Uria Lomvia Research Articles

Within and between species competition in a seabird community: statistical exploration and modeling of time-series data

In a changing environment, the maintenance of communities is subject to many constraints (phenology, resources, climate, etc.). One such constraint is the relationship between conspecifics and competitors. In mixed colonies, seabirds may have to cope with interspecific and intraspecific competition for both space and food resources. We applied competitive interaction models to data on three seabird breeding populations: black-legged kittiwake (Rissa tridactyla), common guillemot (Uria aalge) and Brünnich's guillemot (Uria lomvia) collected over 27-years at Kharlov Island in the Barents Sea. We found a competitive effect only for the kittiwake breeding population size on the common guillemot breeding population size when kittiwakes were abundant. The timing of kittiwake breeding negatively affected the number of breeding Brünnich's guillemots. The timing of breeding was negatively correlated to biomass of the main pelagic fish in the Barents Sea, the capelin (Mallotus villosus), which suggests an indirect action. The community matrix shows that the community was not stable. The kittiwake population did not decrease as seen in north Norwegian populations. Likewise, the common guillemot population, after a crash in 1985, was recovering at Kharlov while Norwegian populations were decreasing. Only the Brünnich's guillemot showed a decrease at Kharlov until 1999. We suggest that the stability of the kittiwake and common guillemot populations at Kharlov is due to better feeding conditions than in colonies of the Norwegian coast, linked to a possible eastward shift of the capelin population with the temperature increase of the Barents Sea.

Isotopic segregation between sympatric seabird species increases with nutritional stress

Dietary segregation is essential for the coexistence of closely related species of animals. However, little is known about how changes in availability of food resources might affect trophic interactions of wild animals breeding in sympatry. Here, we examined how interannual variations in relative food availability (as reflected in blood levels of stress hormone corticosterone, CORT) affect food partitioning (assessed via a comparison of stable isotope δ(15)N and δ(13)C ratios of blood) between the common murre (Uria aalge) and thick-billed murre (Uria lomvia), breeding on a single colony in the Bering Sea. During a 6-year study, CORT varied among years but not between species, whereas stable isotope ratios varied among years and between species. Isotopic distance between species increased with increasing CORT. These results indicate that, when food was not limiting, both species relied on similar food resources. As foraging conditions deteriorated, murres diverged in their diets. We conclude that the degree of dietary segregation between Uria spp. varies with changes in the availability of food and is greatest during food shortages.

Cryptic introgression between murre sister species (Uria spp.) in the Pacific low Arctic: frequency, cause, and implications

As southern species undergo northward range expansions, reports of hybridization between temperate and arctic taxa are increasing, which may have important implications for the evolution, conservation, and management of arctic species. The extent of hybridization between temperate common murres (Uria aalge) and arctic thick-billed murres (U. lomvia), seabirds in the family Alcidae, has been the subject of debate. In a previous survey of variation in mitochondrial DNA (mtDNA) in common and thick-billed murres sampled from throughout the North Pacific and low Arctic, 12 of 327 common murres (~4%) were found to possess DNA sequences characteristic of thick-billed murres. In the present study, we surveyed variation in three nuclear introns in 230 common murres and 56 thick-billed murres and report that these putative hybrids carry various combinations of intron alleles from common and thick-billed murres. Analysis using the program STRUCTURE indicated that nine of these individuals possessed high proportions of thick-billed murre intron alleles, two possessed alleles in F1 and F2 proportions, and one individual possessed predominantly common murre intron alleles. We propose that the asymmetric mtDNA introgression we observed is most likely the result of mate choice at mixed colonies based on differences in male mating behaviours. Our results highlight that hybridization between thick-billed and common murres is more prevalent than previously thought, which may have important implications for the conservation and management of arctic-dwelling thick-billed murres as common murres expand northward.

Modelling state uncertainty with photo series data for the estimation of breeding success in a cliff-nesting seabird

Breeding success in cliff-nesting seabirds has until now been estimated through repeated nest checks by field workers during the breeding season. The use of automatic cameras offers a method for collecting mark–recapture data that can be modelled in order to estimate productivity without making recurring inspections. This saves expense and work hours in the field and allows for more colonies to be monitored. Capture histories for Brunnich’s Guillemot Uria lomvia breeding sites in a colony on Svalbard were generated using a series of photos taken by a time-lapse camera during the breeding season. To account for state uncertainty for the offspring when only the adult could be observed on the breeding site, we applied a multievent model. We estimated egg survival, hatching success and chick survival rates by modelling state transitions. Subsequently, the estimates were used to calculate breeding success. In order to assess the performance of the model, we compared the estimates with field observations of productivity. The observed breeding success in the study plot lay within the confidence intervals of the breeding success estimated by our model. We show that automatic cameras can be used to collect data which, by the application of new modelling techniques, will provide reliable estimates of demographic parameters that are vital for research and management of cliff-nesting birds. The method presented is a very good supplement to physical examination or “manual” around-the-clock monitoring of breeding birds.

