Factors affecting nest site selection and nesting success in the Common Eider Somateria mollissima

Nesting site selection and nesting success in Common Eiders Somateria mollissima were studied over a 3-year period (1991–1993) in the Mast River delta (58^o24'N, 94^o24'W), 40 km east of Churchill, Manitoba, Canada. Eiders preferentially nested on islands that had incubating Lesser Snow Geese Anser caerulescens caerulescens on them; this effect was also seen between years on the same islands. Eiders which nested on islands with geese had a reduced chance of having eggs taken by predators during egg-laying and had a greater chance of hatching once incubation had begun, independent of the number of other eiders nesting on the island. The distance to a goose nest was less in nests which did not lose eggs prior to incubation than in those which did, but there was no difference in the distance to a goose nest in eider clutches which did or did not hatch. Artificial eider eggs placed closer to goose nests had a lower probability of being predated than those placed farther away. Nests on islands farther up the river and farther from the mainland had higher nesting success, presumably because these nesting islands were inaccessible to Arctic Foxes Alopex lagopus.     

Nesting bird species in sun coffee, pasture, and understory forest in southern Costa Rica

We investigated avian nest distribution and success in understoryforest, sun coffee plantations, and pasture in southern Costa Rica. Nestsearching occurred in plantations and forest in 1999 and 2000 and in pastures in2000. Nests were monitored until they failed or fledged young. Antbirds(Thamnophilidae) were the most common understory forest nesters and were notfound nesting in the plantations or pastures. Common nesting species in theplantations included Turdidae, Tyrannidae, Cardinalidae, and Thraupidae, many ofwhich are typical of forest edge/canopy or open, scrubby habitats. Two speciesassociated with forest interior, Henicorhina leucostictaand Buarremon brunneinucha, were found nesting in theplantations. Pastures supported similar types of nesting species as theplantations, with the exception of the forest-interior species. Daily mortalityrates (DMRs) for above-ground cup-nesting species in plantations and pastureswere similar to those for species nesting in forest at our site and a site inPanama. The results indicate that conversion from forest to pastures and suncoffee plantations diminishes nesting habitat for forest-interior species, whilenumerous forest edge/canopy species and open-country species are able to nest inthese agricultural land-cover types. As a group, species nesting in theplantations and pastures do not have unusually high nest mortality rates,although species-specific studies are lacking. Nesting species distributionsacross habitat types and DMRs at our study site may be influenced by the largeamount of forest in the landscape.     

The influence of nest-site characteristics on the nesting success of the Karoo Prinia (Prinia maculosa)

Choice of nest site has important consequences for nest survival. We examined nest-site characteristics relative to nest success in Karoo Prinias breeding in coastal dwarf shrubland, where high nest predation is the main cause of nest failure. Initially, we compared nests that failed during the building, laying, incubation and nestling stages and those from which young were successfully raised, to test whether nests that survived to progressive stages in the nesting cycle differed in their nest-site characteristics. Subsequently, we compared the characteristics of successful nests with those of unsuccessful nests. The nest-site characteristics considered included nest height, nest-plant height, nest-plant species, distance from lateral foliage edge, nest concealment, nest-patch heterogeneity and vegetation cover at four different heights. We were unable to distinguish between the nest-site characteristics of nests that failed during the various stages of the nesting cycle. Concealment was the main nest-site characteristic that differentiated successful nests from unsuccessful nests, with successful nests being located in more concealed sites. The other variables that contributed to the discrimination between successful and unsuccessful nests by discriminant function analysis included nest-plant type and distance from edge, which are also directly related to concealment. This suggests that nest concealment is the most important variable influencing nesting success at this site, which has a preponderance of visually-oriented predators.     

