Preferred Prey Species Research Articles

Does the risk of encountering lions influence African herbivore behaviour at waterholes?

A central question in the study of predator–prey relationships is to what extent prey behaviour is determined by avoidance of predators. Here, we test whether the long-term risk of encountering lions and the presence of lions in the vicinity influence the behaviour of large African herbivores at waterholes through avoidance of high-risk areas, increases in group size, changes in temporal niche or changes in the time spent in waterhole areas. In Hwange National Park, Zimbabwe, we monitored waterholes to study the behaviour of nine herbivore species under different risks of encountering lions. We radio-collared 26 lions in the study area which provided the opportunity to monitor whether lions were present during observation sessions and to map longer-term seasonal landscapes of risk of encountering lions. Our results show that the preferred prey species for lions (buffalo, kudu and giraffe) avoided risky waterholes. Group size increased as encounter risk increased for only two species (wildebeest and zebra), but this effect was not strong. Interestingly, buffalo avoided the hours of the day which are dangerous when the long-term and short-term risks of encountering lions were high, and all species showed avoidance of waterhole use at night times when lions were in the vicinity. This illustrates well how prey can make temporal adjustments to avoid dangerous periods coinciding with predator hunting. Additionally, many herbivores spent more time accessing water to drink when the long- and short-term risks of encountering lions were high, and they showed longer potential drinking time when the long-term risk of encountering lions was high, suggesting higher levels of vigilance. This study illustrates the diversity of behavioural adjustments to the risk of encountering a predator and how prey respond differently to temporal variations in this risk.

Food habits and prey selection of tiger and leopard in Mudumalai Tiger Reserve, Tamil Nadu, India

Food habits and prey selection of tiger (Panthera tigris) and leopard (Panthera pardus) in Mudumalai Tiger Reserve, Tamil Nadu were assessed from January to August 2008. Chital, Axis axis was the most common prey species in the study area with a density of 55.3 ± 6.28 animals/km 2 followed by common langur Presbytis entellus with 25.9 ± 3.59 animals/ km 2 and gaur Bos gaurus with 11.4 ± 2.14 animals/km 2 .The estimated mean biomass of the potential prey species was 8365.02 kg/km 2 . A total of 179 tiger scats and 108 leopard scats were collected and the prey remains were analyzed. Sambar and chital were the principle prey species for tiger and leopard, respectively, as inferred from the relative biomass consumption of prey remains in tiger and leopard scats. The preferred prey species of leopard and tiger were sambar, common langur, wild pig and cattle. The dietary overlap between these two predators was 82% in terms of percentage frequency of occurrence of prey remains in the scats. In terms of biomass consumed, the estimated dietary overlap between tiger and leopard was 72%.

Predator–prey relationships and responses of ungulates and their predators to the establishment of protected areas: A case study of tigers, leopards and their prey in Bardia National Park, Nepal

Interactions among sympatric large predators and their prey and how they respond to conservation measures are poorly known. This study examines predictions concerning the effects of establishing a protected area in Nepal on tigers ( Panthera tigris), leopards ( Panthera pardus), and their ungulate prey. Within a part of the park, after 22 years the total density of wild ungulates had increased fourfold, to ca. 200 animals/km 2, almost exclusively due to a remarkable increase in chital deer ( Axis axis). Tiger density also increased markedly to nearly 20 animals/100 km 2, whereas leopard density did not and was ca. 5 animals/100 km 2. The prediction that grazers should increase more than browsers was only partially supported. The prediction of positive density dependence in prey selection was not supported. Instead, the most abundant species (chital and hog deer, Axis porcinus) were killed less frequently than expected, whereas the lower-density wild boar ( Sus scrofa) was preferred. Predictions that (i) initially rare species suffer highest predation was partially supported, that (ii) predation is highest among the most abundant prey was not supported, and that (iii) predation is highest among the most preferred prey independently of their densities was supported. Clearly, the conservation efforts adopted in Bardia were successful, as both tigers and their natural prey base increased. However, the positive numerical response of tigers limited and depressed the abundance of some prey species. Thus, conservation activities aimed at restoring large predators are likely to change in the composition of the overall mammal community, potentially eliminating rare but preferred prey species.