Movements and wintering areas of breeding age Thick-billed Murre Uria lomvia from two colonies in Nunavut, Canada

The non-breeding movements of marine birds were poorly known until recently, but this information is essential to understanding the risk to different geographical populations from events on the wintering grounds. We tracked the migration routes and wintering areas of Thick-billed Murre Uria lomvia from two breeding colonies in eastern Canada: Coats Island in northern Hudson Bay and The Minarets, Baffin Island, during the period August 2007–May 2008 using geolocation loggers. Birds from The Minarets moved south rapidly post-breeding and wintered principally off Newfoundland and southern Labrador, or between Newfoundland and southern Greenland, remaining south of 55°N until at least the spring equinox. Those from Coats Island remained in Hudson Bay until at least mid-November, after which they moved rapidly through Hudson Strait to winter in southern Davis Strait and the northern Labrador Sea, mostly north of 55°N. Many individuals stayed throughout the winter in areas of heavy ice cover. Adults from the two colonies appear to be completely segregated in winter and those from Coats Island probably did not enter the area of the winter hunt in Newfoundland. Unexpectedly, some birds from The Minarets wintered in waters beyond the continental slope and outside the distribution of pack ice, demonstrating that particular individuals can be wholly pelagic throughout the winter. Coats Island birds returned through Hudson Strait as soon as open water areas became available in spring. Their sojourn in Hudson Bay coincided very closely with the occurrence of areas with <90% ice cover. In spite of the relatively large error in positions obtained from geolocation loggers, our results demonstrated the value of these devices by uncovering a number of previously unknown aspects of Thick-billed Murre non-breeding ecology in the Northwest Atlantic. Comparison of the non-breeding ecology based on SST experienced in winter show that the winter niche is broader than hitherto assumed, demonstrating that separate populations may experience different selection in the face of climate change.

Arctic seabird food chains explored by fatty acid composition and stable isotopes in Kongsfjorden, Svalbard

Marine birds are important predators in the marine ecosystem, and dietary studies can give useful information about their feeding ecology, food webs and oceanographic variability. The aim of this study was to increase our understanding of the diet and trophic level of the seabirds breeding in Kongsfjorden, Svalbard. We have used fatty acids and stable isotopes, both of which integrate diet information over space and time, to determine trophic relationships in marine food webs. Fatty acid compositions of muscle from Little auk (Alle alle), Brunnich’s guillemot (Uria lomvia), Black-legged kittiwake (Rissa tridactyla), Northern fulmar (Fulmarus glacialis) and Glaucous gull (Larus hyperboreus) were determined and compared with their prey species. Canonical analysis (CA) showed that fatty acid composition differed among the five seabird species. Little auk, Black-legged kittiwake and Northern fulmar had high levels of the Calanus markers 20:1n9 and 22:1, indicating that these seabirds are a part of the Calanus food chain. Brunnich’s guillemot differed from the other species with much lower levels of 20:1n9 and 22:1. Brunnich’s guillemot is a pursuit diver feeding on fish and amphipods deeper in the water column, below 30 m. Glaucous gull also differed from the other seabird species, with a larger variation in the fatty acid composition indicating a more diverse diet. Trophic level analysis placed Little auk at the lowest trophic level, Brunnich’s guillemot and Black-legged kittiwake at intermediate levels and Glaucous gull and Northern fulmar at the highest trophic level.

