for 2 or more years in 6 successive years were examined to test whether the isotope profiles of individual warblers appeared to be consistent between years. Similar isotopic signatures in successive years suggested that individual birds tended to return and grow their feathers in Afro-tropical wintering habitats that generate similar δ¹³C, δ¹⁵N and δD signatures. Previous studies have shown that Great Reed Warblers exhibit strong natal and breeding philopatry, with most of the surviving birds returning to the breeding site. The present study of feather δ¹³C, δ¹⁵N and δD isotopic values demonstrate the year-to-year fidelity might also include the African moulting sites in this migratory species.     

Stable isotopes identify the natal origins of a generalist brood parasite, the brown-headed cowbird Molothrus ater

Identifying the natal origins of brood parasites is a major challenge that usually requires labor-intensive searching for nests of host species. Stable isotope analysis of feathers and other body tissues of parasitic young could be a possible tool for determining natal origins if tissues reflect the isotopic composition of the diet fed to nestlings. We measured the carbon (¹³C) and nitrogen (¹⁵N) isotope compositions of feathers for two age-classes of brown-headed cowbirds Molothrus ater at the Konza Prairie Biological Station near Manhattan, Kansas: nestlings raised by five species of songbird hosts in two different habitats, and juveniles captured after independence. Isotope values from cowbird nestlings did not differ among host species and we were unable to assign juvenile cowbirds to their natal hosts. However, nestlings raised in grassland habitat had feathers that contained significantly higher δ¹³C values and lower δ¹⁵N values than nestlings raised in shrub habitats. In addition, independent juveniles had isotopic signatures that were similar to cowbird nestlings raised on shrub habitats. Although dickcissel Spiza americana comprised the majority of samples from shrub habitats, our conclusions reflect the natural pattern of parasitism at the site and should be representative of cowbirds raised at Konza. We conclude that stable isotope analysis of feathers is effective for determining the natal origins of parasitic young if isotope values from nestlings are isotopically distinct among habitats.     

Influence of fine-scale habitat structure on nest-site occupancy,laying date and clutch size in Blue Tits Cyanistes caeruleus

Most birds have specific habitat requirements for breeding. The vegetation structure surrounding nest-sites is an important component of habitat quality, and can have large effects on avian breeding performance. We studied 13 years of Blue Tit Cyanistes caeruleus population data to determine whether characteristics of vegetation structure predict site occupancy, laying date and number of eggs laid. Measurements of vegetation structure included the density of English Oak Quercus robur, European Beech Fagus sylvatica, and other deciduous, coniferous and non-coniferous evergreen trees, within a 20-m radius of nest-boxes used for breeding. Trees were further sub-divided into specific classes of trunk circumferences to determine the densities for different maturity levels. Based on Principal Component Analysis (PCA), we reduced the total number of 17 measured vegetation variables to 7 main categories, which we used for further analyses. We found that the occupancy rate of sites and the number of eggs laid correlated positively with the proportion of deciduous trees and negatively with the density of coniferous trees. Laying of the first egg was advanced with a greater proportion of deciduous trees. Among deciduous trees, the English Oak appeared to be most important, as a higher density of more mature English Oak trees was associated with more frequent nest-box occupancy, a larger number of eggs laid, and an earlier laying start. Furthermore, laying started earlier and more eggs were laid in nest-boxes with higher occupancy rates. Together, these findings highlight the role of deciduous trees, particularly more mature English Oak, as important predictors of high-quality preferred habitat. These results aid in defining habitat quality and will facilitate future studies on the importance of environmental quality for breeding performance.     