Experimental Study of Nest-site Selection in the Biscutate Swift (Streptoprocne biscutata, Aves: Apodidae) in Southern Brazil

Biscutate swift Streptoprocne biscutata nests are usually built on protected rocky cliff walls. Birds often renest at the same location. Remains of previous nests may offer information about potential nests and quality of nest-sites. Here, we experimentally study nest-site selection to test the hypothesis that information from previous nests is used in current nest-site selection. We placed old nest material at artificial nest-sites to test whether new nest-sites are chosen based on the presence of nesting material. We also tested whether the use of natural nest-sites is influenced by nesting material by creating two types of natural sites: previously used natural nest-sites with vestiges of old nests removed and never used natural nest-sites to which vestiges of old nests were added. In the first experiment, in 139 nest-use opportunities, 16 artificial nest-sites were used, all of which included vestiges. In the second experiment, in 91 nest-use opportunities, four nests were in previously unused but natural locations to which vestiges had been added, 22 nests were in previously used sites without vestiges, and the remaining 65 nests remained unused. Two processes are apparently in action: first, prior experience and memory; second, vestiges indicate where nesting has occurred, possibly useful for first breeding, or for imperfect memory. Previous nesting information may explain why swifts use nesting locations for many years and why new nesting colonies seldom form. This transmission of information suggests that swifts tend to be conservative and nest where previous nesting has occurred.     

THE BREEDING BIOLOGY OF THE GREAT WHITE PELICAN PELECANUS ONOCROTALUS ROSEUS AT LAKE SHALA, ETHIOPIA

Pelecanus onocrotalus roseus is known to breed more or less regularly in Africa at Lake Shala, Ethiopia; Lake Rukwa, Tanzania; St. Lucia Bay, Natal; Abou Tougour, Chad; Kapsikis, Northern Cameroons; and Wase Rock, Nigeria; and has been observed or reported breeding at Mweru Marsh, Zambia; Lake Ngami, Botswana; Seal and Dyers Islands, South Africa; and Lake Natron, Tanzania. The Shala colony, after Lake Rukwa's, is the largest known breeding colony in Africa and is probably of crucial importance to the species in Africa. For successful breeding regular colonies of P. onocrotalus must have an assured supply offish and an inaccessible breeding site. These conditions are met at the Lake Shala colony. Assuming that a Great White Pelican consumes about 10% of body weight or about 900-1,200 g per day, the Lake Shala breeding colony with 7,500-12,000 pairs would consume about 3,140-5,040 tonnes in the breeding seasons. The partially unsuccessful breeding of perhaps 10,000 pairs at Lake Natron in 1962 is described. Failure was attributed to sudden failure of the food supply. Breeding of the Great White Pelican on Lake Shala takes place throughout the year although there is a peak of numbers breeding from December to the end of March, which is in the dry season. Observations recorded in this paper cover two full breeding seasons, 1965-6 and 1966-7. Although during the peak period of breeding the flock at Shala is made up of 2,500-5,000 pairs, this mass is composed of smaller units, each made up of 300-1,200 pairs, laying more or less together with little overlap from one group to the next. Three changes occur in the plumage of the Great White Pelican towards the onset of the breeding season: (a) the development of a knob or swelling on the forehead at the base of the beak with associated expanses of brightly-coloured bare skin, pinkish yellow in males and bright orange in females; (b) the development of a crest; and (c) the development of a yellowish band across the chest or larger areas of darker brown in the plumage. Four distinct colour-types of breeding plumage were observed: the dark-brown type, the brown-breasted type, the yellow-banded type, and the type with scarcely any suggestion of a breast patch. No correlation was found between plumage type and sex, size, length of bill or any other obvious physical feature. Group display and individual displays of the adults are described. There appears to be no special pre-copulation display. Elaborate nests are not prepared; the male collects the nesting material, and both sexes build the nest. Nests are small, 35–60 cm in diameter (averaging 46-4 cm), and close together (364 nests averaged 1–55/m²). Average clutch-size is 1–88. Incubation begins with the first egg; both sexes incubate; the incubation period is probably about 38 days. The fledging period is 65–70 days, of which about the first 28–30 days are spent in the nesting area. After that the young form into groups or “pods”. The development of the young is described. When it is very small it is fed by either parent several times each day. When the young is 30–35 days or older, it is probably fed less than once per day. The bright red nail-like tip of the adult's upper mandible serves to direct the begging movements of the young chick, and it probably holds the liquid food on which the chick feeds. It is suggested that pod formation of young both in the middle of the day and at night possibly helps to avoid extremes of temperature, either of heat or cold. Parents recognise their own chicks; and young pelicans, at a later stage, recognise their parents. Indiscriminate feeding of young does not occur in P. o. roseus. Adults often are quiescent but not asleep at night. Activity is stimulated by the earliest light, and most pelicans begin leaving the island by 09.30-10.00 hrs. Most departures and arrivals take place from about 10.00 to 16.00 hrs. The distance from which food is brought to the colony is not known for certain, but most birds probably fly to and from Lake Abiata. It is not known where the young go once they leave the colony; most do not fly to the obvious place, Lake Abiata. Although difficult to determine, we suggest that the breeding success of the pelicans is on the average less than one young per nest. Egyptian Vultures were the most important predators at the colony, but all forms of predation together accounted for less than 10% of the nesting losses. The timing of breeding in relation to food supply, climatic factors and inaccessibility of the breeding site is discussed. Inaccessibility appears to override seasonal factors, but in permanently inaccessible sites the peak of breeding is in the dry season. It is suggested that the Great White Pelican nests in discrete breeding units within the main colony to minimise the effect of predation.     