Philopatry, morphological divergence, and kin groups: structuring in thick-billed murres Uria lomvia within a colony in Arctic Canada

Many seabirds exhibit high natal philopatry despite their extreme dispersal ability and delayed reproduction, and some exhibit phenotypic clustering in colonies and fostering or adoption of neighbouring chicks. Previous investigations of kinship in a small thick-billed murre colony Uria lomvia (Alcidae) in Norway revealed high relatedness among breeders on cliff ledges. To investigate the presence of kin groups and within-colony genetic sub-structuring elsewhere, we investigated kinship within a larger murre colony on Coats Island, Nunavut, Canada. Morphological (five characters) and genetic data (five microsatellite loci and a fragment of the mitochondrial cytochrome b gene) were analysed. Strong morphological differentiation was found among ledges. Genetic structuring was overall weak but significant at the coarse scale for males among ledges and on the east vs. the west side of the colony. Global spatial autocorrelation analyses did not detect consistent, widespread spatial patterns, although local 2D analyses provided some evidence of a tendency for larger neighbourhood sizes for females and a broad range of small to large neighbourhoods for males. Average within-ledge relatedness was low overall, but ranged widely from slightly unrelated to greater than the level of cousins in both sexes. Kin-level relationships occurred on ledges more frequently for same-sex groups than expected by chance, suggesting that recruiting breeders (especially females) avoid or are unable to settle directly adjacent to relatives particularly of the opposite sex. Behavioural studies of natal dispersal of murres at Coats I. indicating that both sexes are highly philopatric, but that up to one-fifth of females may disperse, are concordant with this study. Overall, structuring was weaker than in Norway, and may be explained in part by genetic marker and sampling artifacts, and by the lack of genetic equilibrium suspected in the much larger Canadian Arctic colony. Natal philopatry may be an important factor driving the diversification of seabirds and kin groups in other colonies and species and may be more widespread than is currently acknowledged.

Geographic differences in organic contaminants and stable isotopes (δ13C, δ15N) in thick-billed murre (Uria lomvia) eggs from Alaska

The contents from thick-billed murre (Uria lomvia) eggs collected at four Alaskan colonies in 2002 were analyzed for organic contaminants and carbon δ¹³C) and nitrogen (δ¹⁵N) stable isotopes. Contaminant concentrations in the eggs varied from below detection limits to 230 ng g⁻¹ wet mass for 4,4'-DDE in one egg from St Lazaria Island in the Gulf of Alaska. Eggs from this colony generally contained higher levels of contaminants and exhibited significantly different patterns compared to eggs from the Bering and Chukchi seas. Stable isotope values also varied geographically; however, these differences appeared to be related to differences in C and N baselines in the food webs instead of differences in prey. Contaminant and stable isotope correlations were inconclusive, suggesting that better information on regional food web differences and differential offloading of contaminants and stable isotopes to the eggs must be obtained before these kinds of data can be fully incorporated into seabird egg contaminant monitoring programs.

Monitoring Temperature and Heart Rate during Surgical Field Implantation of PTT-100 Satellite Transmitters in Greenland Sea Birds

Information on cloacae temperature (CT), heart rate (HR), Isoflurane use, and oxygen flow was collected during field implantation of Platform Terminal Transmitters (PTT-) 100 satellite transmitters in Greenland sea birds. Information was obtained from 14 intracoelomic and 5 subcutaneous implantations in thick-billed murres (Uria lomvia) and 9 intracoelomic implantations in common eiders (Somateria mollissima). CT decreased in the order subcutaneous murres > intracoelomic eiders > intracoelomic murres due to the explorative exposure to the surroundings and increased heat loss (murres smaller than eiders) and were preheated to 35°C. During all implantations, heat loss was prevented using electric heat and rescue blankets. Regarding HR, the fluctuations were most pronounced during the intracoelomic murre implantations as a result of lower PTT temperature and lower body size leading to more pronounced digital manipulations and stimulation of the pelvic nerve plexus. Based on these results, we therefore suggest that HR and CT are carefully monitored in order to adjust anaesthesia and recommend the use of an electric heat blanket and preheating of PTTs to body temperature in order to prevent unnecessary heat loss causing physiological stress to the birds.

Changes in body composition during breeding: Reproductive strategies of three species of seabirds under poor environmental conditions

