FOOD AS A FACTOR CONTROLLING THE BREEDING OF PUFFINVS LHERMINIERI

A study of Puffinus Iherminieri on Plaza Island, Galápagos, showed that, although eggs were laid in all months, there were marked peaks and troughs of laying. Intervals between successive layings varied with the success or failure of the first egg, an average of nine months for successful pairs, eight months for pairs which lost a young, and 6–5 months for those which failed to hatch an egg. However the breeding success did not influence the time between the end of one breeding attempt and the next laying. This suggests that birds were breeding as quickly as possible. The critical factor preventing more frequent breeding appeared to be the time required to replace the wing and tail feathers.
Details are given of the breeding biology, all aspects of which were strongly influenced by frequent and unpredictable food shortages. The average incubation period was 49 days but this was prolonged by temporary desertion due to food shortages. Chick growth and fledging periods (62 to 100 days) were variable. There was no well–defined desertion period and birds were experimentally induced to feed young for up to 120 days.
Overall nesting success was low (26%) and adult survival between breeding attempts was high (92–95%).
Food, planktonic fish larvae and Crustacea, appeared to be rarely abundant and details are given of the effect of food shortage on the breeding.
The breeding synchrony was brought about by food shortages. The ultimate factor controlling breeding appeared to be the availability of food for egg formation and there was no possibility of birds timing breeding so that young were being fed at a time of food abundance.
A comparison is made of the breeding of eight Puffinus species.     

The effect of migration strategy and food availability on White Stork Ciconia ciconia breeding success

In the mid 1970s, the breeding populations of the migrant White Stork Ciconia ciconia were close to extinction in the northeastern region of France (Alsace). A re-introduction project was implemented, resulting in the year-round settlement of some individuals in the region, which rely on additional food supplied by humans during the winter. Today, both resident and migrant birds breed in the same areas and take food from rubbish dumps and humans (farmers). The effects of these anthropogenic influences, altering Stork behaviour, on Stork reproductive success are not known. The aim of this study was to test the influence of bird status (resident vs. migrant) and food availability (control nests vs. nests that benefit from high food supply) on reproductive success. In control nests, the mean laying date was earlier in resident than in migrant White Storks. There was also a clear seasonal decline in clutch size. For all nests, the numbers of eggs and hatchlings were higher in resident birds than in migrants, which can be attributed to the earlier breeding of resident Storks. The large broods of resident birds showed a high mortality rate, leading to the same fledgling success (fledglings/hatchlings) and number of fledglings as in migrants. Fledgling success and the number of fledglings were higher for nests close to a reliable food supply. In summary, although resident birds can breed earlier and produce more eggs than migrants, we found no advantage in terms of number of fledglings. The higher mortality rate of chicks found in pairs with a large brood could be caused by the deterioration of their habitat. Thus, the year-round settlement of Storks may not present a biological advantage if the quality of their habitat is not guaranteed by the conservation of their grasslands.     

Distribution of shorebirds at the Berg River estuary, South Africa, in relation to foraging mode, food supply and environmental features

Foraging densities of Curlew Sandpipers Calidris ferruginea and Grey Plovers Pluvialis squa-tarola at the Berg River estuary, South Africa, are examined in relation to prey and habitat attributes.
There is a substantial overlap in the prey species eaten by Curlew Sandpipers and Grey Plovers, but the distribution patterns of the two species within the estuary could not be explained by the same prey attributes.
The distribution of tactilely foraging Curlew Sandpipers was determined largely by the numerical abundance of nereid worms. They foraged preferentialiy on muddy rather than sandy substrata, but their distribution could not be explained on the basis of substratum penetrability. The density of visually foraging Grey Plovers was correlated with the biomass, rather than abundance, of the larger of the two common nereids on the estuary. The favoured foraging area of Grey Plovers had a high vegetation cover, but their foraging dispersion was not influenced by substratum type.
The energy intake rates of Curlew Sandpipers were broadly inversely related to their foraging density, although the rate of prey capture was maximal at high prey density. These birds may use neighbour success rates in selecting foraging sites. Energy intake rates of Grey Plovers were very consistent across the estuary, suggesting that individuals occupy or defend areas which provide access to comparable food resources.     