Do marine birds use environmental cues to optimize egg production? An experimental test based on relaying propensity

According to the environmental cues hypothesis, female birds use information available to them in the early‐season environment to fine‐tune egg production annually. However, support for the hypothesis derives largely from correlational studies. In each year from 2002 to 2006, which spanned a period of extreme variation in environmental conditions, we removed eggs from early‐laying rhinoceros auklets Cerorhinca monocerata, burrow‐nesting seabirds that lay a single‐egg clutch. We then measured their relaying rates, relaying intervals, and breeding success. We also monitored the timing and success of breeding in control pairs, and control chick diets. If the experimental females base their relaying decision on early‐season cues, then we predict that few will relay in years in which early‐laying control birds breed unsuccessfully, and in which a preferred prey species, Pacific sandlance Ammodytes hexapterus, is in short supply in nestling diets. Results matched neither prediction. In each year, almost all (88–90%) of the experimental females relaid, despite that the control pairs’ breeding success (32–87% fledged chicks), and their chicks’ diets (twofold variation in proportion of sandlance), varied markedly. We conclude that female rhinoceros auklets did not modify their relaying decision in response to variation in environmental conditions, although relaying intervals and their own breeding success (0–78%) covaried negatively. Our results may have important implications related to using seabirds as monitors of the marine environment.

The effect of signal complexity on localization performance in bats that localize frog calls

The fringe-lipped bat, Trachops cirrhosus, uses frog mating calls to detect and locate its prey. The tungara frog, Physalaemus pustulosus, a preferred prey species of this bat, produces two types of sexual advertise- ment calls, simple and complex, and both female frogs and predatory bats prefer complex calls to simple ones. Complex calls differ from simple ones in that they contain chucks: short, broadband suffixes with distinct onsets and offsets, acoustic properties that should maximize binaural comparisons and facilitate localization. We investigated the hypothesis that frog-eating bats prefer the complex calls of tungara frogs to simple ones because they find complex calls easier to localize. We tested bats under experimental con- ditions that mirror the conditions they encounter in nature: we broadcast tungara frog calls with and with- out background noise and with and without intervening obstacles. We broadcast calls either continuously during the hunting approach or only prior to the bat's flight to mimic the conditions under which frogs have detected an approaching bat and ceased calling. Bats showed a trend for better localization perfor- mance of complex calls than of simple ones under all treatment conditions. We found significant differ- ences in localization performance in some but not all levels of localization task complexity. This study is the first to offer evidence that an eavesdropping predator shows better localization performance for a pre- ferred signal variant of its prey.

Swan foraging shapes spatial distribution of two submerged plants, favouring the preferred prey species

Compared to terrestrial environments, grazing intensity on belowground plant parts may be particularly strong in aquatic environments, which may have great effects on plant-community structure. We observed that the submerged macrophyte, Potamogeton pectinatus, which mainly reproduces with tubers, often grows at intermediate water depth and that P. perfoliatus, which mainly reproduces with rhizomes and turions, grows in either shallow or deep water. One mechanism behind this distributional pattern may be that swans prefer to feed on P. pectinatus tubers at intermediate water depths. We hypothesised that when swans feed on tubers in the sediment, P. perfoliatus rhizomes and turions may be damaged by the uprooting, whereas the small round tubers of P. pectinatus that escaped herbivory may be more tolerant to this bioturbation. In spring 2000, we transplanted P. perfoliatus rhizomes into a P. pectinatus stand and followed growth in plots protected and unprotected, respectively, from bird foraging. Although swan foraging reduced tuber biomass in unprotected plots, leading to lower P. pectinatus density in spring 2001, this species grew well both in protected and unprotected plots later that summer. In contrast, swan grazing had a dramatic negative effect on P. perfoliatus that persisted throughout the summer of 2001, with close to no plants in the unprotected plots and high densities in the protected plots. Our results demonstrate that herbivorous waterbirds may play a crucial role in the distribution and prevalence of specific plant species. Furthermore, since their grazing benefitted their preferred food source, the interaction between swans and P. pectinatus may be classified as ecologically mutualistic.

