Brandt’s cormorants (Phalacrocorax penicillatus) are medium-sized seabirds found along the Pacific coast of North America. They are distinguished by their glossy black plumage, long neck, and bright blue throat pouch. Brandt’s cormorants nest in large colonies and are highly social birds. Their mating and breeding behaviors have been well-studied by ornithologists. Whether Brandt’s cormorants form lifelong pair bonds has been a topic of scientific interest. This article will examine the evidence regarding mate fidelity and longevity of pair bonds in Brandt’s cormorants.
Courtship and Pair Formation
Brandt’s cormorants engage in elaborate courtship rituals. Males select a nest site, perform nest demonstrations, and attempt to attract a female. Courtship feeding of the female by the male also occurs. Once a pair is established, they begin building a nest together from seaweed, grasses, and feathers. Brandt’s cormorants are seasonally monogamous, with a single mate during breeding season. However, the pair bond only lasts for that breeding season and must be re-established the following year. Some individuals may mate with the same partner in subsequent seasons, while others switch mates between years. The factors that influence mate retention versus switching are not fully understood in this species.
Duration of Pair Bonds
Researchers have studied the duration of pair bonds in banded Brandt’s cormorants. At colonies in the Gulf of California, the average duration of pairs was found to be 1.4 breeding seasons. Less than 15% of pairs remained together for 4-5 seasons. The longest recorded duration for a Brandt’s cormorant pair bond was 15 years. At colonies farther north, the persistence of pairs was even lower. At Southeast Farallon Island off California, no banded pairs stayed together for more than two seasons. These field observations demonstrate that while mate fidelity within a breeding season is high, most Brandt’s cormorant pairs do not remain intact across multiple years.
Reasons for Mate Switching
Why do most Brandt’s cormorant pairs fail to remain bonded long-term? There are several possible explanations. For one, there is a high mortality rate in the population. Average annual survival is estimated at only 25% for adult Brandt’s cormorants. The death of one partner clearly dissolves the pair. Divorce, or the active separation of a pair, also occurs. In some seabird species, divorce is adaptive if one partner underperforms on breeding. Poor incubation or low breeding success may lead to divorce in Brandt’s cormorants. The availability of higher quality mates may also promote mate switching. Younger breeders tend to improve their reproductive performance. Mating with a younger bird can increase fitness. Finally, breeding dispersal likely contributes to dissolution of pairs. Cormorants do not always return to the same colony site each year. When birds nest at new colonies, they must form new pairs.
Do Brandt’s Cormorants Mate for Life?
The evidence overwhelmingly shows that Brandt’s cormorants in natural populations do not mate for life. While seasonal monogamy within a breeding season is the norm, long-term pair bonding over multiple years is rare. Only a small percentage of pairs manage to stay together. The average pair bond lasts just 1-2 breeding seasons. Mate switching is frequent across years. Factors like mortality, divorce, dispersal, and age drive the turnover in mates. Although a small proportion of birds do reunite with previous mates, the overall pattern demonstrates that Brandt’s cormorants are serially monogamous across breeding seasons rather than mating for life. This mating system fits with life history trade-offs in a seabird species with high adult mortality. Annual pair formation allows birds to increase fitness by selecting new mates. So in summary, lifelong pair bonds do not occur in this species as shown by field observation of marked birds.
Bird Ecology and Life History
The behavior, breeding biology and life history of Brandt’s cormorants have been shaped by their ecological context as seabirds that rely on marine food resources. Understanding their ecology provides insight into their mating and reproductive strategies. This article section will provide background on the food habits, habitat needs, mortality rates, and other ecological factors that are relevant to evaluating pair bonding patterns in Brandt’s cormorants. Their ecology illustrates the environmental pressures and trade-offs that have led to seasonally monogamous breeding without lifelong pair bonds.
Diet and Foraging
Brandt’s cormorants are specialized fish-eating birds that dive from the surface to depths of up to 130 feet to catch prey. They feed on small schooling fish including anchovies, sardines, rockfish and perch. Much of their foraging occurs relatively close to breeding colonies. Maximum foraging ranges are around 18 miles. Birds travel from colonies to productive areas of upwelling where prey is abundant. Their diet is influenced by location and ocean conditions. Sardines are important where available, but cormorants switch prey when populations fluctuate. Diving underwater to catch swift fish requires excellent swimming ability, visual acuity and coordination. Cormorants pursue and capture individual prey items one at a time. They are vulnerable to starvation during events like El Niño when fish numbers decline.