BREEDING PERFORMANCE OF PUFFINS FRATERCULA ARCTICA IN RELATION TO NEST DENSITY, LAYING DATE AND YEAR

The paper presents data on the breeding and predation of Puffins in two areas of different nest density within a single colony on Dun, St Kilda group, Outer Hebrides in 1973-78.
Within a season birds laying early had a slightly higher nesting success than birds laying late, but laying date had little influence on the peak and fledging weights of young. The main disadvantage in late laying was a reduced chance of relaying if the first egg was lost.
Breeding success and chick weights varied from year to year. The 1974 season was the least successful with the lowest nesting success, lowest frequency of feeds, lowest calorific value of feeds, lightest chicks and slowest growth. Overall breeding performance was not related to the annual mean laying dates.
In all years pairs nesting in the area of high nest density did better than pairs nesting at low density. The effect is attributed to differential predation and disturbance by predatory gulls. At least 4.2% of adult Puffins breeding in the area of low burrow density were killed by gulls each breeding season; this is higher than the total annual mortality found in three other studies. Only 0.9% of adults from the high density area were found killed. The subpopulation in the low density area cannot survive without much immigration, yet there is no evidence that this happens.     

Annual variation in breeding biology of gentoo penguins, Pygoscelis papua, at Bird Island, South Georgia

The breeding biology of the gentoo penguin, Pygoscelis papua , was studied over a three-year period (1986–1988) at Bird Island, South Georgia, with particular reference to birds of known age or breeding experience. Laying date varied significantly between all three years, being three weeks later in 1987, when the breeding population decreased markedly. Factors involved in the timing of breeding are discussed. Within years egg-laying was highly synchronous: 95% of clutches were initiated in 14·5 days or less. The incubation period was 35 days and the laying interval, between the two eggs, 3·3–3·4 days. Chicks creched when 25–30 days old, and this varied between years, possibly related to food supply and chick growth. Chicks left the colony for the first time between 75 and 85 days of age. The breeding population at Bird Island decreased by 20% and increased by 84% in successive years during the study period. Breeding success (chicks fledged per egg laid) varied between 0·33 and 0·65 within colonies, but for the whole island was very consistent over the three years: 0·45, 0·51 and 0·47. Overall, colony differences were not correlated between years. Disturbance from Antarctic fur seals, Arctocephalus gazella , is suggested as the cause of consistently lower breeding success at one colony. Mean egg weight varied annually, and with age of the breeding bird, nest location and, in one year, with laying date. Young, first-time breeders laid smaller eggs and had lower breeding success compared to older, experienced birds, similar to other seabirds. However, they differed from other species in laying on average earlier than older birds. The relationship between age, egg weight, laying date and breeding success is discussed in relation to predation and seasonal food supply.     

The effect of body condition on the timing and success of breeding in Little Penguins Eudyptula minor

We studied the effect of parental body condition on the breeding biology of Little Penguins Eudyptula minor . Daily attendance patterns and body mass were recorded using an Automated Penguin Monitoring System, which collected arrival and departure masses for approximately 200 breeding birds over the 2000 and 2001 breeding seasons. Breeding success varied between the two years; 2000 was a year of average breeding success (fledging 1.07 chicks per pair), and 2001 a year of poor breeding success (fledging 0.53 chicks per pair). In both years, adult body condition (body mass divided by flipper length) increased significantly prior to laying. The laying period began over a month later in 2000 than in 2001, and birds in 2000 exhibited significantly better body condition at laying. However, the mean laying dates in 2000 were less variable than in the 2001 breeding season. Body condition appeared to influence both the time of breeding and breeding synchrony in Little Penguins. Breeding success was correlated with adult body condition at incubation in 2000 but not in 2001, indicating that success was not solely influenced by adult body condition at incubation.     