Evidence for the persistence of Arabian Wolf (Canis lupus pallipes) in the Ibex Reserve, Saudi Arabia and its preferred prey species

Numbers of Arabian Wolf (Canis lupus pallipes) on the Arabian Peninsula are in decline and intact ecosystems in which large carnivores prey on larger mammals are virtually absent in the Kingdom of Saudi Arabia. Moreover, many canids roaming in the Kingdom are hybrids between feral dogs and wolves. In this study we provide genetic evidence for the persistence of wolves in the Jebel Tuwaiq ecosystem (Ibex Reserve) in central Saudi Arabia and we provide information on the preferred prey species of wolves in this protected area.

Snake Pipefish Entelurus aequoreus are poor food for seabirds

There is now a general consensus that widespread changesare occurring in the world’s oceans, leading to concernsabout the effect that changes in fish stocks are likely tohave on the more obvious and charismatic wildlife, includ-ing seabirds. As regards seabirds, two questions are oftenposed: (1) if environmental change results in the decline ofthe preferred prey species, will numbers of other fish increase?;and (2), if so, then are these possible alternative prey likelyto be as useful to seabirds as the species that are lost?Starting in about 2003, the abundance and range of theSnake Pipefish

Carrying capacity of large African predators: Predictions and tests

Successful conservation initiatives often lead to rapid increases in large carnivore densities to the extent that overpopulation occurs. Yet conservation managers have no way of knowing the carrying capacity of their reserves. Here we derived relationships between the preferred prey (species and weight range) of Africa’s large predator guild and their population densities to predict their carrying capacity in ten South African conservation areas. Conservation managers intervened at several of these sites because of evidence of predator overpopulation and these provided independent tests of our predictions. Highly significant linear relationships were found between the biomass of the preferred prey species of lion, leopard, spotted hyaena and African wild dog, and the biomass of prey in the preferred weight range of cheetah. These relationships are more robust than previous work for lion, cheetah and leopard, and novel for spotted hyaena and African wild dog. These relationships predicted that several predators exceeded carrying capacity at four sites, two where managers expressed concerns about overpopulation due to a decline in wildlife abundance and two where carnivores were actively removed. The ability to predict the carrying capacity of large predators is fundamental to their conservation, particularly in small enclosed reserves. Every predator that preys on large, readily surveyed wildlife can have its carrying capacity predicted in this manner based on the abundance of its preferred prey. This will be beneficial for reintroduction attempts, threatened species management, overpopulation estimation, detecting poaching and in investigating intra-guild competition.

PREY PREFERENCES OF THE AFRICAN WILD DOG LYCAON PICTUS (CANIDAE: CARNIVORA): ECOLOGICAL REQUIREMENTS FOR CONSERVATION

Valuable conservation research on the African wild dog (Lycaon pictus) has identified that its current endangerment is primarily due to human persecution, although habitat alteration, interference competition with other large predators, and disease also are factors. Numerous studies have thus determined what should be avoided to sustain an African wild dog population, yet in this study we identify what is needed to conserve a wild dog population by using Jacobs' index to determine its preferred prey species. Twenty-four assessments of wild dog prey preference were calculated from 18 studies involving 4,874 kills of 45 species from throughout its distributional range. Wild dogs prefer prey within a bimodal body mass range of 16–32 kg and 120–140 kg, which is abundant and less likely to cause injury when hunted. This bimodal range follows that of optimal wild dog pack sizes based on energetic costs and benefits. Greater kudu (Tragelaphus strepsiceros) and Thomson's gazelle (Gazella thomsonii) are killed by wild dogs wherever they coexist and are significantly preferred. Impala (Aepyceros melampus) and bushbuck (Tragelaphus scriptus) also are significantly preferred. Our results allow wildlife managers to more accurately assess the survival chances of reintroduced or small wild dog populations by determining if sufficient preferred prey are available. These techniques are applicable to all adequately studied large predators.