Importance of scale in nest-site selection by Arizona gray squirrels

Nests provide a place for individuals to rest, raise young, avoid predators, and escape inclement weather; consequently, knowledge of habitat characteristics important to nest placement is critical for managing species of conservation concern. Arizona gray squirrels (Sciurus arizonensis) are endemic to mountains of southwestern United States and northwestern Mexico. We investigated factors influencing nest-site selection at 4 spatial scales (forest-type, nest-site, nest-tree, and within-canopy placement) to provide ecological information and management recommendations for this sensitive species. Nest densities were 2.6 times higher in riparian than pine-oak woodlands. Nest sites had more large trees, snags, logs, and canopy cover and had lower slope. Arizona gray squirrels selected tall trees with more interlocking trees and tended to place nests adjacent to the main trunk. Regardless of scale, Arizona gray squirrels seemed to select nesting areas for their ability to provide protection from predators and the elements as well as access to food. Consequently, maintaining large trees with closed canopies and downed logs should be considered when determining land management plans. © 2011 The Wildlife Society.     

ECOLOGY OF THE RED-NECKED FALCON FALCO CHICQUERA IN ZAMBIA

The Red-necked Falcon in southern Zambia is a year-long resident in its breeding territory. The study was conducted on a floodplain and the adjacent acacia savannah in an undisturbed natural environment. The falcon principally nests in natural depressions on frond bases on the leeward side of Borassus Palms but also utilizes old crow and raptor nests. Incubation averaged 33 days and was undertaken by the female. The young fledged after a 36-day nestling period and remained under parental care for up to three weeks after that. Nesting success averaged 1.3 young per nest or 44% of the eggs laid. The post-juvenal moult commenced when the young were five to six months old and continued for six months. Birds comprised 98% of the diet of the falcons. The adults selected larger prey as the nestlings increased in size. The 69-day incubation and nesting period is up to 14 days longer than in similar-sized falcons.     

