Single-brooded Species Research Articles

The biotic and abiotic drivers of timing of breeding and the consequences of breeding early in a changing world

Abstract The decision of when to breed is an important determinant of individual fitness. However, despite a multitude of studies investigating the intraspecific relationship between timing of breeding and reproductive performance, less is known about why the strength of this relationship varies between species. Furthermore, environmental change has the potential to alter the relationship between lay date and fitness, but there is still a limited understanding of what mechanisms drive these differential responses to change environmental conditions. We propose that the potential effects of environmental change on the relationship between timing of breeding and fitness are dependent on 2 primary factors: (1) the potential constraints imposed by breeding early and (2) the drivers of higher fitness of early breeders. We first summarize multiple hypotheses proposed to explain why breeding early, either based on absolute date or relative to conspecifics, increases fitness. We then summarize the factors that may constrain when individuals initiate breeding, including limits on the ability to advance their lay date or extend the length of their breeding season under favorable conditions. Understanding constraints on the timing of breeding allows for the identification of obligate (single-brooded species that do not attempt to breed after a specific date) and facultative (predominantly multi-brooding species that have long breeding seasons) early breeding species that are likely differently affected by climate change. Finally, we propose a simple mathematical formula that incorporates the costs and benefits associated with early breeding to quantify how climate change could influence the benefits of early breeding and either mitigate or exacerbate the costs. Our cost-benefit approach provides a clear framework to predict how species may shift the timing of their breeding to maximize fitness in a changing world.

The effect of climate change on avian offspring production: A global meta-analysis

Climate change affects timing of reproduction in many bird species, but few studies have investigated its influence on annual reproductive output. Here, we assess changes in the annual production of young by female breeders in 201 populations of 104 bird species (N = 745,962 clutches) covering all continents between 1970 and 2019. Overall, average offspring production has declined in recent decades, but considerable differences were found among species and populations. A total of 56.7% of populations showed a declining trend in offspring production (significant in 17.4%), whereas 43.3% exhibited an increase (significant in 10.4%). The results show that climatic changes affect offspring production through compounded effects on ecological and life history traits of species. Migratory and larger-bodied species experienced reduced offspring production with increasing temperatures during the chick-rearing period, whereas smaller-bodied, sedentary species tended to produce more offspring. Likewise, multi-brooded species showed increased breeding success with increasing temperatures, whereas rising temperatures were unrelated to reproductive success in single-brooded species. Our study suggests that rapid declines in size of bird populations reported by many studies from different parts of the world are driven only to a small degree by changes in the production of young.

A review of predation as a limiting factor for bird populations in mesopredator-rich landscapes: a case study of the UK.

The impact of increasing vertebrate predator numbers on bird populations is widely debated among the general public, game managers and conservationists across Europe. However, there are few systematic reviews of whether predation limits the population sizes of European bird species. Views on the impacts of predation are particularly polarised in the UK, probably because the UK has a globally exceptional culture of intensive, high-yield gamebird management where predator removal is the norm. In addition, most apex predators have been exterminated or much depleted in numbers, contributing to a widely held perception that the UK has high numbers of mesopredators. This has resulted in many high-quality studies of mesopredator impacts over several decades. Here we present results from a systematic review of predator trends and abundance, and assess whether predation limits the population sizes of 90 bird species in the UK. Our results confirm that the generalist predators Red Fox (Vulpes vulpes) and Crows (Corvus corone and C. cornix) occur at high densities in the UK compared with other European countries. In addition, some avian and mammalian predators have increased numerically in the UK during recent decades. Despite these high and increasing densities of predators, we found little evidence that predation limits populations of pigeons, woodpeckers and passerines, whereas evidence suggests that ground-nesting seabirds, waders and gamebirds can be limited by predation. Using life-history characteristics of prey species, we found that mainly long-lived species with high adult survival and late onset of breeding were limited by predation. Single-brooded species were also more likely to be limited by predation than multi-brooded species. Predators that depredate prey species during all life stages (i.e. from nest to adult stages) limited prey numbers more than predators that depredated only specific life stages (e.g. solely during the nest phase). The Red Fox and non-native mammals (e.g. the American Mink Neovison vison) were frequently identified as numerically limiting their prey species. Our review has identified predator-prey interactions that are particularly likely to result in population declines of prey species. In the short term, traditional predator-management techniques (e.g. lethal control or fencing to reduce predation by a small number of predator species) could be used to protect these vulnerable species. However, as these techniques are costly and time-consuming, we advocate that future research should identify land-use practices and landscape configurations that would reduce predator numbers and predation rates.