Prey preferences of the spotted hyaena (Crocuta crocuta) and degree of dietary overlap with the lion (Panthera leo)

AbstractSpotted hyaenas Crocuta crocuta were once considered mere scavengers; however, detailed research revealed that they are very efficient predators. Information on what spotted hyaenas actually prefer to prey on and what they avoid is lacking, as well as the factors that influence prey selection. Data from 14 published and one unpublished study from six countries throughout the distribution of the spotted hyaena were used to determine which prey species were preferred and which were avoided using Jacobs' index. The mean of these values for each species was used as the dependent variable in multiple regression, with prey abundance and prey body mass as predictive variables. In stark contrast to the rest of Africa's large predator guild, spotted hyaenas do not preferentially prey on any species. Also surprisingly, only buffalo, giraffe and plains zebra are significantly avoided. Spotted hyaena most prefer prey within a body mass range of 56–182 kg, with a mode of 102 kg. The dietary niche breadth of the spotted hyaena is similar to that of the lion Panthera leo, and the two species have a 58.6% actual prey species overlap and a 68.8% preferred prey species overlap. These results highlight the flexible and unselective nature of spotted hyaena predation and are probably a reason for the species' success throughout its range, despite a large degree of dietary overlap with lions.

Food-limitation in a generalist predator

Investigating food-limitation in generalist predators is difficult, because they can switch to alternative prey, when one of their staple prey becomes scarce. Apart from data on the dynamics of the predator population, a robust study requires: (i) a documentation of the predator's entire prey base; and (ii) an experimental or natural situation, where profitable dietary shifts are impossible, because several preferred prey species decline simultaneously. Here, we provide a detailed description of how food-supply has limited a generalist avian top predator, the northern goshawk Accipiter gentilis. In our study area, populations of several principal goshawk prey species crashed simultaneously during 1975-2000, whereas other extrinsic factors remained essentially unchanged. The breeding and non-breeding segments of the local goshawk population declined markedly, associated with a significant increase in nest failures. Brood size of successful pairs remained unaffected by changes in prey availability. Breeding recruitment ceased at a time when potential replacement birds ('floaters') were still present, providing a rare empirical demonstration of an 'acceptance threshold' in raptor territory choice. To investigate how goshawk diet changed in response to varying food-supplies, we make novel use of an analytical tool from biodiversity research-'abundance-biomass-comparison curves' (ABC curves). With increasing levels of food-stress, the dominance of principal prey species in the diet decreased, and the number of small-bodied prey species increased, as did intra-guild predation. Our finding that breeder and non-breeder segments declined in concert is unexpected. Our results carry the management implication that, in food-limited raptor populations, externally induced breeder mortality can rapidly depress population size, as losses are no longer buffered when floaters reject breeding opportunities.

Epilimnetic rotifer community responses to Bythotrephes longimanus invasion in Canadian Shield lakes

We studied whether the effects of Bythotrephes longimanus, a new predatory cladoceran zooplankton species to North America, can extend beyond the macrozooplankton to the rotifer community, which is known to be an important controller of the microbial loop. During summer 2003, 14 Canadian Shield lakes were sampled biweekly to evaluate the effect of Bythotrephes on epilimnetic rotifers. Rotifer densities, particularly those of Conochilus unicornis, significantly increased, with elevated Bythotrephes densities. Conochilus became the dominant rotifer species in the pelagic community of lakes with high B. longimanus populations. Bythotrephes densities were a better estimator of rotifer densities than physical and chemical lake properties. Bythotrephes has indirect effects beyond its preferred prey species, cladocerans. The potential for increased rotifer densities to act as a secondary source of food for Bythotrephes was also investigated. In a 24-h feeding experiment, Bythotrephes did not prey on various common rotifer species, suggesting that Bythotrephes ignores rotifer species as a prey even when they form the sole food source. Rotifer increases are likely either due to competitive release when crustacean zooplankton decline or predatory release when native invertebrate predator populations decline.