Carry‐over effects of winter habitat vary with age and sex in yellow warblers Setophaga petechia

We use stable isotope data to investigate the role of winter habitat use in altering the breeding phenology of yellow warblers Setophaga petechia. We first confirm that δ¹³C and δ¹⁵N isotopic signatures vary with winter habitat use in this species. We then examine the relationship between winter habitat use, breeding phenology and productivity within four age‐sex‐classes, since life history theory would predict that carry‐over effects should vary with age and gender. The δ¹³C signatures of yellow warblers using riparian habitats over winter were more depleted than the signatures of those using agricultural or scrub habitat. Individuals on the Pacific coast of Mexico were also more δ¹⁵N enriched than those on the southern Gulf of Mexico. δ¹³C and δ¹⁵N signatures were only correlated with earlier clutch initiation and subsequent higher productivity in first‐breeding‐season females. We estimate that shifts in δ¹³C equivalent to a shift from scrub to riparian winter habitat would be associated with the production of 0.8 more fledglings by yearling females. Pre‐breeding events that influence the timing of breeding could also influence the reproductive performance of older males and females, but we found little evidence that winter habitat use influenced breeding season phenology in these birds.     

Does avian malaria infection affect feather stable isotope signatures?

It is widely accepted that stable isotope ratios in inert tissues such as feather keratin reflect the dietary isotopic signature at the time of the tissue synthesis. However, some elements such as stable nitrogen isotopes can be affected by individual physiological state and nutritional stress. Using malaria infection experiment protocols, we estimated the possible effect of malaria parasite infections on feather carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope signatures in juvenile common crossbills Loxia curvirostra. The birds were experimentally infected with Plasmodium relictum (lineage SGS1) and P. ashfordi (GRW2), two widespread parasites of passerines. Experimental birds developed heavy parasitemia of both parasites and maintained high levels throughout the experiment (33 days). We found no significant difference between experimental and control birds in both δ¹³C and δ¹⁵N values of feathers re-grown. The study shows that even heavy primary infections of malaria parasites do not affect feather δ¹³C and δ¹⁵N isotopic signatures. The results of this experiment demonstrate that feather isotope values of wild-caught birds accurately reflect the dietary isotopic sources at the time of tissue synthesis even when the animal’s immune system might be challenged due to parasitic infection.     

Stable isotope ratios in winter‐grown feathers of Great Reed Warblers Acrocephalus arundinaceus,Clamorous Reed Warblers A. stentoreus and their hybrids in a sympatric breeding population in Kazakhstan

Analyses of the stable isotope composition of feathers can provide significant insight into the spatial structure of bird migration. We collected feathers from Great Reed Warblers Acrocephalus arundinaceus, Clamorous Reed Warblers A. stentoreus and a small sample of their hybrids in a sympatric breeding population in Kazakhstan to assess natural variation in stable isotope signatures and delineate wintering sites. The Great Reed Warbler is a long‐distance migrant that overwinters in sub‐Saharan Africa, whereas the Clamorous Reed Warbler performs a short‐distance migration to the Indian sub‐continent. Carbon (δ¹³C), nitrogen (δ¹⁵N) and deuterium (δD) isotope signatures were obtained from winter‐grown feathers of adult birds. There were highly significant differences in δD and less significant differences in δ¹³C between Great and Clamorous Reed Warblers. Thus, our results show that the stable isotope technique, and in particular the deuterium (δD) signal, resolves continental variation in winter distribution between these closely related Acrocephalus species with sympatric natal origin. The isotope signatures of hybrid Great × Clamorous Reed Warblers clustered with those of the Great Reed Warblers. Hence, a parsimonious suggestion is that the hybrids undergo moult in Afrotropical wintering grounds, as do the Great Reed Warblers. The observed δD values fell within the range of expected values based on available precipitation data collected at precipitation stations across the wintering continents of each species. However, the power to predict the winter origin of birds in our study system using these data was weak as the expected values ranged widely at this broad continental scale.     