Seasonal fecundity and costs to λ are more strongly affected by direct than indirect predation effects across species

Increased perceived predation risk can cause behavioral and physiological responses to reduce direct predation mortality, but these responses can also cause demographic costs through reduced reproductive output. Such indirect costs of predation risk have received increased attention in recent years, but the relative importance of direct vs. indirect predation costs to population growth (λ) across species remains unclear. We measured direct nest predation rates as well as indirect benefits (i.e., reduced predation rates) and costs (i.e., decreased reproductive output) arising from parental responses to perceived offspring predation risk for 10 songbird species breeding along natural gradients in nest predation risk. We show that reductions in seasonal fecundity from behavioral responses to perceived predation risk represent significant demographic costs for six of the 10 species. However, demographic costs from these indirect predation effects on seasonal fecundity comprised only 12% of cumulative predation costs averaged across species. In contrast, costs from direct predation mortality comprised 88% of cumulative predation costs averaged across species. Demographic costs from direct offspring predation were relatively more important for species with higher within-season residual-reproductive value (i.e., multiple-brooded species) than for species with lower residual-reproductive value (i.e., single-brooded species). Costs from indirect predation effects were significant across single- but not multiple-brooded species. Ultimately, demographic costs from behavioral responses to offspring predation risk differed among species as a function of their life-history strategies. Yet direct predation mortality generally wielded a stronger influence than indirect effects on seasonal fecundity and projected λ across species.

Changes in breeding phenology and population size of birds

Although the phenology of numerous organisms has advanced significantly in response to recent climate change, the life-history and population consequences of earlier reproduction remain poorly understood. We analysed extensive data on temporal change in laying date and clutch size of birds from Europe and North America to test whether these changes were related to recent trends in population size. Across studies, laying date advanced significantly, while clutch size did not change. However, within populations, changes in laying date and clutch size were positively correlated, implying that species which advanced their laying date the most were also those that increased their clutch size the most. Greater advances in laying date were associated with species that had multiple broods per season, lived in nonagricultural habitats and were herbivorous or predatory. The duration of the breeding season increased for multibrooded species and decreased for single-brooded species. Changes in laying date and clutch size were not related to changes in population size (for resident or migratory species). This suggests that, across a wide variety of species, mismatches in the timing of egg laying or numbers of offspring have had relatively little influence on population size compared with other aspects of phenology and life history.