Prey preferences of the lion (Panthera leo)

AbstractLions Panthera leo are generally thought to prey on medium to large ungulates. Knowledge of which species are actually preferred and which are avoided is lacking, however, as is an understanding of why such preference or avoidance may arise. An analysis of 32 studies over 48 different spatial locations or temporal periods throughout the distribution of the lion shows that it preferentially preys upon species within a weight range of 190–550 kg. The most preferred weight of lion prey is 350 kg. The mean mass of significantly preferred prey species is 290 kg and of all preferred species is 201 kg. Gemsbok, buffalo, wildebeest, giraffe and zebra are significantly preferred. Species outside the preferred weight range are generally avoided. Species within the preferred weight range that are not significantly preferred (such as roan, sable and eland) generally have features that reduce predation either morphologically (e.g. sable horns), ecologically (e.g. roan and sable occurring at low density), or behaviourally (e.g. the large herd size and increased vigilance of eland). Warthog are below the preferred weight range yet are taken in accordance with their availability and this is probably due to their sympatry with lion, their relatively slow evasion speed and their lower level of vigilance. Plots of prey preference against prey body mass follows a bell curve with a right skew that, we argue, is caused by collective hunting by lions of larger‐bodied prey. Our methods can be used on all large predators and are likely to be useful in assessing competition in sympatric communities of predators, cooperative hunting and predicting predator diets. This will allow us to move beyond descriptive dietary studies to improve our predictive understanding of the mechanisms underlying predator–prey interactions.

PREDATION POTENTIAL OF THE INVASIVE GREEN CRAB (CARCINUS MAENAS) AND OTHER COMMON PREDATORS ON COMMERCIAL BIVALVE SPECIES FOUND ON PRINCE EDWARD ISLAND

The prey and size-class preferences of four marine epibenthic predators was examined in a laboratory study by determining the mortality rates of four commercial bivalve species in 4-d trials where predators were present or absent. Bivalve species used were quahogs (Mercenaria mercenaria), eastern oysters (Crassostrea virginica), blue mussels (Mytilus edulis) and soft-shell clams (Mya arenaria). Prey size-classes investigated were 0–15, 15–25 and 25–40 mm. A first set of experiments compared the mortality rates of prey in the presence of the invasive green crab (Carcinus maenas) and the native rock crab (Cancer irroratus). The second set of experiments compared the predation behavior of the green crab, the common starfish (Asterias vulgaris) and the moon snail (Euspira heros). Single- and multiple-choice experiments were carried out in relation to the prey species being challenged by the predator. Results from the first set of experiments showed that green crabs preyed on all prey species from all size-classes in single and multiple-choice trials. Mussels and clams were the preferred prey species. Rock crabs preyed on mussels in single- and multiple-choice trials. Rock crabs preyed heavily on soft-shell clams in single-choice trials, whereas no clams were eaten in multiple-choice trials. Rock crabs did not prey on quahogs in single- and multiple-choice trials. In both trials, small individuals were preyed on more often by both crab species. The second set of experiments confirmed results observed for the green crab in the first set of experiments. The common starfish was a very active predator as well. Blue mussels were the preferred prey species of the starfish. Moon snails displayed a much lower predation activity than the other predators. The multiple-choice experiment showed different results in that predation rates were lower than in cases where predators were facing one prey species at a time. Overall, our results showed that the two crab species and the common starfish displayed a generalist feeding behavior. Results also showed that the invasive green crab might represent a new predation threat to commercial bivalves as well as a competing threat to the native rock crab.