A quantitative analysis of habitat preferences of Woodcock Scolopax rusticola in the breeding season

In Britain, prior to breeding, Woodcock Scolopax rusticola switch from feeding on pastures by night to feeding in woodland by day. Their major food is earthworms (Lumbricidae). In April and May 1985, vegetation structure and composition and soil parameters including earthworm numbers (30 habitat variables) were recorded for quadrats containing 50 feeding locations of six radio-tagged birds, seven nests and 50 random locations in Whitwell Wood, northeast Derbyshire.
Significantly different mean values between feeding and random sites were found for 11 habitat variables. Feeding sites were in younger stands, with a higher percentage ground cover of dog's mercury Mercurialisperennis and consistently high values for pH. Earthworm biomass was on average 82% greater than in random plots. Areas of beech Fagus sylvaticus were avoided. A discriminant function based on six habitat variables correctly classified 85-5% of the feeding and random sites; 70% of the feeding locations were correctly classified by a jack-knife procedure. The important factors determining where feeding occurs are probably safety from avian predators and high availability of earthworms.
Discriminant analysis of the habitats used for feeding by solitary birds, broods and for nesting produced significant functions which correctly classified 84% of sites. Nests were in areas with a high percentage cover of brambles Rubus spp. and more open ground vegetation. Broods and solitary birds used similar areas characterized by denser ground vegetation than nest sites.
The results of this study indicate that both habitat structure and food availability influence the distribution of Woodcock in the breeding season.     

Effects of experimental food provisioning on reproduction in the Jackdaw Corvus monedula, a semi-colonial species

Two Jackdaw Corvus monedula colonies were given supplementary food before and during breeding in 1983. Breeding density and cavity use were compared with those of the same colonies in previous years, when no food was provided. Predation rate and reproductive parameters were compared with those in the same colonies in previous years and with those of two control colonies, without experimental food. Jackdaws preferred safe cavities with small minimum nest-entrance dimensions and avoided those with a high risk of nest predation. In experimental (fed) colonies, however, there was a tendency to use all cavities, which resulted in an increased breeding density. No nests were preyed upon by Ravens Corvus corax in the experimental colonies because supplemental food favoured group defence by increasing colony size and by increasing the time the Jackdaws spent in the colony. Additional food advanced laying date, increased clutch size independently of laying date and increased fledging success. Supplementary food significantly increased fledging success in less than half of all experimental studies on birds. We suggest that the key to this problem is the species' breeding strategy, and we show that supplementary food significantly increased fledging success in brood-reduction strategist species but not in species which directly adjusted their clutch size.     

SOCIAL STIMULATION OF REPRODUCTION IN HERRING AND LESSER BLACK-BACKED GULLS

The date of the first egg laid and the position of nests were recorded for all pairs in 10 study areas within the mixed Herring and Lesser Black-backed Gull colony on the southern end of Walney Island, Lancashire. The study areas differed from one another in density of breeding birds and in proportion of each species. These data were collected in order to determine if evidence could be found for synchrony of reproduction by social stimulation (the "Fraser Darling Effect"). The gulls in different study areas showed considerable variability in onset and spread of laying; however, attempts to correlate a number of variables, such as density of breeding birds in each area with median date or spread of laying, average nearest neighbour distance with median date of egg-laying, and median laying date for one species with that of the other species in the same section of the gullery, failed to produce convincing evidence for inter- or intraspeciiic social stimulation of reproduction.     

SYNCHRONOUS FLUCTUATIONS IN POPULATIONS OF SOME RAPTORS AND THEIR PREY

Raptorial birds which depend on a small number of prey species, especially those living in open habitats in the tundra, semi-desert and desert belts of the Palaearctic, undergo local fluctuations in numbers which are synchronous with the fluctuations in numbers of their main prey species. In this they contrast with the asynchronous or lagging type of predator-prey oscillation which has received wider attention in the literature.
Evidence from ringing recoveries shows that the species which fluctuate locally in synchrony with their prey tend to move greater distances from their birth-place or previous breeding place than those with more stable populations. The former species also tend to be split into fewer sub-specifically distinct forms than the latter. It is argued that the species which are subject to a fluctuating food supply have evolved, as an important adaptation, the ability to undertake more or less extensive 'searching migrations', which enable them to find and settle in areas of adequate food supply, often far removed from the previous breeding area. In the longer evolutionary aspect, this ability may be a factor promoting food specialization.
The synchronous population fluctuations of raptorial birds with their prey are compared with the asynchronous or lagging oscillations of carnivorous mammals.     