Bird ringing and nest recording in Britain and Ireland in 2012

This is the 76th annual report of the British Trust for Ornithology's Ringing Scheme, covering work carried out and data processed in 2012. Demographic integrated population monitoring has been the focus of the Scheme priorities in recent years and models that integrate ringing and nest record data and abundance trends were built for ten species for which sufficient data were available. Population drivers vary between species but they can be split between changes in breeding success, survival, or both. Multi-brooded species exhibited more seasonal variation in reproductive output than single-brooded species. In particular, migratory species showed a seasonal decrease in fledgling output, as do single-brooded species while residents generally showed a seasonal increase in productivity. Constant Effort Site (CES) data showed long-term (1984–2011) declines in four long-distance migrants and five resident species. However, four species (two short-distance migrants and two residents) have increased over the same period. Productivity of migrants monitored by CES was poor in 2012: seven of the eight species showed significant reductions in productivity compared to the five-year average and had the lowest productivity since CES began in 1983. Breeding performance of resident birds was low in 2012, with juvenile:adult ratios significantly lower than those calculated over the preceding five years for 10 of the 16 species. The number of CE sites increased in 2012, with data submitted for 125 sites, the highest total for a decade. The number of active RAS projects increased to 163 in 2012, with 21 new projects, eight of which involved RAS target species. In 2012 the total number of birds ringed (970,957) was the second highest recorded, and included 408,225 first-year birds, 377,199 adults (birds in their second calendar year or older), 23,772 unaged birds and 161,761 nestlings. In addition, there were 249,551 recaptures of ringed birds at or near the ringing site. The reduction in numbers ringed compared to 2012 was probably the result of the record wet weather that most of Britain & Ireland experienced in spring and summer 2012. A total of 18,693 BTO-ringed birds were recovered in 2012, which was lower than in 2011 but higher than in any previous year. This is the result of an increased number of birds ringed in the past 10 years, an increasing number of colour-ringed and tagged birds and subsequent resightings, more ringers within the Ringing Scheme, efforts to increase reports from members of the public of both metal and colour-ringed birds and an improvement in the way recoveries are processed. Recoveries of 22 BTO-ringed and 11 foreign-ringed individual birds that have extended our knowledge of movement of the species or constitute a new longevity record are presented. In total 44,895 nest records were submitted in 2012, the highest annual submission since the scheme began. Of these, 81% originated from England, 9% from Wales and 9% from Scotland, the remaining 1% divided equally between Northern Ireland and the Republic of Ireland. Submissions from England, Scotland and Northern Ireland have all risen by similar proportions since 2007 (18–22%), while those from Wales have increased by 34% over the same period. Long-term annual productivity trends have been calculated for three additional species. Long-term productivity trends were significant for 40 of the 72 species monitored and nine of these trends were negative, indicating a decline in breeding success over time.

Mixed life-history strategies in a local population of the ectoparasitic flyCarnus hemapterus

A major issue for the proper understanding of the evolution of life-cycle histories is the regulation of voltinism and its variation. Diapause characteristics are known to regulate voltinism, but the underlying mechanisms are poorly understood. This paper studies diapause duration and voltinism variation in a haematophagous diptera parasitizing 2 sympatric hosts with very different breeding phenologies. We hypothesize that bivoltinism will be more frequent in carnid flies parasitizing an early breeding, multi-brooded species than in flies parasitizing a late breeder, single-brooded species. We obtained evidence of the co-occurrence of uni- and bivoltinism in both clutches of the multi-brooded Spotless starling (Sturnus unicolor) as well as in clutches of the single-brooded European roller (Coracias garrulus). Unexpectedly, the proportion of bivoltine flies was similar in both host species. A remarkable degree of host-parasite synchronization at the population level was found for bivoltine flies. Our findings reveal the facultative nature of diapause in Carnus. We discuss the influence of abiotic conditions and host availability on polymorphism in life-history cycles and the consequences both for the parasite and the host.

Use of Fecundity Measured Directly Throughout the Breeding Season to Test a Source-Sink Demographic Model