Comparative life history traits of three neotropical phytoseiid mites maintained on plant-based diets

Abstract Plant-based foods may serve as alternative resources in meeting the nutritional requirements of predatory mites, when the preferred prey species is scarce. In laboratory studies, we evaluated the effect of various plant-based food resources associated with cassava on development and reproduction of Neoseiulus (= Amblyseius ) idaeus Denmark and Muma, Typhlodromalus manihoti Moraes, and Typhlodromalus aripo DeLeon, three predatory mites that have been used in classical biological control of the herbivorous mite Mononychellus tanajoa (Bondar) in Africa. The tests included five treatments (diets): pollen of three plant species— Zea mays L. (maize), of Leucaena leucocephala (Lam.) De Wit, and of Ricinus communis L.—cassava foliar exudate, and cassava leaf tissue (as the control). Neoseiulus idaeus did not complete development (hence no reproduction occurred) on any of the offered foods. Both T. aripo and T. manihoti successfully completed immature development—albeit in small proportions—on pollen of maize and L. leucocephala and on cassava foliar exudate. Only T. aripo completed immature development on pollen of R. communis and cassava leaf tissue; on the latter two foods, however, only males reached the adult stage. Moreover, of the five diets, none except pollen of maize was suitable for reproduction, and only for T. aripo , as T. manihoti did not reproduce on any of these diets. The duration of immature development of T. aripo was shorter on pollen of maize (6.9 days) than on cassava foliar exudate (11.1 days), while an intermediate value was recorded on pollen of L. leucocephala (8.6 days). Typhlodromalus manihoti developed faster on pollen of maize and cassava foliar exudate (6.8 days) than on pollen of L. leucocephala (9.8 days). Typhodromalus aripo lived much longer on pollen of maize and cassava foliar exudate compared with T. manihoti , whereas the two predator species had nearly similar female longevity on pollen of L. leucocephala . These results suggest that consumption of pollen of maize, pollen of L. leucocephala , or cassava foliar exudate can enhance development of T. aripo and T. manihoti in the absence of M. tanajoa , their preferred prey. Pollen of maize was suitable for T. aripo in completing oogenesis and hence allowing the predator to increase in abundance during shortages of M. tanajoa . The implications of our findings for the persistence of the acarine predator–prey system on cassava, and for the success of biological control of M. tanajoa in Africa are discussed.

Feeding ecology of the Indo-Pacific bottlenose dolphin ( Tursiops aduncus) incidentally caught in the gillnet fisheries off Zanzibar, Tanzania

The stomach contents of 26 Indo-Pacific bottlenose dolphins ( Tursiops aduncus) incidentally caught in gillnet fisheries around Unguja Island (Zanzibar) between February 2000 and August 2002 were examined. The relative importance of each prey species was assessed through indices of relative importance. In total, 1403 prey items comprising 50 species of bony fish and three species of squid were identified from food remains. Five species of fish, Uroconger lepturus, Synaphobranchus kaupii, Apogon apogonides, Lethrinus crocineus, Lutjanus fulvus, and three species of squid, Sepioteuthis lessoniana, Sepia latimanus and Loligo duvauceli, were the most important prey species. Based on an index that included frequency of occurrence, percentage by number and by weight, Uroconger lepturus proved to be the most important prey species of mature dolphins whereas Apogon apogonides was the preferred prey of immature dolphins. These results indicate that Indo-Pacific bottlenose dolphins off the coast of Zanzibar forage on a relatively large number of prey species, but that only a few small- and medium-sized neritic fish and cephalopods contribute substantially to the diet. Further, the ecology and behavior of the preferred fish prey species indicate that the dolphins forage over reef or soft bottom substrata and near the shore.

