Reduced Breeding Success Research Articles

Ecosystem responses to short-term climate variability in the Gulf of the Farallones, California

We conducted an integrated study from physics to upper trophic-level predators in the Gulf of the Farallones, California. We hypothesized that decreased zooplankton abundance for upper trophic-level predators in the Gulf of the Farallones during 2004 and 2005 was a response to reduced upwelling-favorable winds and primary production. Based on their trophic ecology, we hypothesized that planktivorous auklets and omnivorous murres will show differential responses to upwelling variability. We examined these hypotheses by analyzing time series on oceanographic variables associated to upwelling and the biological responses at low, mid and high trophic-levels. We found that reduced upwelling-favorable wind was correlated with anomalously high SST and low chlorophyll a concentration from July 2004 to August 2005. During 2005, low chlorophyll concentrations were related to reduced krill abundance in the upper water column and decreased seabird abundance in the vicinity of the breeding colony in the study area. Decreased krill abundance was associated with late timing of nesting and reduced breeding success, with auklets showing a more pronounced response. This study shows how short-term climate variability can affect primary through tertiary productivity, and supports an interpretation of “bottom-up” control of ecosystem dynamics.

Reproductive Senescence in a Long‐Lived Seabird: Rates of Decline in Late‐Life Performance Are Associated with Varying Costs of Early Reproduction

Evolutionary theories of senescence predict that rates of decline in performance parameters should be shaped by early-life trade-offs between reproduction and somatic maintenance. Here we investigate factors influencing the rate of reproductive senescence in a long-lived seabird, the common guillemot Uria aalge, using data collected over a 23-year period. In the last 3 years of life, individual guillemots had significantly reduced breeding success and were less likely to hold a site or attempt to breed. Females senesced at a significantly faster rate than males. At the individual level, high levels of reproductive output earlier in life were associated with increased senescence later in life. This trade-off between early- and late-life reproduction was evident independent of the fact that as birds age, they breed later in the season. The rate of senescence was additionally dependent on environmental conditions experienced earlier in life, with evidence that harsh conditions amplified later declines in breeding success. Overall, individuals with intermediate levels of early-life productivity lived longer. These results provide support for the antagonistic-pleiotropy and disposable-soma theories of senescence and demonstrate for the first time in a wild bird population that increased rates of senescence in reproductive performance are associated with varying costs of reproduction early in life.

Spatial heterogeneity of mesopredator release within an oceanic island system

Predator-prey communities are ubiquitous in ecology, but introduced predators can drive native species to extinction within island systems, prompting the eradication of such exotics. Ecological theory predicts that elimination of top-introduced predators from islands can lead to the counterintuitive decline of native prey populations through the ecological release of smaller introduced species in a process termed "mesopredator release." We show, in accordance with mesopredator release theory and counter to conservation goals for a New Zealand island reserve, that initial eradication of cats on Little Barrier Island led to reduced breeding success of Cook's petrels, which also are vulnerable to predation by a mesopredator, the Pacific rat. The rat's impact on prey productivity varied with elevation within the island. Rat eradication was followed by a rise in petrel productivity, in support of both ecological theory and practical conservation management goals. It appears that interactions among introduced predators, native prey, and environmental gradients can drive counterintuitive and spatially heterogeneous responses to predator eradications from islands. Location-specific, ecosystem-level understanding is essential for predicting the outcomes of such restoration management techniques.