Carotenoid plumage hue and chroma signal different aspects of individual and habitat quality in tits

We hypothesized that Blue Tits Cyanistes caeruleus and Great Tits Parus major from low quality habitat (small woods) would have less yellow ventral plumage than those from high quality habitat (large woods) because they moult faster and/or their diet contains fewer carotenoids. They moult faster because they moult later in the season and are subject to more rapidly shortening daylengths. We tested this using a database of the plumage coloration (chroma, hue and lightness) of birds breeding in woods of different sizes, by manipulating the speed of moult in captive Blue Tits, and by counting the abundance and size of caterpillars (the major source of dietary carotenoids) in the diet of nestlings. In accordance with our hypothesis, juveniles of both species (which moult about three weeks later than adults) were about 8% less saturated in colour (lower chroma) than adults, but there was no significant difference in chroma between habitats. However, both species did differ significantly in hue between large and small woods. Blue Tits forced to moult faster in captivity, at a rate similar to that caused by a month's delay in the start of moult, had yellow flank feathers that were 32% less saturated in colour than those allowed to moult more slowly. Blue Tit nestlings in large woods consumed 47% more caterpillar flesh (per gram of faecal material voided) than those in small woods, and Great Tit pulli 81% more. When habitat effects were controlled for in ANOVAs, Blue Tits mated assortatively on the basis of flank hue and Great Tits on the basis of flank lightness. Flank colour therefore has the capacity to provide information about the potential quality of both habitats, and individual birds, to potential colonists and sexual partners.     

Spatial heterogeneity of trophic pathways in the invertebrate community of a temperate bog

Summary 1. To examine spatial heterogeneity of trophic pathways on a small scale (<5 m diameter), we conducted dual stable isotope (δ¹³C and δ¹⁵N) analyses of invertebrate communities and their potential food sources in three patchy habitats [sphagnum lawn (SL), vascular‐plant carpet (VC) and sphagnum carpet] within a temperate bog (Mizorogaike Pond, Kyoto, Japan). 2. In total, 19 invertebrate taxa were collected from the three habitats, most of which were stenotopic, i.e. collected from a single habitat. Amongst the habitats, significant variation was observed in the isotopic signatures of dominant plant tissues and their detrital matter [benthic particulate organic matter (BPOM)], both of which were potential organic food sources for invertebrates. Site‐specific isotopic variation amongst detritivores was found in δ¹³C but not in δ¹⁵N, reflecting site‐specificity in the isotopic signatures of basal foods. The eurytopic hydrophilid beetle Helochares striatus was found in all habitats, but showed clear site variation in its isotopic signatures, suggesting that it strongly relies on foods within its own habitat. 3. The most promising potential foods for detritivores were the dead leaf stalks of a dominant plant in the VC and BPOM in the SL and carpet. An isotopic mixing model (IsoSource version 1.3.1) estimated that aquatic predators rely on unknown trophic sources with higher δ¹³C than detritus, whereas terrestrial predators forage on allochthonous as well as autochthonous prey, suggesting that the latter predators might play key roles in coupling between habitats. 4. Our stable isotope approach revealed that immobile detritivores are confined to their small patchy habitats but that heterogeneous trophic pathways can be coupled by mobile predators, stressing the importance of habitat heterogeneity and predator coupling in characterising food webs in bog ecosystems.     

Linking isotopic and migratory patterns in a pelagic seabird

The value of stable isotope analysis in tracking animal migrations in marine environments is poorly understood, mainly due to insufficient knowledge of isotopic integration into animal tissues within distinct water masses. We investigated isotopic and moult patterns in Cory’s shearwaters to assess the integration of different stable isotopes into feathers in relation to the birds’ transoceanic movements. Specimens of Mediterranean Cory’s shearwater Calonectris diomedea diomedea caught accidentally by Catalan longliners were collected and the signatures of stable isotopes of C (δ¹³C), N (δ¹⁵N) and S (δ³⁴S) were analysed in 11 wing and two tail feathers from 20 birds, and in some breast feathers. Based on isotopic signatures and moult patterns, the feathers segregated into two groups (breeding and wintering), corresponding to those grown in the Mediterranean or Atlantic regions, respectively. In addition, feathers grown during winter, i.e. moulted in Atlantic waters, were grouped into two isotopically distinct profiles, presumably corresponding to the two main wintering areas previously identified for Mediterranean Cory’s shearwater in tracking studies. N signatures mainly indicated the Mediterranean-to-Atlantic migration, whereas C and S signatures differed according to the Atlantic wintering area. Our results indicate that isotopic signatures from distant oceanic regions can integrate the feathers of a given bird and can indicate the region in which each feather was grown. This study thus underscores how stable isotope analysis can link marine animals to specific breeding and wintering areas, and thereby shed new light on studies involving assignment, migratory connectivity and carry-over effects in the marine environment. Xavier Ruiz deceased 27 April 2008.     