Seabirds differ dramatically in life history traits and breeding strategies. For example, gulls have short incubation shifts (several hours) and high metabolic rates, auks have medium-length incubation shifts (12-24h) and high metabolic rates, and petrels have long incubation shifts (days) and low metabolic rates. How these different strategies affect the dynamics of body components is poorly known. We compared body, organ and lipid mass changes among three different seabirds (gull: black-legged kittiwake Rissa tridactyla; auk: thick-billed murre Uria lomvia; petrel: northern fulmar Fulmarus glacialis) at Prince Leopold Island, Nunavut, Canada during 2002 (a year with low reproductive success and poor chick growth across all three species). This study is among the first to compare mass and lipid dynamics among different species foraging in the same food web and at similar trophic levels during the same breeding season (same environmental conditions). In fulmars and murres, most of decreases in body mass reflected decreases in lipid mass while in kittiwakes the increase in body mass reflected an increase in lean mass, especially the muscle. The species with the longest fasting endurance (incubation shift length) had the highest percent body lipids during incubation (fulmars: 13.3%, murres: 7.3%, kittiwakes: 6.9%), the highest variability in body lipids, tended to regulate body mass primarily through lipid stores and tended to regulate exercise and digestive organs separately. In contrast, in the species with the highest metabolic rate, all organ systems were adjusted similarly and in relation to body mass, and in a similar manner between incubation (stress due to heavy ice conditions) and chick-rearing (lower stress due to ice-free conditions). In high metabolic rate species, we suggest that organ size varies in response to environmental stress. We conclude that the organ dynamics of seabirds are set by a combination of key life history traits (like incubation shift and metabolic rate) and environmental conditions.

The genome sequence of an H11N2 avian influenza virus from a Thick-billed Murre (Uria lomvia) shows marine-specific and regional patterns of relationships to other viruses

Influenza A viruses infect a range of host species, including various mammals and more than 100 species of birds. For avian influenza viruses (AIV), prevalence varies between different groups of birds, with waterfowl showing the highest prevalence. We have sequenced the complete genome of A/Thick-billed Murre/Newfoundland/031/2007(H11N2), an AIV identified in the pelagic seabird, Thick-billed Murre (Uria lomvia). This represents the first complete genome sequence of an AIV from this host species, and only the second complete genome sequence from a seabird in the alcid group. All of the virus segments fall within the American avian lineage. Several of the segments show a close relationship to AIV identified in other marine host species, and also a strong geographic association with other AIV sequences from the northeastern coast of North America from recent years. The identification of this virus, and the growing number of AIV identified in seabird species, indicates these marine birds could be underappreciated host species. This has potential consequences for global influenza dynamics because of the seasonal distributions and migratory patterns of this group of birds.

Ingested plastic in a diving seabird, the thick-billed murre (Uria lomvia), in the eastern Canadian Arctic

Plastic debris has become ubiquitous in the marine environment and seabirds may ingest debris which can have deleterious effects on their health. In the North Atlantic Ocean, surface feeding seabirds typically ingest high levels of plastic, while the diving auks which feed in the water column typically have much lower levels. We examined 186 thick-billed murres from five colonies in the eastern Canadian Arctic for ingested plastic debris. Approximately 11% of the birds had at least one piece of plastic debris in their gastrointestinal tracts, with debris dominated by user plastics. This is the first report of ingested plastics in an auk species in Canada’s Arctic, and the highest incidence of plastic ingestion to date for thick-billed murres (Uria lomvia).

High Arctic sea ice conditions influence marine birds wintering in Low Arctic regions

Ocean climate change is having profound biological effects in polar regions. Such change can also have far-reaching downstream effects in sub-polar regions. This study documents an environmental relationship between High Arctic sea ice changes and mortality events of marine birds in Low Arctic coastal regions. During April 2007 and March 2009, hundreds of beached seabird carcasses and moribund seabirds were found along the east and northeast coasts of Newfoundland, Canada. These seabird “wrecks” (i.e. dead birds on beaches) coincided with a period of strong, persistent onshore winds and heavily-accumulated sea ice that blocked bays and trapped seabirds near beaches. Ninety-two percent of wreck seabirds were Thick-billed Murres ( Uria lomvia). Body condition and demographic patterns of wreck murres were compared to Thick-billed Murres shot in the Newfoundland murre hunt. Average body and pectoral masses of wreck carcasses were 34% and 40% lighter (respectively) than shot murres, indicating that wreck birds had starved. The acute nature of each wreck suggested that starvation and associated hypothermia occurred within 2–3 days. In 2007, first-winter murres (77%) dominated the wreck. In 2009, there were more adults (78%), mostly females (66%). These results suggest that spatial and temporal segregation in ages and sexes can play a role in differential survival when stochastic weather conditions affect discrete areas where these groups aggregate. In wreck years, southward movement of Arctic sea ice to Low Arctic latitudes was later and blocked bays longer than in most other years. These inshore conditions corresponded with recent climate-driven changes in High Arctic ice break-up and ice extent; coupled with local weather conditions, these ice conditions appeared to be the key environmental features that precipitated the ice-associated seabird wrecks in the Low Arctic region.