Alarm calls of the Southern House Wren Troglodytes musculus: variation with nesting stage and predator model

Alarm calls given by parents when risk is detected during nesting may be considered a form of parental defense. We analyzed variations in callings of breeding pairs of the Southern House Wren Troglodytes musculus during the nesting cycle and when faced with different predator models. Nesting birds were exposed to stuffed models at different nesting stages (early and late during incubation, and nests with younger and older nestlings). Nests were also exposed to different predator models where the calling response of breeding adults and acoustic structure variations of the calls were analyzed. The presence of a predator model increased the parents’ alarm calls along the nesting stage. This result supports the hypothesis that the higher the nest reproductive value, the higher the nest defense performed by the Southern House Wren. However, it also supports the notion that alarm calls could be used by parents to silence nestlings and reduce their detectability. Alarm calls also varied according to the predator model presented. We suggest that alarm calls variations of Southern House Wrens could encode information about the kind of predator and the risk envisaged through variations of call rates.     

THE AFRICAN SKIMMER RHYNCHOPS FLAVIROSTRIS: POPULATION COUNTS AND BREEDING IN THE NSEFU GAME RESERVE

Craig, A. J. F. K., Hulley, P. E. &; Walter, G. H. 1989. Nesting of sympatric Redwinged and Pale-winged Starlings. Ostrich 60:69-74.

Observations were made over four breeding seasons at Cradock, South Africa, where Redwinged Starlings Onychognathus morio and Palewinged Starlings O. nabouroup nest on the same cliffs. Nests were not accessible, and the stage of breeding was determined by the behaviour of the birds. Both species reuse the same nest sites, and only the females incubate, but both sexes feed the young. The timing of breedingMaybe more variable in the Palewinged Starling. A review of the available data on nest site selection and nest construction shows apparent species-specific differences: Redwinged Starlings usually nest on ledges, often on buildings, and use mud in the nest base; Palewinged Starlings favour vertical crevices, and do not use mud.     

Factors determining nest-site selection of surface-nesting seabirds: a case study on the world's largest pelagic bird,the Wandering Albatross (Diomedea exulans)

Several factors may drive bird nest-site selection, including predation risk, resource availability, weather conditions and interaction with other individuals. Understanding the drivers affecting where birds nest is important for conservation planning, especially where environmental change may alter the distribution of suitable nest-sites. This study investigates which environmental variables affect nest-site selection by the Wandering Albatross Diomedea exulans, the world's largest pelagic bird. Here, wind characteristics are quantitatively investigated as a driver of nest-site selection in surface-nesting birds, in addition to several topographical variables, vegetation and geological characteristics. Nest locations from three different breeding seasons on sub-Antarctic Marion Island were modelled to assess which environmental factors affect nest-site selection. Elevation was the most important determinant of nest-site selection, with Wandering Albatrosses only nesting at low elevations. Distance from the coast and terrain roughness were also important predictors, with nests more generally found close to the coast and in flatter terrain, followed by wind velocity, which showed a hump-shaped relationship with the probability of nest occurrence. Nests occurred more frequently on coastal vegetation types, and were absent from polar desert vegetation (generally above c. 500 m elevation). Of the variables that influence Wandering Albatross nest location, both vegetation type and wind characteristics are likely to be influenced by climate change, and have already changed over the last 50 years. As a result, the availability of suitable nest-sites needs to be considered in light of future climate change, in addition to the impacts that these changes will have on foraging patterns and prey distribution. More broadly, these results provide insights into how a wide range of environmental variables, including wind, can affect nest-site selection of surface-nesting seabirds.     

Dynamics of the abundance of seagulls (Charadriiformes: Laridae) in the northern Tatar Strait

Field observations on colonies of seagulls (Laridae) were carried out annually on islands of the northern Tatar Strait during 2001–2008. A general tendency towards a reduction in the number of nesting birds was observed on all islands surveyed. The anxiety factor and direct destruction of bird eggs and fledglings caused the whole or the greater part of colony to migrate to “quieter” sites on islands or the continental coast. The replacement of Larus crassirostris by L. schistisagus was observed in sites where the two species nest in neighborhood. Nesting specimens of L. schistisagus were found on islands of Lake Bolshoe Kizi, suggesting that the birds are in search of alternative nesting places that are less accessible to humans and predators.     