Populations of landbirds (bird species that occupy terrestrial habitats for most of their life cycle) are declining throughout North America (north of Mexico) and Europe, yet little is known about how demography is driving this trend. A recent model of 5 geographically separated populations of Cerulean Warblers (Dendroica cerulea) that was based on within-season sampling of nest survival and fledgling success shows that all populations are sinks (annual reproduction is consistently less than annual adult mortality). I tested this indirect model by directly measuring fecundity (number of female fledglings/female) during the breeding season for 2 years in a Cerulean Warbler population occupying a mature forest in southwestern Michigan (U.S.A.) I determined territories of male birds on the basis of male plumage characters and phases of the nesting cycle (2007) and on uniquely color-banded males (2008). I transferred locations of identified males to topographic maps. I counted all fledglings in territories from May to July each year. The model I tested may apply only to single-brooded species; therefore, I searched the literature to estimate the percentage of single-brooded species in North America. The breeding season of Cerulean Warblers was short- nearly all nests were initiated from mid-May to late June. Nest predation and brood parasitism were primary and rare causes of nest failure, respectively. Significantly fewer Cerulean Warblers fledged from parasitized than from nonparasitized nests. Fledgling survival required to maintain the population size was well above previously published values for Neotropical migrants. Single-brooded species comprise 62% of North American breeding bird species for which the number of broods per year is known; I believe my results may apply to these species. The consistency between identification of populations as sources or sinks on the basis of either model estimates or direct measurements suggests that a demographic model relying on within-season sampling of fecundity is adequate to determine population status of single-brooded avian populations. In addition, on the basis of results of previous studies, annual adult survival rate of the Cerulean Warbler is typical of parulid warblers that are not declining. Thus, low fecundity, here determined with different quantitative methods, can drive status of landbird species with high-observed survival.

Broodedness, not latitude, affects the response of reproductive timing of birds to food supplementation

IntroductionMultiple food supplementation experiments during the pre-laying period have shown that the availability of additionalfood at this time can advance laying date (laydate) and/orincrease the number and/or quality of the eggs laid (Nageret al. 1997). It would therefore appear that food is a lim-iting factor at this critical time of year, although the extentof this limitation varies between years, locations, andspecies (Nager et al. 1997). Svensson (1995) and Dhondtet al. (2002) showed that the response to supplementaryfeeding depends on the number of clutches a female usu-ally lays in a season: single-brooded bird species onlyrespond weakly to supplementary feeding whereas multi-brooded species respond more strongly. In two recentpapers, Schoech and Hahn (2007, 2008) documented alatitude effect on responses to supplemental feeding andconcluded that birds advance laying in response to feedingmore as the distance from the equator decreased. If theseauthors are correct, this is indeed an important observationas it suggests that birds would be more food limited in thepre-breeding season closer to the equator than furthernorth.In this paper I re-analyze the dataset used by Schoechand Hahn in order to test if latitude, number of broods, orboth influence the effect of supplemental feeding on lay-date. The response to food supplementation has beenmeasured in four species at three or more latitudes, makingit possible to determine if latitudinal variation of theresponse to supplemental food can be confirmed withinsingle species, which is a stronger test than that in which allspecies are combined. If latitude drives the response tosupplemental food, the differences between control birdsand birds provided with additional food should be largest inpopulations furthest south (closest to the equator). If thenumber of clutches a species usually lays in a season drivesthe response, the effect of food supplementation should notvary systematically with latitude.Materials and methodsTo explore to what extent broodedness influences theresponse to supplemental food, I added this information tothe dataset Schoech and Hahn (2008) provided in theirElectronic Supplementary Material (see Electronic Sup-plementary Material of this article). For the within-speciescomparison, I added results on food supplementationexperiments in the Corsican Blue Tit Cyanistes caeruleus(Bourgault et al. 2009). Single-brooded species very rarelylay a second clutch after having fledged a first brood (13species); double-brooded species often lay a second clutchafter having raised a first brood successfully (8 species);triple-brooded species often lay three clutches or more perseason (3 species). I assigned species to a category

Climate change affects the duration of the reproductive season in birds

1. The duration of the reproductive season may depend on the duration of the growing season, with recent amelioration in spring temperatures allowing earlier start of reproduction. Earlier start of reproduction may allow a longer breeding season because of more broods a longer interval between broods for multi-brooded species. 2. We analysed extensive long-term data sets on timing of breeding in 20 species of birds from Denmark, based on records of over 100 000 individual offspring, showing considerable heterogeneity among species in temporal change in duration of the breeding season. 3. Multi-brooded species increased the duration of their breeding season by 0.43 days year(-1) while single-brooded species decreased the duration of their breeding season by 0.44 days year(-1). This implies that recent climate change has allowed more broods or better temporal spacing of broods in multi-brooded species, while the time window for reproduction has become narrower in single-brooded species. 4. The single-most important predictor of change in duration of the breeding season was change in the date breeding started; there was no change in the date of end of breeding. Species advancing their breeding date the most also expanded the duration of the breeding season. In contrast, long-distance migration and generation time did not predict change in duration of the breeding season.