Recruitment of grey and red-legged partridges (Perdix perdix and Alectoris rufa) in relation to population density and habitat

The recruitment of first-year grey and red-legged partridges into subsequent breeding populations was investigated on 17 study areas in Britain.
The density of birds recruited per unit area increased with the density of birds available in their first autumn for potential recruitment in the following spring but decreased with adult density. The relationship between recruitment and adult density, while always negative, differed significantly between areas. These differences were correlated with variation in the amount of nesting habitat available to both species of partridge and with variation in nesting habitat quality. Recruitment of grey partridges was related to length of available nesting habitat, the amount of residual dead grass in the nesting cover, and earth bank height at the base of the nesting cover. Recruitment of red-legged partridges was also related to length of nesting habitat, together with the amount of nettle in the nesting cover.
It is suggested that the attractiveness of different areas to settling recruits is determined by the amount and quality of nesting habitat available.     

Selection for synchronous breeding in the European starling

Colonial birds often demonstrate considerable breeding synchrony. In southern Sweden the semi-colonial European starling initiated the vast majority of clutches within one week. Laying dates were positively skewed so that many birds initiated clutches at similar dates early in the season. Breeding was further synchronised by a particularly strong clutch-size reduction equivalent to one third of an egg per day during the first part of the breeding season. The decline in clutch size with season also held true for separate age-classes of females, for individual females laying at different times at different years and for individual females laying at different times the same year. Trends in breeding success during nestling rearing were unlikely to explain the high degree of breeding synchrony or the seasonal decline in clutch size; nestling survival and growth were weakly related or unrelated to reproductive timing. In contrast recruitment success of fledged offspring declined sharply with season. Even within the synchronous laying period, defined as clutches initiated during the first week each year, local recruitment success declined. It is suggested that the early seasonal decline is caused by selection for synchronous fledging permitting the immediate formation of flocks after fledging, whereas the late seasonal trends may be caused by either population differences in female quality or deteriorating conditions for raising young.     

Conspecific attraction and nest site selection in a nomadic species,the zebra finch

Conspecific nesting density affects many aspects of breeding biology, as well as habitat selection decisions. However, the large variations in breeding density observed in many species are yet to be fully explained. Here, we investigated the settlement patterns in a colonial species with variable breeding density and where resource distribution could be manipulated. The zebra finch, Taeniopygia guttata, is a classic avian model in evolutionary biology but we know surprisingly very little about nest site selection strategies and nesting densities in this species, and in fact, in nomadic species in general. Yet, important determinants of habitat selection strategies, including temporal predictability and breeding synchrony, are likely to be different in nomadic species than in the non‐nomadic species studied to date. Here, we manipulated the distribution of nesting sites (by providing nest boxes) and food patches (feeders) to test four non‐exclusive habitat selection hypotheses that could lead to nest aggregation: 1) attraction to resources, 2) attraction to breeding conspecifics, and 3) attraction to successful conspecifics and 4) use of private information (i.e. own reproductive success on a site). We found that wild zebra finches used conspecific presence and possibly reproductive success, to make decisions over where to locate their nests, but did not aggregate around water or food within the study areas. Moreover, there was a high degree of inter‐individual variation in nesting density preference. We discuss the significance of our results for habitat selection strategy in nomadic species and with respect to the differential selection pressures that individuals breeding at different densities may experience.     

Numerical and behavioural responses of migrant passerines to experimental manipulation of resident tits (Parus spp.): heterospecific attraction in northern breeding bird communites?