Eggshell pigmentation indicates pesticide contamination

Summary Persistent pesticide residues remain a significant environmental pollutant, reducing breeding success of predatory birds even where such pesticides have been banned for many years. The ability to detect such residues inexpensively without destructive sampling would be a valuable tool for ecotoxicological monitoring. We suggest that assaying eggshell colour might provide such a tool. Recent studies have revealed two mechanisms affecting eggshell pigmentation in, and believed to be specific to, passerine birds. In the great tit Parus major, reddish protoporphyrin speckles on their otherwise white eggs coincide precisely with thin shell and are related to calcium shortage. In the pied flycatcher Ficedula hypoleuca, the intensity of the eggshell's blue‐green ground‐colour (pigment biliverdin) is related to the female's health (immunocompetence) at laying. We studied the effect of the insecticide DDT (notorious for reducing eggshell thickness in raptors by blocking calcium availability to the shell gland) on the eggshell pigmentation of Eurasian sparrowhawks Accipiter nisus (Falconiformes), whose eggshells show both protoporphyrin spotting and a blue‐green biliverdin‐based ground colour. Eggshells showing protoporphyrin spots as an internalized pigment layer also showed a strong correlation between DDE levels (DDT metabolite) and shell thickness. The shells were also significantly thinner at these spots. Eggshell thickness was uncorrelated with DDE levels in eggs that lacked such spotting. The green‐ness of the ground colour, related to biliverdin pigment, also increased with DDE levels but, unlike the protoporphyrins, this increase was independent of eggshell thickness. We also noted that the colour of eggs collected before 1930 was consistent with the environmental absence of DDT. Synthesis and applications. DDT occurrence in predatory birds indicates ecosystem‐wide contamination. Correlation between eggshell colour and DDT concentration indicates that pigmentation might be used as a non‐destructive assay for DDT (which is still in widespread use, particularly for combating malaria) in eggs. Further studies should allow our findings to be applied to other raptor species. Our results corroborate the associations of protoporphyrin pigmentation with shell thinning and biliverdin with female immunocompetence described for passerines, but their occurrence in a falconiform implies that these effects evolved long before the appearance of the order Passeriformes.

Impact of Forestry Practices on Fitness Correlates and Population Productivity in an Open‐Nesting Bird Species

In the boreal forests of Fennoscandia, over 99% of forest area has been altered by forestry practices, which has created forest with age structures and stand characteristics that differ from primary forest stands. Although many researchers have investigated how forestry affects species abundance, few have assessed how forestry practices affect fitness correlates of species living in altered habitats, and this has negatively affected management efforts. We experimentally addressed the effect of standard forestry practices on fitness correlates of an open-nesting, long-lived bird species typical to boreal forests of Eurasia, the Siberian Jay (Perisoreus infaustus). We used a before-after comparison of reproductive data on the level of territories and found that standard forestry practices had a strong negative effect on the breeding success of jays. Both partial thinning of territories and partial clearcutting of territories reduced future breeding success by a factor of 0.35. Forestry practices reduced territory occupancy. Thus, over the 15 years of the study, productivity of the affected population declined over 50% as a result of territory abandonment and reduced breeding success. Results of previous studies on Siberian Jays suggest that the strong effect of forest thinning on fitness is explained by the fact that most common predators of nests and adults are visually oriented and thus thinning makes prey and nests more visible to predators. The consequences of thinning we observed are likely to apply to a wide range of species that rely on understory to provide visual protection from predators. Thus, our results are important for the development of effective conservation management protocols and for the refinement of thinning practices.

Persistence of Forest Birds in the Costa Rican Agricultural Countryside

Understanding the persistence mechanisms of tropical forest species in human-dominated landscapes is a fundamental challenge of tropical ecology and conservation. Many species, including more than half of Costa Rica's native land birds, use mostly deforested agricultural countryside, but how they do so is poorly known. Do they commute regularly to forest or can some species survive in this human-dominated landscape year-round? Using radiotelemetry, we detailed the habitat use, movement, foraging, and nesting patterns of three bird species, Catharus aurantiirostris, Tangara icterocephala, and Turdus assimilis, by obtaining 8101 locations from 156 individuals. We chose forest birds that varied in their vulnerability to deforestation and were representative of the species found both in forest and human-dominated landscapes. Our study species did not commute from extensive forest; rather, they fed and bred in the agricultural countryside. Nevertheless, T. icterocephala and T. assimilis, which are more habitat sensitive, were highly dependent on the remaining trees. Although trees constituted only 11% of land cover, these birds spent 69% to 85% of their time in them. Breeding success of C. aurntiirostris and T. icterocephala in deforested habitats was not different than in forest remnants, where T. assimilis experienced reduced breeding success. Although this suggests an ecological trap for T. assimilis, higher fledgling survival in forest remnants may make up for lower productivity. Tropical countryside has high potential conservation value, which can be enhanced with even modest increases in tree cover. Our findings have applicability to many human-dominated tropical areas that have the potential to conserve substantial biodiversity if appropriate restoration measures are taken.