Migratory behaviour constrains the phenological response of birds to climate change

Migratory birds may be unable to respond to climate change at their breeding grounds in the same way as residents because they do not experience local environmental conditions until migration has been completed, or because of limited plasticity in response to environmental variation. Indeed, several studies have shown that long-distance migrants have advanced their phenology less than short-distance migrants in recent decades, suggesting a slower phenological response to ongoing climate warming. However, few studies compared the phenological response of species that live year-round within the same climatic region (including residents) with that of species migrating over longer distances. We used an extensive data set on temporal change in spring first singing date (FSD) during 1977-2006 of 56 resident and migratory bird species in Northern Germany to test whether migratory status affected 2 aspects of the phenological response: the temporal trend in FSD and the response of FSD to local temperatures. Species migrating <4° latitude (ca. 450 km) (less migratory species) showed greater temporal advancement in FSD compared to more migratory species. In addition, the variance in temporal trends was larger among less migratory species. Higher local temperatures were associated with earlier FSD, and multi-brooded species showed a stronger advancement in FSD with higher temperatures than single-brooded species. However, the tempera- ture response was unaffected by migratory status, suggesting that the difference in the temporal trend in FSD between less migratory and more migratory species was mainly due to climatic and/or environmental cues regulating departure of the latter from the wintering areas and/or temporal scheduling of migratory flights, rather than to differential responses of FSD to local temperatures. Therefore, migratory behaviour should be regarded as a life-history trait possibly constraining the phenological response to rapid climatic changes in avian species.

Impact of season and social challenge on testosterone and corticosterone levels in a year-round territorial bird

Plasma testosterone increases during breeding in many male vertebrates and has long been implicated in the promotion of aggressive behaviors relating to territory and mate defense. Males of some species also defend territories outside of the breeding period. For example, the European nuthatch ( Sitta europaea) defends an all-purpose territory throughout the year. To contribute to the growing literature regarding the hormonal correlates of non-breeding territoriality, we investigated the seasonal testosterone and corticosterone profile of male (and female) nuthatches and determined how observed hormone patterns relate to expression of territorial aggression. Given that non-breeding territoriality in the nuthatch relates to the reproductive context (i.e., defense of a future breeding site), we predicted that males would exhibit surges in plasma testosterone throughout the year. However, we found that males showed elevated testosterone levels only during breeding. Thus, testosterone of gonadal origin does not appear to be involved in the expression of non-breeding territoriality. Interestingly, territorial behaviors of male nuthatches were stronger in spring than in autumn, suggesting that in year-round territorial species, breeding-related testosterone elevations may upregulate male–male aggression above non-breeding levels. In females, plasma testosterone was largely undetectable. We also examined effects of simulated territorial intrusions (STIs) on testosterone and corticosterone levels of breeding males. We found that STIs did not elicit a testosterone response, but caused a dramatic increase in plasma corticosterone. These data support the hypothesis that corticosterone rather than testosterone may play a role in the support of behavior and/or physiology during acute territorial encounters in single-brooded species.