Has early ice clearance increased predation on breeding birds by polar bears?

Past studies suggest that polar bears (Ursus maritimus) consume terrestrial food only opportunistically and derive little nutritional benefit from it. Here, we present observations of at least 6 bears consuming large numbers of snow goose (Chen caerulescens) eggs at two locations in the eastern low Arctic in 2004 and 2006. We also report two records of a polar bear eating the eggs and chicks of cliff-nesting thick-billed murres (Uria lomvia) in 2000 and 2003. Climatic warming has resulted in progressively earlier ice break-up in Hudson Bay, forcing bears ashore much earlier than historical records indicate. Advancement in the nesting dates of birds has been more modest, and this mismatch in timing could lead to an increasing overlap between the nesting period of birds and the period during which bears are on land. At these sites in these years, bears were on land prior to the hatch of nests, and the predation that ensued was catastrophic for the birds at a local scale. Although anecdotal, our observations highlight the complexity of trophic interactions that may occur in a changing Arctic.

Annual variation in the foraging behaviour of thick-billed murres in relation to upper-ocean thermal structure around St. George Island, Bering Sea

We studied the foraging behaviour of adult thick-billed murres Uria lomvia as they reared their chicks at St. George Island, Alaska, USA, relative to the thermal structure of the nearby ocean water column in the summers 2004, 2006, and 2007 using data recorders attached to the birds. The thermal structure of the upper ocean varied substantially among the years. Satellite-derived sea surface temperature (SST) and water temperature at depths >40 m were higher in 2004 than in 2006 and 2007 (9.1, 8.3 and 7.8°C for mean SST; 5.1, 4.4, and 2.2°C for median bottom temperature, respec- tively). We recorded a strong thermocline in 2004 and 2007, but not in 2006. Nonetheless, the thermo- cline was one of the important foraging habitats in all years. The foraging behaviour of thick-billed murres appeared to vary with annual variation in the intensity of the thermocline and water temper- ature at depth and evidently with associated vertical distribution of prey. Birds spent more time forag- ing in stratified waters and dived to around thermocline depth (and deeper in 2007) in 2004 and 2007. However, the birds used both stratified and mixed waters in 2006 and were less likely to dive to ther- mocline depth. Main prey items delivered to chicks varied among years. Sandlance (53% of observed items) and pollock (23%) predominated in 2004, compared with cephalopods (23%), flatfishes (17%) and pollock (15%) in 2006, and cephalopods (62%), pollock (16%) and sculpins (9%) in 2007.

Foraging behavior of incubating and chick-rearing thick-billed murres Uria lomvia

The varying demands associated with egg incubation and chick-rearing are known to have a corresponding effect on the foraging behavior of seabirds. We deployed data loggers on incu- bating and chick-rearing thick-billed murres Uria lomvia to examine differences in their diving behavior and characteristics of habitats used for foraging. To compare diets of incubating and chick- rearing birds we collected their stomach contents using a water offloading technique. We found that incubating birds performed longer foraging trips than chick-rearing birds (incubating: 19.0 ± 7.2 h; chick-rearing: 9.9 ± 5.6 h). Incubating birds foraged in the offshore stratified water masses (sea sur- face temperature (SST) > 9°C) and frequently dived to the depth of the thermocline (20 to 50 m). Chick-rearing birds spent more time foraging in the inshore, well-mixed water masses (SST 60 m. Small juvenile walleye pollock Theragra chalcogramma, squid and euphausi- ids were the dominant prey of incubating and chick-rearing birds. Distributions of these small prey were commonly associated with the thermocline, while larger fish, which parents brought back to feed their chicks, were distributed below the thermocline. Results suggest that incubating murres mainly foraged at shallow depths near the thermocline with higher concentrations of small prey, while chick-rearing murres feed their chicks large prey caught on deep dives while feeding them- selves on small prey caught on shallow dives.