Contributions of Weather and Predation to Reduced Breeding Success in a Threatened Northern Loggerhead Shrike Population

Collister, D. M., and S. Wilson. 2007. Contributions of weather and predation to reduced breeding success in a threatened northern Loggerhead Shrike population. Avian Conservation and Ecology - Écologie et conservation des oiseaux 2(2): 11.http://dx.doi.org/10.5751/ACE-00193-020211

Factors Affecting the Between-Season Divorce Rate in the Urban Populations of the European BlackbirdTurdus merulain North-Western Poland

The between-season divorce rate of the European Blackbird was studied in 1997–2003 in two city parks in northwest Poland. Within the population studied, 52 and 51% of pairs in each park were observed to divorce. Of the eight parameters included in the analysis, the divorce rate was found to depend on marriage training and the time of territory acquisition. Territory quality, breeding success, synchrony in territory acquisition, age and size of the partners were unimportant. 19.5% of pairs with marriage training were observed to divorce, whereas 65.5% of pairs without marriage training did so. Birds acquiring their territories earlier divorced their partners significantly less frequently than those acquiring a territory at a later date. During the first post-divorce breeding season, young males that stayed in the park started breeding earlier than in the preceding year; old males started breeding later, whereas the difference was not significant in young or old females. Regardless of their age, the divorced birds showed a reduced number of fledglings raised with a new partner. The reduced breeding success may be a result of poor adaptation to increased pressure from predators. Similar divorce rates in pairs with and without breeding success and the rarer divorces among pairs claiming their territories earlier strongly support the habitat-mediated hypothesis. However, the more frequent desertion of poor territories by females, as well as differences found between older males and females in the timing of the onset of breeding before and after divorce, indicate that an individual may choose divorce to maximize fitness.

Increased adult mortality and reduced breeding success with age in a population of common guillemot Uria aalge using marked birds of unknown age

Although there is general consensus about the existence of senescence in vertebrates, empirical evidence of senescence in demographic parameters in wild populations is limited. Data on breeding success and survival of breeding common guillemots Uria aalge were collected over 20 years on the Isle of May (Scotland) using a pool of individuals marked as adults. Because the years of hatching of individuals were not known, we used the time (years) elapsed since first capture (TFC) as a measure of age. The use of this proxy did not create any bias in estimating senescence in the case of a linear decline, nor did it greatly decrease the power of a test for senescence. Breeding success declined significantly from 0.81 (95% CI: 0.77–0.84) to 0.62 (0.54–0.68) over the study period. It also varied in relation to age, initially increasing (from 0.62 (0.54–0.68) at TFC of 0 year to 0.76 (0.73–0.79) at TFC of 9 years) up to a plateau (from TFC of 9 years with 0.76 (0.73–0.79) until TFC of 13 years with 0.77 (0.74–0.79) before declining in later life to 0.70 (0.61–0.78) at TFC of 19 years). Survival was generally high and varied significantly from year to year. It also declined with TFC: survival of birds marked in 1982 decreased from 0.92 (0.85–0.96) at a TFC of 0 year to 0.88 (0.82–0.92) at a TFC of 19 years. Resighting probabilities also declined with TFC suggesting that the oldest birds do not come back to the colony to breed as regularly as younger individuals. These findings indicate that individual common guillemots on the Isle of May showed both actuarial and reproductive senescence.