Summary We studied experimentally interspecific competition among foliage-gleaning passerine birds by manipulating the density of resident tits. In 1988 tit density was experimentally increased on three small islands in a central Finnish lake, and decreased on three other islands by tit removal. In order to avoid the effects of between-island differences in habitat quality, the role of the islands was reversed when the experiment was repeated in the following year. Censuses and observations on foraging and feeding behaviour were conducted to assess the numerical and behavioural responses of migrant conguilders (mainly chaffinches and willow warblers) with respect to the manipulated abundance of the tits. We also measured whether variation in food consumption of tits affected the frequency with which the migrants found food by calculating average intervals between successful prey captures, time lags to prey-capture and giving-up times. Our results indicate that interspecific competition is of minor importance in structuring breeding bird assemblages and species feeding ecologies on the study islands. No consistent difference in foraging or feeding niches of chaffinches and willow warblers was found between low and high tit density conditions. Niche overlap analysis showed no avoidance by chaffinches and willow warblers of the microhabitats which tits used. Tit abundance had no significant effect on feeding success or behaviour. Experimentally increased abundance of resident birds was associated with increased abundance of breeding migrants, however. This pattern was found not only in the foliage gleaning guild but also with all passerine birds, indicating that food was not an important contributor to this pattern. We elaborate a hypothesis suggesting heterospecific attraction in northern breeding bird assemblages. Habitat generalist migrants may use the presence of residents as an indicator of safe and/or productive breeding sites in northern unpredictable circumstances.     

Nest site characteristics, duration of use and breeding success in the Guillemot Uria aalge

The fate of all eggs laid by Guillemots Uria aalge in seven study areas on the Isle of May was recorded in 12 consecutive seasons. The average success at each site was related to its physical characteristics, bird density, position on the cliff and availability of suitable habitat for ticks. The numbers of neighbours and walls, type of site, slope where the egg was incubated and distance from the top of the cliff all had a significant effect on both hatching and breeding success. Potential tick habitat had no significant effect. Much variation in hatching and breeding success remained unexplained. Sites used when the colony was less than half its current size and most frequently in the period 1984–1995 were the most successful. Since an average Guillemot pair remains together at a site for only 3–4 years. the consistently high success of many sites over a period of 10–12 years suggests that the most used sites were occupied by a succession of high-quality birds.     

Spatial distribution and settlement strategies in willow ptarmigan

In heterogeneous landscapes individuals select among several habitat patches. The fitness rewards of these choices are assumed to play an important role in the distribution of individuals across landscapes. Individuals can either use environmental cues to directly assess the quality of breeding sites, or rely on social cues to guide the settlement decision. We estimated the density of adult birds and per capita reproductive success of willow ptarmigan over 5–15 years in 42 survey areas, nested within 5 spatially separated populations in south-central Norway. Our aims were to (1) examine spatial and temporal patterns of variation in densities of adult birds (i.e., the breeding densities) and reproductive success (juveniles/pair) measured in autumn and (2) evaluate which habitat distribution model best described the distribution of willow ptarmigan across heterogeneous mountain landscapes. Variation in density of adult birds was primarily attributable to variation between survey areas which could arise from spatial heterogeneity in adult survival or as a consequence of spacing behavior of juveniles during the settlement stage. In contrast, reproductive success was more variable between years and did not vary consistently between survey areas once year effects were accounted for. The lack of any relationship between the density of adult birds and reproductive success supported the predictions of an ideal free distribution (IFD), implying that within years, the mean reproductive success was approximately equal across survey areas. However, analysis based on Taylor's power law (i.e., the relationship between logarithms of spatial variance and mean density of adult birds) suggested that aggregation was stronger than expected under IFD. This implies that the relative change in density of adult birds was larger in areas with high mean densities than in areas with low densities. The exact mechanisms causing this statistical pattern are unclear, but based on the breeding biology of willow ptarmigan we suggest that yearlings are attracted to areas of high densities during the settlement period in spring. Our study was conducted during a period of low overall density and we suggest that this pattern might be particular to such situations. This implies that the presence of conspecifics might represent a cue signaling high adult survival and thus high habitat quality.     