Carotenoid-based coloration correlates with the hatching date of Blue Tit Cyanistes caeruleus nestlings

Carotenoid-based coloration occurs predominantly in adult birds, yet in some species from the family Paridae, this trait is also present at the nestling stage. One of the factors proposed to affect the expression of this trait in immature birds is hatching date. Here, using the avian tetrahedral colour space model, we examined the influence of hatching date on the breast carotenoid-based plumage coloration of the Blue Tit Cyanistes caeruleus nestlings. Because Blue Tits are sexually dichromatic, we also investigated the potential interaction between hatching date and sex that could arise from differences in condition dependence of this trait between males and females. We found a positive relationship between UV chroma of breast feathers and hatching date. The amount of UV reflectance is thought to be negatively related to carotenoid content in feathers. The observed increase of UV chroma through the breeding season might therefore be caused by a seasonal decline in the availability and quality of Lepidoptera larvae – the main source of carotenoids in food of the Tits. We also observed a sex difference in the relationship between brightness of breast feathers (achromatic, structural component) and hatching date, which in males was negative and in females not significant. Our study provides further evidence that the timing of breeding is related to the expression of nestling carotenoid-based coloration, a potentially meaningful element of offspring–parent communication, and suggests a sex-specific effect of hatching date on its structural component.     

The diet of great tit nestlings: Comparing observation records and stable isotope analyses

The diet of wild animals has been studied using many different strategies, approaches and methods in recent decades. In this regard, stable isotopes analysis (SIA) is becoming a widespread tool, but no study has yet, to our knowledge, compared diet estimations from SIA with direct observations of the diet of passerine nestlings. Accordingly, our aim was to test the predictive power of SIA for this purpose and identify potential confounding factors such as habitat effects. To do this, we compared isotopic signatures of δ¹³C and δ¹⁵N in the feathers of great tit (Parus major) nestlings, and the corresponding estimates of their diet based on stable isotope mixing models, with prey proportions delivered by their parents obtained through video-recordings. Between-nest differences in isotopic signatures of δ¹⁵N were larger than within-nest differences. We found that δ¹⁵N signatures of nestling feathers correlated positively with the proportion of spiders and negatively with the proportion of caterpillars in the nestlings’ diet, the most important prey types. On the other hand, between-nest and within-nest differences in δ¹³C ratios were of similar magnitude and δ¹³C ratios correlated mainly with the proportion of trees surrounding nest-boxes that were Quercus spp. Estimates of diet composition based on mixing models correlated with the observed nestling diet, yet effect sizes were quite low. Although mixing models are commonly used to ascertain diets, our data show that they can provide valuable information on the relative intake of prey types from different trophic levels; but when complex dietary patterns are recorded (e.g. due to the confounding effects of habitat and/or temporal variation) it can be difficult to draw firm conclusions about diet composition.     