Anti-predator adaptations in nesting Marsh Tits Parus palustris: the role of nest-site security

A proposal that nest predation is the most important selective pressure shaping nest-site use of Marsh Tits Parus palustris was checked in a long-term study (almost 500 nests) carried out in the Białowieża National Park (eastern Poland) in primeval conditions, where the birds breed in natural holes and their nests are at risk from a variety of predators. It was predicted that predation rates would depend on hole attributes, so that Marsh Tits should use the most secure holes. Predation was responsible for 70% of total nest losses. Predation risk depended on hole attributes. Nests in dead wood were predated significantly more often than those in live wood. Nests in old woodpecker holes were predated more frequently than nests in holes of other origin, and nests situated closer to the entrance were more at risk than ones further from it. The entrance size did not influence overall predation risk, but small entrance size was important in preventing access by larger predators. These differences can account for the very rare use of holes in dead wood or of woodpecker holes, the fact that they nest in holes with small entrances and relatively far from the entrance. It is concluded that the patterns of nest-site use found in this species are best explained as anti-predator adaptations, which have evolved and are maintained by the pressure of nest predators. This study also indicates the possible limits of, and constraints on, these adaptations.     

BREEDING BEHAVIOUR OF OSPREYS PANDION HALIAETUS IN SCOTLAND

Ospreys Pandion haliaetus nested at a site near Loch Garten, Inverness-shire continuously from 1959 to 1973. Each year the Royal Society for the Protection of Birds has organized a continuous watch on the eyrie in the breeding season. The detailed records kept of the activities of Ospreys at the nest by those participating in the watch were analysed and the results presented here. Ospreys are migratory and arrived in the breeding area in early April. Nesting material was usually added to an existing eyrie platform. The male collected more material than the female. The female lined the nest cup. The extent of nest building activity and the frequencies of mating and other activities prior to laying varied markedly from year to year. These differences may have been related to changes in the identity of the nesting female, but the birds were not individually marked. Both sexes incubated but the female took the greater share and normally incubated at night. When the young hatched they were brooded by the female. The female stayed in the vicinity of the nest for most of the time until the young fledged at about 53 days old. The male Osprey caught almost all the fish eaten by his mate and young during the breeding season. The number of fish caught per day increased markedly after the young hatched. Pike Esox lucius and Trout Salmo trutta were the main species taken, and some Rainbow Trout Salmo gairdnerii were identified. There were seasonal and diurnal changes in the size and the species composition of the catch. The effects of weather conditions on hunting are examined. The occurrence of Ospreys other than the resident birds at the nest site is described. The behaviour of another pair of Ospreys which repeatedly failed to hatch eggs is described. There was an instance of egg eating in this pair, and some differences in behaviour were found between these birds and those at Loch Garten whose breeding success was good. The breeding biology of Ospreys is compared with that of other British diurnal birds of prey. In other species the female leaves the young unguarded at some stage in the nestling period and hunts food for them, whereas female Ospreys do not usually hunt in the nesting period.     

Nest location, clutch size and nest success in the Scarlet Ibis Eudocimus ruber

Breeding success and nest-site characteristics were studied during the 1996–1997 breeding season in a colony of Scarlet Ibises Eudocimus ruber in south-eastern Brazil to test the hypothesis that nest-site characteristics and clutch size affect nest success. Two nesting pulses produced young, the earlier being more successful. Predation accounted for most failures during the first pulse, wind destruction during the second. A third pulse with few nests produced no young. Adult Ibises abandoned nests when they lost sight of other incubating birds. Logistic regression analysis indicated that nest success during the first pulse was positively related to clutch size, number of nests in the nest tree and in the nearest tree, and negatively to the distance to the nearest neighbour. During the second pulse there were significant negative associations between success, nest height and distance to the fourth nearest nest, and a positive association between success and nest cover. The results agree with the 'selfish herd' hypothesis, indicating that nest aggregation may increase breeding success, but the nest-site characteristics affecting success can differ over the course of one breeding season.     