Working beneath the surface: interplay of biomechanics, physiology and behavioural ecology in diving seabirds

ABSTRACT: This Theme Section looks at new approaches in the study of seabird foraging ecology,with particular emphasis on the use of recently developed recording devices attached to birds. It in-cludes studies with energy as a theme, such as the energetics of foraging in Magellanic penguins,how dive efficiency varies with depth in emperor penguin, optimality of stroke frequency in divingseabirds, and how using energy to counteract depth-dependent buoyancy may limit pursuit speedsin hunting imperial shags. A more physiological approach is adopted in two studies, one on the divecapacity of emperor penguins and one on planktivorous auklets. Foraging ecology is examined morebroadly in a study on the foraging behaviour of breeding thick-billed murres while a modellingapproach is used to describe foraging profitability in aucklets (the smallest marine endotherms) feed-ing on pelagic patches. Finally, consideration is given to bird-attached instrument artefacts in pathreconstruction of diving birds using dead-reckoning.KEY WORDS: Diving seabirds · Biomechanics · Energy efficiency · Foraging ecology · Recordingdevices

Trends in the harvest of Brünnich's guillemots Uria lomvia in Newfoundland: effects of regulatory changes and winter sea ice conditions

AbstractThe harvest of Brünnich's guillemots (thick‐billed murres) Uria lomvia off Newfoundland and Labrador is the only legal hunt of seabirds by non‐natives in Canada and the United States. Ringing programmes at Arctic breeding colonies have been used to track changes in numbers and age composition of harvested birds. In recent years, the numbers of rings reported by hunters have fallen steeply. We examined recoveries by hunters of rings from a colony in northern Hudson Bay during 1984‐2006 to assess the possible reasons for the decline in recoveries. Because recoveries of common guillemots have remained stable over the same period, it seems unlikely that a change in reporting rates is involved. Instead, it appears that a combination of a reduction in hunting pressure and a change in the behaviour of the birds due to more northerly termination of the winter pack‐ice boundary account for the observed reduction in recovery rates. The pattern of first year recovery rates, in particular, appears consistent with an explanation based on accommodation to ice conditions. Recovery rates of older birds appear less affected by ice conditions and in recent years, have, in any case, been very low. Our study demonstrates that under conditions of changing weather and climate, harvest management decisions may not always have the impact expected.

Wintering Areas and Recovery Rates of Brünnich’s Guillemots &lt;i&gt;Uria lomvia&lt;/i&gt; Ringed in the Svalbard Archipelago

We mapped wintering areas and estimated the recovery rates of Brünnich’s guillemots (thick-billed murres) ringed in Svalbard during 1954–98. Recoveries were reported from Iceland, Greenland, and Newfoundland. An intensive hunt occurs annually off Greenland and Newfoundland, and more than 95% of the recovered birds were reported as shot. Birds recovered as immatures differed from birds recovered as adults in their temporal and spatial distribution. Immatures were more exposed to hunting because they arrived at the hunting areas earlier in the autumn and formed a higher proportion of the population in hunting areas farthest away from Svalbard. The recovery rate of birds ringed in Svalbard as chicks was comparable to the recovery rate reported from Canada. Interestingly, no Svalbard birds have ever been recovered in the southern part of the Barents Sea or along the Norwegian coast, suggesting that these areas are not important to the Svalbard population. The distribution of winter recoveries of birds ringed in Svalbard was compared to the winter recovery areas of Brünnich’s guillemots ringed elsewhere in the North Atlantic. Only birds ringed in Svalbard have been recovered in Iceland in winter.

Sex-specific behavior by a monomorphic seabird represents risk partitioning

The presence of sex-stereotyped behavior in monomorphic animals, where there are no sexual differences in form to account for sexual differences in function, is often attributed to intraspecific competition or to differential parental investment. The possibility that the use of different behavioral strategies by each parent may increase reproductive success for both partners through risk partitioning is seldom considered. We studied thick-billed murres (Uria lomvia), where the male exclusively feeds the offspring during the late chick rearing. During the period of biparental care, males fed on “risk-averse” prey (consistent across time and space; unitized risk = 0.29), whereas females fed on “risk-prone” prey (risk = 0.59). Males fed at night at 1 colony, during the day at 2 colonies, and there was no pattern at another colony. We suggest that these differences reflect the availability of risk-prone prey. Modeling suggested that mixed-risk pairs had higher success than “risky” or “riskless” pairs. Males accumulated reserves and reduced chick provisioning just prior to fledging. Thus, sex-specific patterns at 1 period (male-only care during postfledging) may have led to sex-specific patterns at earlier periods through the need for specialization in foraging habits and risk. We propose that risk partitioning may contribute to the prevalence of sex-specific behaviors in monomorphic animals and that patterns are likely context specific rather than species specific.