Tits on the move: exploring the impact of environmental change on blue tit and great tit migration distance

1. In response to warmer spring conditions in Central Europe many migratory bird species have shifted their timing of breeding. Environmental change has also led to warmer winters, shortening the distance between the breeding grounds of migratory birds and their overwintering areas. 2. Here, we show that in response to warmer winters, blue tits (Cyanistes caeruleus), but not great tits (Parus major), breeding in Germany decreased their migration distance between 1964 and 1996. Understanding this difference provides insight into possible constraints and selection pressures involved in how species respond to environmental change. Here, we focus on their breeding ecology. 3. In a nest box population in southern Germany, both species laid their first clutch earlier with increasing spring temperature, but over the study period (1974-1999) blue tits showed a significant and stronger advancement in laying date than great tits. For both species, selection for earlier breeding did not vary with environmental change, indicating that early laying pairs did not do better than later laying pairs as spring temperature increased. 4. Blue tits in the nest box population were single-brooded and existing hypotheses state that single-brooded species likely advance their laying date to match timing of reproduction with the advancing food peak in spring. We hypothesize that this might be one reason why blue tits adjusted their migration strategy as closer proximity to the breeding grounds in winter allows better prediction of the onset of spring. Ten per cent of great tits successfully produced second broods and their first clutch laying date is a compromise between first and second clutch laying date, which might be why great tits had not advanced their laying date nor altered their migration strategy.

Predicting Annual Fecundity from Nest Success

Nest success, the probability that a nest will fledge offspring, is a widely measured parameter in ornithological studies, and is usually estimated by monitoring the fates of nests throughout a breeding season. Because nest success es- timates are commonly reported in the avian literature, a key question in population studies is how to derive annual fecun- dity rates, F (a vital parameter in population biology), from nest success data when breeding individuals are not marked. In this manuscript, we describe a simple, recursive model that generates an estimate of the frequency distribution of an- nual fecundity rates that can be achieved for a species, given precise and unbiased estimates of nest success, the average number of young that fledge per successful nest, and three life history parameters: the maximum number of possible suc- cessful broods per breeding season (J), the maximum number of possible nesting attempts per breeding season (K), and the maximum brood size (B). We illustrate the model for 3 hypothetical species in which the average young that fledge per successful nest is 2 offspring: (1) a single-brooded species in which J =1, K = 3, B = 3; (2) a double-brooded species in which J = 2, K = 3, B = 3, and (3) a triple-brooded species in which J =3, K = 3, B = 3. In general, the frequency distributions of acceptable fecundity solutions for single-, double-, and triple-brooded species are all approximately symmetric, and are defined by Fave (the average acceptable solution), Fmin (the minimum acceptable solution), and Fmax (the maximum acceptable solution). The breadth of these distributions, or the difference between Fmin and Fmax, appears to be controlled largely by solutions where an unequal number of young fledge across attempts. However, when examining relationships between annual fecundity and nest success across the full spectrum of nest suc- cess values, we find that, more often than not, non-linear relationships characterized the association between nest success, Fmax, Fmin, and Fave. The only case where nest success predicts annual fecundity in a linear fashion occurred when J = K. Thus, the assumption that nest success is an adequate indicator of annual fecundity should be viewed with caution.

OCCURRENCE AND IMPLICATIONS OF DOUBLE BROODING IN A SOUTHERN POPULATION OF TREE SWALLOWS

Abstract Double brooding is the initiation of a second clutch of eggs after successfully raising young from the first clutch. Migratory birds that nest in temperate North America are often single-brooded, but there is widespread intra- and interspecific variation. Tree Swallows (Tachycineta bicolor), which are becoming a model species in biology, are typically classified as a single-brooded species. We documented 18 cases of double brooding in a population of Tree Swallows recently established in the Shenandoah Valley, Virginia. Double brooding may be underreported in this species or may be increasing as a result of climate change or range expansion. If double brooding is as common elsewhere as it was in our study population, it could significantly alter estimates of seasonal or lifetime reproductive success in this widely studied bird.