Use of exotic conifers as nesting sites by Black-faced Ibis (Theristicus melanopis melanopis) in an urban area of southern Chile

We studied the influence of seven habitat variables, including tree species, for nesting by the Black-faced Ibis (Theristicus melanopis melanopis) in an urban area of southern Chile. Variables were compared between 30 trees with nests and 30 randomly selected trees without nests. Nests were found in big trees with large diameters and heights. However, the only variable found to have a significant effect on site selection was tree species, which explained 57.9% of data variability (deviance) and suggested a selection of exotic conifers, mainly Douglas fir (Pseudotsuga menziesii). Tree species and tree diameter also had significant effects upon the number of nests per tree, jointly explaining 68.9% of data deviance. Our results suggest that in urban environments the Black-faced Ibis uses larger trees that provide greater nest stability and protection.     

Strategies for Nest-Site Selection by King Eiders

ABSTRACT Nest site selection is a critical component of reproduction and has presumably evolved in relation to predation, local resources, and microclimate. We investigated nest-site choice by king eiders (Somateria spectabilis) on the coastal plain of northern Alaska, USA, 2003–2005. We hypothesized that nest-site selection is driven by predator avoidance and that a variety of strategies including concealment, seclusion, and conspecific or inter-specific nest defense might lead to improved nesting success. We systematically searched wetland basins for king eider nests and measured habitat and social variables at nests (n = 212) and random locations (n = 493). King eiders made use of both secluded and concealed breeding strategies; logistic regression models revealed that females selected nests close to water, on islands, and in areas with high willow (Salix spp.) cover but did not select sites near conspecific or glaucous gull (Larus hyperboreus) nests. The most effective nest-placement strategy may vary depending on density and types of nest predators; seclusion is likely a mammalian-predator avoidance tactic whereas concealment may provide protection from avian predators. We recommend that managers in northern Alaska attempt to maintain wetland basins with islands and complex shorelines to provide potential nest sites in the vicinity of water.     

THE BREEDING OF COLLOCALIA GIGAS

The nest of Collocalia gigas is a solid cup with a shallow depression for the egg, constructed of plant material with moist cement. The main nest materials are living liverworts (Mastigophora and Herberta spp.) with green fern fronds or whole ferns (Hymenophyllaceae and Asplenium sp.) in the nest wall. Minor components are moss (Aerobryopsis, Ectropothecium and Meteorium spp.) and aerial roots of epiphytes (Orchidaceae). Nests are always associated with falling water, being usually placed on ledges or in crevices behind or very close to waterfalls. Nests are single or in small colonies of up to ten nests. The clutch consists of a single egg. Egg-weight is 13-7% of the adult's body-weight, rather lower than in other Collocalia species. In nest-site and habits, and in some morphological characters there is a marked convergence, or a relationship, between C. gigas and the tropical American genus Cypseloides.     

The role of conservative versus innovative nesting behavior on the 25‐year population expansion of an avian predator