Experimentally increased aggressiveness reduces population kin structure and subsequent recruitment in red grouse Lagopus lagopus scoticus

Summary According to the ‘territorial behaviour’ hypothesis, red grouse population cycles are caused by delayed density‐dependent changes in male aggressiveness influencing recruitment. These lagged changes in aggressiveness might be caused by changes in the kin structuring of male populations and differential aggressive behaviour between kin and non‐kin (‘kinship’ hypothesis). A population‐level manipulation of male aggressiveness in autumn affected the kin structure of male populations, their subsequent aggressiveness, and recruitment of both sexes. On two moors, we implanted the old territorial cocks in autumn with testosterone on an experimental area (T‐areas) and with sham implants on a control area (C‐areas). Increased aggressiveness in autumn t reduced recruitment in autumns t and t+ 1, and breeding density of both sexes in springs t+ 1 and t+ 2, confirming previous studies elsewhere. A new observation was that cocks on T‐areas had bigger combs (an ornament whose size is testosterone‐dependent) than those on C‐areas for at least 1·5 years after treatment, evidence that they remained more aggressive. Increased aggressiveness reduced not only subsequent density but also kin structuring among territorial cocks. This is consistent with the ‘kinship’ hypothesis that changes in the kin structure of male populations mediate year‐to‐year changes in male aggressiveness. Increased aggressiveness did not increase intensity of infection by the dominant intestinal nematode Trichostrongylus tenuis, which might have affected recruitment through reduced breeding success. Moreover, breeding success after treatment was no lower on the T‐ than on the C‐areas. The results show for the first time that increased aggressiveness affects both kin structure and subsequent recruitment, supporting a key assumption of the kinship hypothesis for red grouse population cycles.

Warmer springs advance the breeding phenology of golden plovers Pluvialis apricaria and their prey (Tipulidae)

Most studies of climate-driven changes in avian breeding phenology have focused on temperate passerines, yet the consequences of such environmental change may be more deleterious for other avian taxa, such as arctic and sub-arctic waders (Charadrii). We therefore examine large-scale climatic correlates of the breeding phenology of one such species (golden plover Pluvialis apricaria), and the timing of emergence of their adult tipulid prey, to assess the potential for climate change to disrupt breeding performance. Golden plover first-laying dates were negatively correlated with both March and April temperature, the mean laying date of first clutches was additionally negatively correlated with March rainfall. The timing of final laying dates were negatively correlated with April temperature only. The timing of tipulid emergence was negatively correlated with May temperature. In combination with historical climatic data, these models suggest a 9-day advancement of golden plover first-laying dates occurred during the 1990s, although this remains within the range of natural variation for the twentieth century. The magnitudes of predicted changes in mean and final laying dates, and the timing of tipulid emergence, were smaller. Climate predictions for 2070-2099 suggest potential advances in first-laying dates by 25 days, whilst the timings of mean and final laying dates are predicted to change by 18 days and 13 days, and tipulid emergence by 12 days. Given the importance of adult tipulids to young golden plover chicks, these changes may result in a mismatch between the timing of first-laying dates and tipulid emergence, so reducing the success of early breeding attempts. Modelling suggests that these changes could reduce breeding success in a South Pennines population by about 11%.

Low energy values of fish as a probable cause of a major seabird breeding failure in the North Sea

Many seabirds in the North Sea feed on lesser sandeels Ammodytes marinus during the breed- ing season. Unprecedented breeding failures were recorded at many seabird colonies on the east coast of Britain in 2004. We used demographic, dietary and behavioural data from a long-term study of a colony of common guillemots Uria aalge, the most abundant seabird species in the North Sea, to set the 2004 season in context. Birds at this colony showed greatly reduced breeding success and those chicks that did survive left the colony in very poor condition. The main prey item fed to chicks in 2004 was sprat Sprattus sprattus rather than sandeels, and parents increased the amount of time spent foraging, frequently leaving chicks unat- tended in order to maintain a normal feeding rate. The calculated daily food intake of chicks derived from these values did not differ markedly from previous years and therefore the magnitude of the impact on chick growth and breeding success appeared dispro- portionately large. However, nutrient analyses of fish collected from birds in 2004 revealed them to be of sig- nificantly lower energy value than expected. Poor food quality therefore appeared to be the proximate cause of seabird breeding failure in 2004 giving support to the 'junk-food' hypothesis. Single-prey loaders such as guillemots will be particularly sensitive to reductions in the energy value of food items. The reasons for the poor fish condition in this part of the North Sea are currently unknown, but the results provide further evidence of major changes in the North Sea food web.