Parid foraging choices in urban habitat and their consequences for fitness

Urban environments are habitat mosaics, often with an abundance of exotic flora, and represent complex problems for foraging arboreal birds. In this study, we used compositional analysis to assess how Blue Tits Cyanistes caeruleus and Great Tits Parus major use heterogeneous urban habitat, with the aim of establishing whether breeding birds were selective in the habitat they used when foraging and how they responded to non‐native trees and shrubs. We also assessed whether they showed foraging preferences for certain plant taxa, such as oak Quercus, that are important to their breeding performance in native woodland. Additionally, we used mixed models to assess the impact of these different habitat types on breeding success (expressed as mean nestling mass). Blue Tits foraged significantly more in native than non‐native deciduous trees during incubation and when feeding fledglings, and significantly more in deciduous than evergreen plants throughout the breeding season. Great Tits used deciduous trees more than expected by chance when feeding nestlings, and a positive relationship was found between the availability of deciduous trees and mean nestling mass. Overall, the breeding performance of both species was poor and highly variable. Positive relationships were found between mean nestling mass and the abundance of Quercus for Great Tits, but not for Blue Tits. Our study shows the importance of native vegetation in the complex habitat matrix found in urban environments. The capacity of some, but not all, species to locate and benefit from isolated patches of native trees suggests that species vary in their response to urbanization and this has implications for urban ecosystem function.     

Habitat and diet variability of two coexisting tit species in central European forests

Capsule: Blue Tits and Great Tits occupied different habitats within forests in Central Europe but their nestlings shared a similar diet.

Aims: To quantify the differences in offspring diet and territory habitat between Great Tits Parus major and Eurasian Blue Tits Cyanistes caeruleus in two European forests, and to test whether the ecological niches of the two species overlap.

Methods: Research was conducted on Great Tits and Eurasian Blue Tits, breeding in nest boxes in two forests near Kraków, Poland, during years: 2009, 2011 and 2012. Nine days after hatching, food items were collected from offspring using neck-collars. Habitat parameters surrounding each nest box were quantified.

Results: Great Tit territories were in old Oak-Hornbeam forest, whereas Blue Tits often nested in mixed forest. There were no significant differences between the two bird species in the variation in their caterpillar diets for which both species were highly variable. Great Tits collected more caterpillars of Noctuidae per nest than did Blue Tits in 2009 and 2011 in Niepo?omice Forest; Blue Tits collected more Tortricidae in 2011 and more spiders every year. In Krzyszkowice Forest in 2012, tits fed their nestlings in different periods and did not differ in the proportion of caterpillars. Habitat affected diet differently in each species.

Conclusion: Although Great Tits and Blue Tits occupied different territories in each forest and year of research, the diets of both species’ nestlings contained similar species of invertebrates. The overlaps of the birds’ environmental needs are specific at a local scale.     

Discriminating geographic origins of migratory waders at stopover sites: insights from stable isotope analysis of toenails

In this study we test the potential of stable isotope analysis to reveal wintering origins of waders mixing at stopover sites, using the dunlin Calidris alpina as a case study. We determined stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope signatures of toenails of dunlins captured during winter at reference sites along the East‐Atlantic Flyway, from Mauritania to the United Kingdom. Afterwards, during spring migration, dunlins were sampled at the Tagus estuary, Portugal, and assigned to their wintering grounds according to their stable isotope signatures. Toenails from wintering dunlins at different sites had significantly different δ¹³C and δ¹⁵N signatures, despite some overlap in isotopic carbon ratios of birds from Morocco, Portugal and the UK. Among birds sampled during migration in Portugal, we found a clear bimodal pattern in δ¹³C values, corresponding to passage migrants from Mauritania (enriched δ¹³C values) and wintering birds from the Tagus estuary (depleted δ¹³C values). The first passage migrants from Mauritania appeared at the Tagus estuary by the end of March, with peak numbers during late April and early May. Our study provides evidence that isotopic signatures of toenails can play a determinant role in tracing the wintering origins of migrant dunlins at their stopover areas. Toenails, instead of feathers, can be the powerful and innovating tissue to sample in wader studies, allowing to bridge the gap in the field of migratory connectivity between sites used in different phases of the life cycle of waders.     