Species ranges often change in relation to multiple environmental and demographic factors. Innovative behaviors may affect these changes by facilitating the use of novel habitats, although this idea has been little explored. Here, we investigate the importance of behavior during range change, using a 25‐year population expansion of Bonelli's eagle in southern Portugal. This unique population is almost exclusively tree nesting, while all other populations in western Europe are predominantly cliff nesting. During 1991–2014, we surveyed nest sites and estimated the year when each breeding territory was established. We approximated the boundaries of 84 territories using Dirichlet tessellation and mapped topography, land cover, and the density of human infrastructures in buffers (250, 500, and 1,000 m) around nest and random sites. We then compared environmental conditions at matching nest and random sites within territories using conditional logistic regression, and used quantile regression to estimate trends in nesting habitats in relation to the year of territory establishment. Most nests (>85%, n = 197) were in eucalypts, maritime pines, and cork oaks. Nest sites were farther from the nests of neighboring territories than random points, and they were in areas with higher terrain roughness, lower cover by agricultural and built‐up areas, and lower road and powerline densities. Nesting habitat selection varied little with year of territory establishment, although nesting in eucalypts increased, while cliff nesting and cork oak nesting, and terrain roughness declined. Our results suggest that the observed expansion of Bonelli's eagles was facilitated by the tree nesting behavior, which allowed the colonization of areas without cliffs. However, all but a very few breeding pairs settled in habitats comparable to those of the initial population nucleus, suggesting that after an initial trigger possibly facilitated by tree nesting, the habitat selection remained largely conservative. Overall, our study supports recent calls to incorporate information on behavior for understanding and predicting species range shifts.     

Nesting biology of the Short-tailed Paradigalla Paradigalla brevicauda

Six nesting attempts of a ringed Short-tailed Paradigalla Paradigalla brevicauda between September 1986 and February 1989 in Papua New Guinea are reported. Nests are described and one illustrated. Previously undescribed eggs, of two single-egg clutches from the same female, lack broad longitudinal streaks typical of most Paradisaeinae eggs. The hatchling is naked, dark-skinned, with fully formed facial wattles. Fifty seven hours of observation at one nest confirmed single parent attendance. Of 66 identified nestling meals 65% were animal, including frogs and skinks, the remainder fruit. Growth and development of a nestling, that fledged at 26 days old, are described. Another young was fed by its parent 108 days after leaving the nest. Breeding and feeding of P. brevicauda are reviewed. The presumed female studied attended six nests within 100 m² over 29 months. Uniparental nesting suggests males are promiscuous, which is discussed with respect to proposed Paradigalla hybrids.     

THE BIOLOGY OF THE WAVED ALBATROSS DIOMEDEA IRRORATA OF HOOD ISLAND, GALAPAGOS

As a nesting species, the Waved Albatross Diomedea irrorata is restricted to Hood Island in the Galapagos archipelago where 12,000 pairs bred in 1971. Outside the islands the species occurs over the northern parts of the Humboldt Current. Two colonies were studied in detail (1970–1971). At the start of a season, males returned first to the colonies and defended a small territory. Copulation occurred without any elaborate ceremony and the female spent little time on land before laying. There was no fixed nest-site, even within a season, and birds moved their eggs considerable distances. This resulted in heavy egg losses. Younger birds bred later than older birds and laid longer but narrower eggs. The average incubation spell varied from four to five days at the extremes of the incubation period to 19 days in the middle. The average incubation and fledging periods were 60 and 167 days respectively. Pairs which lost an egg sometimes adopted the abandoned egg of another bird and successfully reared the chick. Most pairs nested in both seasons. Nesting success was extremely variable, both between years and between colonies. Between 1961 and 1971 at Punta Suarez, virtually no young were reared in four seasons. Even in 1970–71, where nesting success was good, some groups of birds deserted their eggs en masse whereas in neighbouring areas up to 80% of the pairs reared young. The main foods of the young were squid and fish. Birds did not moult wing and tail feathers at the breeding colonies, and about 50% retained some primaries for more than one season, suggesting that successful pairs had difficulty in fitting in a complete moult between breeding attempts. Old feathers were normally found among the inner primaries and at the next moult were preferentially replaced, though adjacent newer feathers were sometimes retained for another season. Some birds bred in their fourth years, but most not until a year or two older. Immatures were present at the colonies late in the breeding cycle, the youngest returning latest and remaining until the last young fledged. Survival of adults and young averaged at least 95% and 93% per annum over many years. Adults and young ringed in 1961 survived equally well. The significance of the timing of the return of immatures and of the large-scale desertion of eggs, apparently not due to food shortage or disturbance, is discussed.