Cowbird parasitism of Pale-headed Brush-finch Atlapetes pallidiceps: implications for conservation and management

Pale-headed Brush-finch Atlapetes pallidiceps is a restricted-range species that is threatened with extinction due to habitat loss. The total population of 60–80 individuals achieved a reproductive output of only 0.74 young per breeding pair in 2002. Brood parasitism by Shiny Cowbird Molothrus bonariensis was a major factor reducing breeding success, affecting 38.5% of broods. Parasitism rates reached 50% in an ungrazed reserve, but only 14% on an adjacent grazed plot. The resulting difference in breeding success was not, however, attributable to vegetation parameters used to describe microhabitat use. Cowbird parasitism rates therefore seem to be influenced largely by factors operating at the landscape level. These may include grazing scheme, topography, humidity and host availability. It is suggested that lower species diversity and bird abundance rendered the grazed site less attractive to cowbirds. Current parasitism rates are of great conservation concern due to the low population size of Pale-headed Brush-finch, and the initiation of controlling measures is pressing. Management options described from intensive cowbird control programmes in North America are reviewed and evaluated for their applicability here. To combine the possibility of further data collection with commencement of immediate conservation action, we consider two alternative approaches. Nest monitoring and cowbird egg removal would enable the study of the distribution of parasitism in relation to landscape and vegetation variables, whereas cowbird shooting and nest monitoring might provide a larger short-term benefit to reproductive output. Habitat management, resumption of some grazing in the reserve and cowbird removal should be considered for the intermediate future.

Effect of egg type on the estimation of nest predation in passerines

We compared predation rates of artificial nests baited with different egg types with the predation rates on nests of the Brown-and-yellow Marshbird (Pseudoleistes virescens), a Neotropical passerine common in the same area, in Argentina. We used deserted natural nests as experimental nests, baited with either Japanese Quail (Coturnix coturnix) or Shiny Cowbird (Molothrus bonariensis) eggs. Each experimental nest was baited with one natural and one paraffin-filled egg. Experimental nests baited with quail eggs suffered lower predation and egg losses than those baited with cowbird eggs. The main cause of egg loss and nest mortality in “cowbird nests” was pecking behavior of cowbirds. However, when we removed the effect of cowbirds, differences still held. In this area, the Shiny Cowbird is common and is the main cause of reduced breeding success of several host species. “Quail nests” had higher nest and egg survival than active Brown-and-yellow Marshbird nests. In contrast, “cowbird nests” and active Brown-and-yellow Marshbird nests had similar nest and egg survival. Experimental nests baited with natural passerine eggs such as those of Shiny Cowbirds seem to approximate the natural nest and egg predation risk for passerines in our study area.

Does supplementary calcium reduce the cost of reproduction in the Pied Flycatcher Ficedula hypoleuca?

Studies in acidified as well as in naturally base‐poor areas have recently revealed that availability of extra calcium‐rich food items is an important component of habitat quality affecting breeding performance in several bird species. However, these mostly short‐term studies have provided equivocal results concerning the exact consequences of calcium shortage on different species in different regions. We studied the effect of calcium availability on reproduction of the Pied Flycatcher Ficedula hypoleuca breeding in pine forests in Estonia, NE Europe, over a period of 4 years. Experimental pairs were provided with supplementary calcium‐rich material when breeding, while control pairs were left unsupplemented. Experimental females laid larger eggs and their nestlings had longer tarsi than those of controls. Moreover, the mass and condition of females tending larger than average clutches were increased by calcium‐supplementation. Our results provide the first experimental evidence that calcium availability may affect the overall cost of reproduction in free‐living passerines. We compared these results with similar data for the Great Tit Parus major, collected from the same area during the same study period. Great Tits responded to low calcium availability mainly by restrained reproductive behaviour and reduced breeding success, while Pied Flycatchers invested significantly more in current reproductive effort despite the increased cost of reproduction. Thus, the effects of calcium deficiency on birds seem to be species‐specific or population‐specific. This partly explains discrepancies between the results of earlier studies.