CLIMATE CORRELATES OF 20 YEARS OF TROPHIC CHANGES IN A HIGH-ELEVATION RIPARIAN SYSTEM

The consequences of climate change for ecosystem structure and function remain largely unknown. Here, I examine the ability of climate variation to explain long-term changes in bird and plant populations, as well as trophic interactions in a high-elevation riparian system in central Arizona, USA, based on 20 years of study. Abundances of dominant deciduous trees have declined dramatically over the 20 years, correlated with a decline in overwinter snowfall. Snowfall can affect overwinter presence of elk, whose browsing can significantly impact deciduous tree abundance. Thus, climate may affect the plant community indirectly through effects on herbivores, but may also act directly by influencing water availability for plants. Seven species of birds were found to initiate earlier breeding associated with an increase in spring temperature across years. The advance in breeding time did not affect starvation of young or clutch size. Earlier breeding also did not increase the length of the breeding season for single-brooded species, but did for multi-brooded species. Yet, none of these phenology-related changes was associated with bird population trends. Climate had much larger consequences for these seven bird species by affecting trophic levels below (plants) and above (predators) the birds. In particular, the climate-related declines in deciduous vegetation led to decreased abundance of preferred bird habitat and increased nest predation rates. In addition, summer precipitation declined over time, and drier summers also were further associated with greater nest predation in all species. The net result was local extinction and severe population declines in some previously common bird species, whereas one species increased strongly in abundance, and two species did not show clear population changes. These data indicate that climate can alter ecosystem structure and function through complex pathways that include direct and indirect effects on abundances and interactions of multiple trophic components.

Interval between clutches, fitness, and climate change

Timing of optimal reproduction can be affected by the presence of multiple broods, with multi-brooded species breeding earlier (and later) than the optimal timing of breeding as compared with single-brooded species that only need to optimize the timing of a single brood. Approximately two-thirds of barn swallows Hirundo rustica produce 2 broods per year, and I tested whether the constraints on timing of reproduction were affected by climate change because climatic amelioration would allow both an earlier start and a later termination of reproduction. The duration of the interval between first and second clutches and the variance in the duration increased during 1971--2005 when temperature during spring, but not summer, increased rapidly. Interclutch interval was shorter when mean date of breeding was late and also among late-breeding individuals during individual years. When clutch size and brood size of the first clutch were large, interval until the second brood increased. Pairs with a long interval produced more fledglings than pairs with a short interval. Pairs with first broods with strong mean T-cell--mediated immune responses took shorter time to start their second clutch, whereas mean body mass or tarsus length of first broods were not significantly related to interclutch interval. Interclutch interval increased with the size of a secondary sexual character, the length of the outermost tail feathers of adult male barn swallows, but not with tail length of females, or with size of several other phenotypic characters in either sex. These findings are consistent with the hypothesis that the duration of the interclutch interval is determined by a combination of environmental conditions, reproductive effort, and sexual selection. Copyright 2007.

Seasonal patterns in the productivity of Meadow Pipits in the uplands of Scotland

In many birds, it is assumed that conditions for breeding follow a seasonal pattern of improvement and then decline and that for each individual in the population the optimal clutch size is larger when conditions for breeding are more favorable. While this is assumed to be the case for single-brooded species, the situation is more complex for multi-brooded species, which start breeding before the date when their optimal clutch size is greatest and conditions are best for raising nestlings. The seasonal patterns of productivity for multi-brooded species are not generally understood. We compared the clutch size, egg size and weight, and the nesting success of Meadow Pipits (Anthus pratensis) nesting during the early and late stages of the breeding season in Scotland. We also compared the provisioning rates, diet, and number of prey items fed to nestlings. Eggs laid late in the season were significantly larger and heavier than eggs laid earlier in the season. There were generally no differences in diet or provisioning rates of nestlings throughout the season. Despite producing larger eggs late in the season, the probability of such an egg producing a nestling that successfully fledged was 17% lower than an egg laid early in the season. This was mainly due to a significant increase in nest predation late in the season.