The effects of grazing on spider assemblages in upland heather moorland

Invertebrates are of particular interest on heather moorlands because of their rapid response to small scale habitat changes. Spiders constitute a variable proportion of the diet of heather moorland bird, mammal and reptile species. Furthermore, spiders may be an important indicator of habitat change resulting from management practices, especially those that exert a large- scale spatial impact, e.g. grazing. The component families and species of spider assemblages indicate, through their differing preference for web attachments and web structure, the vegetation density, height and structure (Marc et al., 1999). This effect may be especially pronounced where continued grazing at a particular stocking rate results in characteristic patterns in the vegetation structure. Species and family specific hunting strategies determine prey type and so the presence of some spider species may indicate the presence of a preferred prey species. This study aims to investigate the effect of grazing with sheep alone or in combination with cattle grazing on an upland heather moorland in the north of England.

Reed cutting affects arthropod communities, potentially reducing food for passerine birds

Winter cutting of Common Reed Phragmites australis (Poaceae) is increasing in Camargue, southern France, potentially affecting wetlands of high conservation value. We studied its impact on arthropods by comparing four cut and uncut sites with special respect to avian prey availability in the breeding season. The two most important prey groups for breeding passerines, spiders (Araneida) and beetles (Coleoptera), were identified to species in trap catches and diet samples. The arthropod assemblages differed significantly between cut and uncut sites. Phytophagous and saprophagous species showed contrasting responses. Numbers of homopterans increased in cut reed beds, where green Phragmites stem density was higher. Saprophagous woodlice decreased, presumably due to the reduced amount of litter. Densities of some vegetation-dwelling spider and beetle species were lower at cut sites, including two of the most preferred prey species for passerine birds. Consequently, large-scale mechanically harvested reed beds host altered arthropod communities, missing major food components used by vulnerable passerines. However, reed cutting on a small scale may increase habitat heterogeneity and species richness on a landscape level. To contribute to reed bed conservation, EU schemes should reward management that leaves uncut reed patches in the proximity of cut areas to permit their recolonization by arthropods.

Rates and consequences of relaying in Cassin's auklets Ptychoramphus aleuticus and rhinoceros auklets Cerorhinca monocerata breeding in a seasonal environment

We removed first eggs from early‐laying females to measure rates and consequences of relaying in Cassin's auklets Ptychoramphus aleuticus and rhinoceros auklets Cerorhinca monocerata at Triangle Island, British Columbia, Canada. Based on egg size and composition, the investment that Cassin's auklets made in first eggs was very close to that predicted from adult body mass, whereas rhinoceros auklets invested more. In both species, a high percentage of females relaid (90% of Cassin's and 87% of rhinoceros auklets). Breeding success declined weakly with later laying among control Cassin's auklet pairs, but pairs that we induced to relay bred more successfully than naturally late pairs, and similar to values predicted from laying dates of their first eggs. Their chicks also fledged heavier and younger than late control chicks, and similar to values in early control chicks, but followed the population‐wide seasonal decline in wing length at fledging. Nestling diets were dominated by Neocalanus copepods until late in the season, a sign that feeding conditions remained favourable until late. In contrast, rhinoceros auklet pairs induced to relay followed the population‐wide seasonal decline in breeding success, which was driven by a decline in hatching success. Pacific sandlance Ammodytes hexapterus, thought to be a preferred prey species, virtually disappeared from nestling diets in mid‐to‐late season, yet there was no seasonal decline in fledging mass. However, chicks from replacement eggs followed the declines among control chicks in both age and wing length at fledging. Despite the female having produced a replacement egg, and despite delayed breeding, there appeared to be little immediate consequence associated with relaying for Cassin's auklets, except for a tendency for their chicks to fledge with short wings. Consequences were more marked in rhinoceros auklets (greatly reduced hatching success, and having their chicks fledge with short wings), and this may have been due to the large investment made in eggs, and/or to delayed breeding. Results of this study show that attributes of Cassin's and rhinoceros auklets that lay at different times in the season can be important in driving seasonal declines in breeding performance, as found in studies on other Alcidae. They also show how decisions taken during the egg stage can have variable yet potentially important implications for fitness, even in relatively long‐lived species that lay single‐egg clutches.