Comparative breeding biology of Hen Harrier and Montagu’s Harrier: an 8‐year study in north‐eastern France

Hen and Montagu’s Harriers breed in the same cultivated areas of eastern France. We present data from an extensive study conducted in three adjacent areas where 757 nests of the two harriers were monitored between 1993 and 2000, with the aim of comparing the breeding ecology of these two species and to evaluate their possible future trends. Breeding habitat for harriers consisted nearly exclusively of winter cereals, causing great conservation concern in this intensively farmed region. The Hen Harrier was almost absent from two of the study areas. This species showed significantly larger clutch size and higher breeding success than the Montagu’s Harrier, and an earlier breeding phenology. It was thus less adversely affected by harvesting activities than Montagu’s Harrier. Both species showed a reduced breeding success with increasing laying date. There was a large diet overlap between the two species, possibly leading to competitive interactions. Overall, the Montagu’s Harrier should be considered as the more vulnerable of the two species, necessitating conservation measures, such as protection of nests from early harvesting activities. Nevertheless, to maintain both species in agricultural habitats, farming practices that preserve sufficient food should also be promoted.

Birds Breeding in a Changing Farmland*

As a result of intensified agriculture, farmland in Europe is the habitat where the most pronounced changes have occurred in recent decades. In parallel, breeding populations of birds have been declining over much of Europe. Today, farmland has the largest proportion of Red-List species. This paper reviews studies on the impact of agriculture on birds breeding in the farmland ecosystems of Europe: breeding bird density in relation to farming practice, effects of agriculture on foraging and feeding ecology while rearing young, and consequences for breeding success.Specialised bird species are most affected by farming practice. They are rare or even absent in intensively managed farmland with a much reduced habitat and structural diversity. As crops grow fast and as their vegetation is very dense, large fields become inaccessible to or unprofitable for ground feeding birds. To exploit alternative food resources, parents feeding nestlings have to cover larger distances to isolated and distant food patches. Reduced food availability in modern farmland and increased time and energy costs of foraging may result in lower body conditions of parent birds and their broods, in a reduced breeding success and lower survival. Some species breeding in intensively farmed areas are at least locally unable to produce sufficient recruits to maintain their numbers in the long term.Our knowledge of the breeding ecology and population dynamics of farmland birds is growing, but it is still based mainly on short-term and small-scale case studies on a few species. Little is known about whether measures to improve habitat quality (e.g. set-asides) are adequate to halt negative trends in bird populations. Hence, there is a need for internationally coordinated scientific work with important implications for conservation.

Variable responses of seabirds to change in marine climate: California Current, 1985-1994

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 212:265-281 (2001) - doi:10.3354/meps212265 Variable responses of seabirds to change in marine climate: California Current, 1985-1994 Cornelia S. Oedekoven*, David G. Ainley, Larry B. Spear H. T. Harvey & Associates, 3150 Almaden Expwy, Suite 145, San Jose, California 95118, USA *Present address: Southwest Fisheries Science Center, La Jolla, California 92037-0271, USA. E-mail: coedekoven@yahoo.com ABSTRACT: We conducted annual ship-board surveys to determine the density and distribution of seabirds off central California in relation to marine climate, from 1985 to 1994. Summarized here are results for the sooty shearwater Puffinus griseus, the common murre Uria aalge, and Cassin¹s auklet Ptychoramphus aleuticus, the 3 most abundant seabirds in the central part of the California Current (91% of seabird abundance and biomass). During the study, sea-surface temperature, wind speed and thermocline depth all increased, salinity decreased and thermocline intensity (slope) showed no consistent trend. Periods of cool water and warm water, as well as offshore and inshore excursions of the shelfbreak front, alternated and were mediated by the Southern Oscillation and upwelling intensity. The responses to climate variation by the 3 seabird species were in accord with their respective morphologies and natural history patterns. All moved closer to shore and away from the shelf-break front (which also weakened). Abundance of the shallow-diving shearwater and auklet decreased dramatically, but those of the deeper-diving murre did not. The shearwater, which nests in the southern hemisphere and is the most mobile of the 3 seabirds, likely changed its migration routes and reduced its association with the California Current. The auklet, which breeds within the study area and lacks mobility, declined in number, most likely due to reduced breeding success and subsequent reduced population size. Remaining auklets moved away from the shelfbreak, but not as far inshore as the shearwater. The murre, which also breeds locally, is the most adaptable owing to its deeper-diving capabilities. It shifted distribution within the study area to feed on alternative prey found throughout the water column. KEY WORDS: Aleutian Low Pressure System · Cassin¹s auklet · Climate change · Common murre · El Niño · La Niña · Sooty shearwater · Upwelling Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 212. Online publication date: March 08, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.