Regional patterns of recruitment success and failure in two endemic California oaks

Oak woodlands and savannas are key defining landscapes in the California Floristic Province, making up almost a quarter of the region's forests and woodlands. Two endemic Californian oak species, valley oak (Quercus lobata Neé) and blue oak (Quercus douglasii Hook. & Arn.), are widely considered at risk of decline from persistent recruitment failures in the last century. However, decades of research have produced no definitive conclusion about the existence, extent, or causes of this ‘regeneration problem’. Underlying causes of perceived recruitment failure are unclear and could include drivers at distribution‐wide to local scales including climate and atmospheric changes, habitat fragmentation, altered herbivore populations, changing fire regimes, exotic plant and animal invasions, livestock grazing, and soil conditions altered by past land uses. We performed meta‐analyses of existing stand‐scale data from the published and grey literatures on seedling and sapling recruitment in blue and valley oaks throughout each of their distributions. We sought to evaluate whether distribution‐wide regeneration ‘problems’ exist for either species and to assess what factors correlate with distribution‐wide recruitment patterns. Nearly 80% of sites surveyed for blue oaks but fewer than 50% of sites surveyed for valley oaks contained some evidence of seedling or sapling recruitment. A majority of sites surveyed for both species appear to have insufficient recruitment to replace adult populations, though further demographic work would be required to quantify minimum replacement recruitment rates. Reserve sites were seven times more likely than non‐reserve sites to contain valley oak populations with evidence of recruitment. Blue oak recruitment patterns were weakly related to climate and geographical factors and strongly variable at subregional scales. We suggest several lines of additional research that could fill gaps in the existing literature and clarify the patterns emerging from this analysis.     

Skuas (Stercorarius spp.) moult body feathers during both the breeding and inter‐breeding periods: implications for stable isotope investigations in seabirds

Seabirds are mostly thought to moult during the inter‐breeding period and the isotopic values of their feathers are often therefore assumed to relate to their assimilated diet during such periods. We observed Brown Skuas Stercorarius antarcticus lonnbergi and South Polar Skuas Stercorarius maccormicki moulting on a breeding site at King George Island, Antarctica. This raises concerns about the reliability of using stable isotopes in feathers to infer feeding localities of birds during the inter‐breeding period. We analysed the δ¹³C and δ¹⁵N values of growing and fully grown body feathers collected from the same individuals. For both species, δ¹³C values of growing feathers indicated feeding areas in the Antarctic zone (breeding grounds), whereas most fully grown feathers (100% for South Polar Skuas and 93.3% for Brown Skuas) could be assigned to northern latitudes (non‐breeding grounds). However, a few fully grown body feathers of Brown Skuas (6.7% of the feathers, belonging to two birds) showed isotopic values that indicated moult in the Antarctic zone. As the growth period of those feathers was unknown, they could not be used with confidence to depict the foraging behaviour of the birds during the non‐breeding period. Although precautions must be taken when inferring dietary information from feathers in seabirds where the moulting pattern is unknown, this study shows that if the development stage of a feather (growing/fully grown) is identified, then dietary information from both breeding and non‐breeding seasons can be obtained on the same individual birds.     

Incorporation of marine-derived nutrients from petrel breeding colonies into stream food webs