SURVIVAL OF MANX SHEARWATERS PUFFINUS PUFFINUS

The breeding success of Manx Shearwaters at Skokholm Island, Pembrokeshire, was followed in study burrows, and data on survival of the young were augmented by ringing large numbers as they were about to leave the island and recapturing them in later years.
The weight of the young at fledging and the date at which they leave affect their chances of survival; overall, as many as 30% of the young birds may survive to reach breeding age, which is thought to be normally about 5–6 years. Adult mortality varies between wide limits of about 5 and 20%, but juvenile survival appears to be of about the right order to balance adult losses.
The significance of the one-egg clutch is discussed. At the normal time of laying the female may not be able to obtain sufficient food to form a second egg. A two-egg clutch, laid at a later date, would not produce more surviving young since these would fledge too late in the season to have a good chance of survival.
The advantage of leaving the island at a heavy weight is discussed, and it is concluded that the fat stores laid down give heavy young a greater chance than light young of migrating a long distance without food. It is possible that the young may even reach their winter quarters (off Brazil) without having to feed on passage if they depart with enough stored fat.     

Reduced parental effort in relation to laying date in house sparrows (Passer domesticus): a study under controlled conditions

It is known that breeding success in birds declines with an advance in the laying date. In this paper, breeding success diminished with laying date in a house sparrow (Passer domesticus) population breeding in captivity. The principal cause of the cline in breeding success with date was a decrease in the female feeding rate. The parental effort in females decreased with the advance of the laying date, although food access was ad libitum throughout the breeding season. Consequently, this cline in breeding success may be due to a reduced parental investment with the advance of the laying date or to exhaustion due to previous reproductive events in the season. Hypotheses usually used to explain the cline in breeding success with date can not explain results in this study.     

Effects of supplementary food on density-reduced breeding in an African eagle: adaptive restraint or ecological constraint?

Increased population density often reduces reproductive output in breeding birds, but the underlying mechanisms (adaptive restraint v reduced food resources) behind decreased productivity are poorly understood. Here I correlatively and experimentally investigated the roles of food, breeding density, latitude, altitude and rainfall in limiting productivity of Wahlberg's Eagles Aquila wahlbergi throughout Africa. Breeding success in equatorial and subtropical Africa (0°–30°S) was highly density-dependent but showed no latitudinal or rainfall-related trends. Pairs in dense populations produced half as many young annually as pairs in low-density populations. Density (but not rainfall or latitude) also explained much of the geographic variation in the mean proportion of pairs attempting to breed each year and the incidence of two-egg clutches.
Breeding within populations was consistent with these density-dependent trends: incidence of two-egg clutches increased in a declining population, and productivity was inversely related to breeding density and rainfall combined. To determine if reduced food resources accounted for reduced output in dense populations, eight pairs were food supplemented: supplementary food failed to induce nonbreeding pairs to breed: nor did it induce earlier laying or increase egg size or clutch-size. Population density itself was unrelated to two correlates of food resources, rainfall and latitude. I conclude that population density influences most aspects of breeding in Wahlberg's Eagles, and reduced food resources do not appear to explain these trends. Hence, adaptive restraint may account for decreased annual reproduction in this species.     

Do voles make agricultural habitat attractive to Montagu's Harrier Circus pygargus?

Loss and degradation of habitat threatens many bird populations. Recent rural land-use changes in the Netherlands have led to a shift in habitat use by breeding Montagu's Harriers Circus pygargus . Since the 1990s, unprecedented numbers of this species have bred in farmland compared with numbers in natural habitat. Destruction of nests by agricultural operations, however, compromises breeding success. Between 1992 and 2005, the number of breeding pairs in the northeastern Netherlands was positively, though weakly, correlated with previous-year estimated abundance of voles, mostly Microtus arvalis . In good vole years, the onset of laying was earlier and mean clutch size was larger. Vole abundance was relatively higher in set-aside land and in high and dense vegetation. We suggest that agri-environmental schemes aimed at increasing the availability of voles in agricultural breeding areas may be an effective management tool for the conservation of Montagu's Harriers in the northeastern Netherlands.