Effects of agricultural intensification on the breeding success of corn buntings Miliaria calandra

1. Corn buntings Miliaria calandra have declined steeply in Britain and north‐western Europe since the mid‐1970s; changes in farming practice are believed to have been partly responsible. 2. We studied nesting corn buntings on the South Downs in west Sussex between 1995 and 1997 to examine the possible effects of agricultural intensification on breeding success. The abundance of invertebrates around individual nests was sampled by sweep‐netting in July. 3. Corn buntings provisioning nestlings foraged in grassy margins more than any other habitat relative to their availability within the maximum foraging range. The other habitats used more than expected were spring‐sown barley, unintensified grass and set‐aside. Those used less than expected included winter‐sown wheat and intensively managed grassland. The invertebrates most commonly fed to chicks were more abundant in foraging areas than elsewhere. Their density was negatively correlated with the number of insecticide applications both when cereal fields only were considered and when all foraging habitats were included. 4. The lower the abundance of chick‐food invertebrates close to nests, the greater the distance from the nest at which parents foraged, and the longer such trips were in duration. The weights of nestlings, corrected for age using tarsus length, were positively correlated with the abundance of chick‐food invertebrates. 5. The probability of nest survival was negatively correlated with the abundance of chick‐food invertebrates close to the nest, apparently as a result of an increased risk of predation. 6. Agricultural intensification in Britain, including the increased use of pesticides, has led to a widespread decrease in the availability of chick‐food invertebrates on lowland farmland. If our results are typical of corn buntings in an arable environment, this decrease correlates with reduced breeding success. Depending on the mortality rates for fledged chicks and older birds, this reduction may have contributed to the corn buntings' decline and may hamper recovery. 7. Farming practices that increase invertebrate availability ought to benefit breeding corn buntings. Large‐scale measures such as set‐aside and the spring‐sowing of cereals (especially if undersown with grass) depend heavily on overall agricultural policy. Small‐scale initiatives might therefore be more feasible; these include the provision of grassy margins or beetle banks and selective spraying of headlands.

Experimentally activated immune defence in female pied flycatchers results in reduced breeding success.

Traditional explanations for the negative fitness consequences of parasitism have focused on the direct pathogenic effects of infectious agents. However, because of the high selection pressure by the parasites, immune defences are likely to be costly and trade off with other fitness-related traits, such as reproductive effort. In a field experiment, we immunized breeding female flycatchers with non-pathogenic antigens (diphtheria-tetanus vaccine), which excluded the direct negative effects of parasites, in order to test the consequences of activated immune defence on hosts' investment in reproduction and self-maintenance. Immunized females decreased their feeding effort and investment in self-maintenance (rectrix regrowth) and had lower reproductive output (fledgling quality and number) than control females injected with saline. Our results reveal the phenotypic cost of immune defence by showing that an activated immune system per se can lower the host's breeding success. This may be caused by an energetic or nutritional trade-off between immune function and physical workload when feeding young or be an adaptive response to 'infection' to avoid physiological disorders such as oxidative stress and immunopathology.