Evidence of re‐nesting after brood loss in Red Grouse Lagopus lagopus scoticus

The Red Grouse Lagopus lagopus scoticus is a single-brooded species that can re-lay if a clutch is lost during the nesting period (Cramp & Simmons 1980). However, there is no reported evidence regarding second clutches being attempted after a brood of young chicks is lost. Indeed, there is only one record of re-nesting after brood loss in Galliformes and this refers to the Ring-necked Pheasant Phasianus colchicus (Dumke & Pils 1979). In that study four ‘wild’ Pheasant hens in Wisconsin were recorded as having re-nested after brood loss. However, doubts about the short time intervals between suspected re-laying, incubation and hatching noted for at least two of the birds suggest that nest appropriation played some part in the result (Dumke & Pils 1979). During the summer of 2003, we monitored a number of radiotagged hen Red Grouse on a heather Calluna vulgaris moor managed for Red Grouse in the Scottish central highlands. Without disturbing the hen, we used radio-telemetry on alternate days through May to monitor whether the hen was incubating and thus determine hatch date. After hatch, we located hens by radiotelemetry and used trained pointing dogs to find chicks on days 5, 10 and 15 after hatching. Thirty-three hens successfully hatched chicks from their first nesting attempt. Of these, 15 lost their entire brood in the first 15 days after hatching. However, we found that two of these hens (A and B) produced a second brood after the loss, for unknown reasons, of their first brood between days 5 and 10. The presence of a second brood was established on the 50th day after each first clutch had hatched, as hens were located at this time to determine survival rates. Hens A and B were found to have chicks judged, from the juvenile primaries, to be under 12 days of age (Parr 1975). In comparison with other chicks of known ages, they were estimated to be 7–10 days of age for the four chicks with Hen A and 10–12 days for the five chicks with Hen B. We estimate that Hen A would have re-laid between 4 and 9 days after brood loss and Hen B between 3 and 8 days after brood loss. These second broods would have hatched in mid-June 2–3 weeks after the main hatching period. Hence Red Grouse can, if rarely, re-lay and produce second broods after losing the first. Although we cannot exclude nest appropriation, we think that the time between brood loss and re-laying, along with the well-documented parental abilities of Red Grouse, support our conclusion. We thank Phones and Ralia estates for access, David Howarth, Nadine Clark and Scott Newey for help during the field season, Kate Graham and two anonymous referees for comments on the manuscript, and the Game Conservancy Scottish Research Trust for funding.

Reduced parental care, compensatory behaviour and reproductive costs of thick-billed murres equipped with data loggers

Theoretical models predict that in species with obligate biparental care, individuals will partially compensate for decreased parental effort by their partners as a stable evolutionary strategy. Full compensation may occur when breeding success is an accelerating function of parental effort, especially in long-lived bird species. We experimentally examined the effect of time-depth recorders (TDRs) on body mass and parental behaviour of thick-billed murres, Uria lomvia, and evaluated the effect of TDRs as handicaps to test whether individuals compensate for decreased partner effort and whether compensation is sex biased. Compared with control birds, TDR-equipped birds had reduced body mass, offspring attendance, number of foraging trips and feeding rates, and males had increased foraging trip duration. In general, males lost mass at a higher rate and made longer foraging trips than females. Partners of TDR-equipped birds compensated for their mates' reduced parental effort by increasing offspring attendance and by increasing chick provisioning above average rates of control birds. Although partners of TDR-equipped birds fully compensated for their mates' reduced offspring attendance (i.e. chicks were never observed to be left unattended), total provisioning rates of pairs with a TDR-equipped bird ( X ¯ ± SE = 4.38 ± 0.26 meals / day ) were significantly lower than those of control pairs (5.74 ± 0.31 meals/day). However, fledgling success of control and TDR-equipped birds did not differ between reproductive seasons. TDR-equipped birds also had a significantly lower rate of return to breed than their non-TDR-equipped partners or control birds, and those that did return the following season were more likely to change mates (32%) compared with controls (0%). Taken together, our results underline the need to quantify the effects of monitoring equipment used to measure seabirds' activities, and indicate the ability of female and male thick-billed murres to compensate for reduced partner effort due to handicapping. Compensatory behaviour seems to be a necessary response of this single-brooded species to ensure current breeding success; however, it may affect the stability of pair bonds in some individuals.