1. Stable isotope ratios of aquatic invertebrates, aquatic mosses and leaves of riparian plants were used to determine whether marine‐derived nutrients from breeding colonies of the Westland petrel (Procellaria westlandica) were incorporated into the food webs of small streams in New Zealand. 2. The δ¹⁵N signatures of all plants and animals examined were higher by 3.6–4.6‰ in small streams draining catchments with petrel colonies than in nearby streams where petrels were absent. δ¹³C values of leaves from terrestrial plants were also enriched by about 2‰ where petrels were present, but the carbon ratios of aquatic species were depleted in ¹³C, rather than enriched, suggesting that any marine signal was over‐ridden by isotopic shifts related to photosynthetic fractionation. 3. A high marine‐nitrogen signal was maintained along the 3 km length of Scotchman Creek with the δ¹⁵N values of leptophlebiid mayflies and predatory insects ranging from 7.4–9.5 and 9.2–11.9‰, respectively. 4. Most nutrients derived from petrels are likely to be translocated to streams via the soil, which they enter in the form of excreta, spilled food, feathers, dead chicks, and abandoned eggs. However, because changes in δ¹⁵N values are brought about by soil processes such as volatilisation of ammonia, nitrification and denitrification, it is difficult to predict the exact isotope signature of nitrogen entering a stream. Tentative estimates of the proportion of marine‐derived nitrogen in stream biota, calculated using a mass‐balance approach, ranged from 28–38%. 5. Our findings indicate that marine nutrients transported inland by seabirds can be incorporated into the food webs of streams. In pre‐human times when there were many more seabird colonies on mainland New Zealand than exist today, marine‐derived nutrients introduced by birds may have had significant effects on nitrogen cycling and the productivity of New Zealand streams.     

Pair bond and breeding success in Blue Tits Parus caeruleus and Great Tits Parus major

Data from 939 nests of the Blue Tit Parus caeruleus and 1008 nests of the Great Tit P. major from nestboxes provided in superabundance in mixed forest study sites between 1976 and 2001 were analysed to examine the effects of mate retention on breeding success and the relationship between mate fidelity and site fidelity. Most birds retained their former partner (76% in Great Tits and 65% in Blue Tits). The probability of a pair divorcing was affected by male age in Great Tits, divorce being more likely in pairs with first‐year males. Great Tit pairs breeding together for a second season bred earlier, but had no higher breeding success than pairs breeding together for the first time. In Blue Tits laying date and start of incubation tended to be earlier in pairs breeding together for a second season, but hatching and fledging dates were not earlier than in other pairs. Great Tit pairs breeding together for two consecutive seasons bred earlier in the second season than in the first, but breeding success did not differ significantly between years. In both species, breeding performance did not differ between pairs that divorced after a season and pairs that stayed together. Thus breeding success did not determine whether a pair divorced or bred together again. Neither Blue Tits nor Great Tits improved their breeding performance through divorce. Blue Tit females even had fewer fledglings in the year after divorce than in the year before. Mate retention affected breeding site fidelity. Blue Tit females had greater breeding dispersal distances between consecutive years when re‐mating than when breeding again with the same mate. In Great Tits both males and females dispersed more when re‐mating than when retaining the former partner, suggesting that mate retention increased the chance of retaining the breeding site. In both species, breeding dispersal distances did not differ between pairs that divorced and pairs in which one mate disappeared. Because no major advantage of mate retention was evident, we suggest that mate retention evolved under different conditions than those found in study sites with high breeding densities and a superabundance of artificial nesting sites.     

Differences in the onset of laying between island and mainland Mediterranean Blue Tits Parus caeruleus: phenotypic plasticity or genetic differences?

Blue tits Parus caeruleus breeding in deciduous or in evergreen woodlands on the mainland of Europe start to lay 3–4 weeks earlier than Blue Tits on the island of Corsica breeding in evergreen woodlands. A similar difference in average laying date between the two Blue Tit populations has been found in a relatively small sample of captive birds held in outdoor aviaries on the mainland. To provide data on the habitat contrast in Corsica for comparison with that on the mainland, a new study area was started in one of the few deciduous woodlands on Corsica. In addition, new data on laying dates of captive Blue Tits were gathered to increase the sample size of independent pairs in aviaries. This study shows that the habitat effect (deciduous ν evergreen woodland) on the average laying date of Blue Tits is relatively small in comparison with the geography effect (European mainland ν Corsica). The laying date differences between mainland and insular Blue Tits persisted whatever the type of habitat (deciduous, evergreen or outdoor aviaries on the mainland). The data support the hypothesis that the observed differences in the onset of laying between the mainland and insular Blue Tits were mainly influenced by genetic effects and relatively little by non-genetic maternal effects of by genotype—environment interactions.