Breeding Habitat
Nesting habitat requirements further limit breeding distribution and populations. Brandt’s cormorants nest colonially on offshore rocks, small islands and cliffs along the coast. Breeding sites must be free of mammalian predators. Colonies are concentrated in central and southern California and the Gulf of California where suitable habitat occurs. Nests are built right on the substrate with no embellishment. Nesting areas with high densities of birds accumulate guano droppings. Habitats can deteriorate over time due to buildup of guano, erosion, storms or rockfalls. Cormorants may abandon declining sites and move to alternate colonies.
Life History Trade-Offs
Seabirds like Brandt’s cormorants have life history strategies adapted to allow reproduction in challenging marine environments. They are long-lived species with high adult survival under normal conditions. Brandt’s cormorants do not start breeding until at least 3 years old and have maximum lifespans over 15 years. However, breeding itself incurs costs such as energy expenditure and predation risk. So seabirds have low reproductive rates, laying only 1-3 eggs per season. Incubation lasts 25-35 days and chicks fledge in 4-5 weeks. Parents make extensive foraging trips to provision young. Breeding success and chick growth depend critically on adequate food supply. During frequent years of food scarcity, seabirds may skip breeding entirely. Their life history balances adult survival with just enough reproductive investment to replace themselves. This ecology favors serial monogamy, with annual mate choice to maximize fitness according to conditions. Any mortality or failed breeding dissolves the pair bond.
Mortality and Threats
Despite natural high adult survival, Brandt’s cormorant numbers fluctuate markedly over decades mainly due to food limitation and mortality during El Niño events. Their restricted nesting habitat and colony locations also leave them vulnerable. Expanding sea lion populations displace nesting cormorants at many sites. Colonies are impacted by disturbance, oil spills, and human activities. Catastrophic mortality events are not uncommon, including mass strandings from algal blooms. Stochastic factors that affect survival and breeding success can eliminate one or both members of a pair. Persistent pair bonds cannot form under such unstable ecological conditions. Serial monogamy allows annual optimization of pair compatibility.
Conclusions
In summary, the ecology and life history traits of Brandt’s cormorants do not favor lifelong monogamy. Their reliance on unpredictable marine food supplies, constrained breeding habitat, trade-off between adult survival and reproduction, and high mortality risk all select for a mating system based on serial monogamy rather than permanent pair bonds. Ecological context provides the reasons why brief seasonal associations have proven adaptive in this seabird species. The evidence for serial monogamy accords well with the environmental challenges Brandt’s cormorants face in reproducing successfully.
Mate Choice and Reproductive Success
Brandt’s cormorants do not mate randomly each year but exhibit active mate choice. Some evidence indicates reproductive benefits to selecting particular partners. This section reviews research on mate selection and evaluates whether pair compatibility affects breeding performance in Brandt’s cormorants. Understanding mate choice provides further behavioral evidence that lifetime monogamy is not the norm in this species.
Criteria for Mate Selection
Studies of banded Brandt’s cormorants reveal that females show preference for certain males as mates. Older males with the most breeding experience are significantly more likely to form pairs. Males that occupy nest sites surrounded by other nesting birds are also preferred, since this indicates territorial males of higher quality. Relative arrival timing at colonies is another criterion, with females favoring early-arriving, more dominant males. Males also select more fecund females as evidenced by egg volume. Mate age, synchronization of breeding stage, and past reproductive history appear to mediate mate choice in Brandt’s cormorants.
Do Pairs Breed More Successfully?
Research tested whether mate selection criteria relate to increased breeding success. Older male age, early colony arrival, and larger female egg size were all associated with larger clutch sizes and heavier chicks in Brandt’s cormorant pairs. Pairs matched by breeding experience also coordinated egg laying better. So certain mate choice pairings did improve reproductive outcomes. This demonstrates an adaptive basis to nonrandom mate selection in Brandt’s cormorants. Choosing compatible mates each year based on traits like experience, age and condition is likely more important for birds than maintaining lifelong pair bonds.
Site Tenacity Versus Mate Tenacity
An interesting question is whether cormorants are more likely to return to a previous nesting site or a previous mate between years. In the Gulf of California, both male and female Brandt’s cormorants showed higher site tenacity than mate tenacity. Over 75% returned to the same colony versus 15% reuniting with a prior mate. At Southeast Farallon Island off California, mate tenacity was extremely low with no banded pairs reforming between years. But approximately 60% of males and 42% of females returned to the same nesting site. This shows that Brandt’s cormorants choose colony locations based more on site quality and familiarity than mate fidelity.
Conclusion
The evidence for selective mate choice and greater site tenacity indicates that serial monogamy and annual pair formation are key breeding strategies in Brandt’s cormorants. Choosing mates based on traits related to higher reproductive potential provides fitness benefits. Prioritizing nest site suitability over mate fidelity also makes ecological sense for a species reliant on constrained breeding habitats. Overall the importance of annual mate selection and lack of long-term pair bonds aligns well with the biology and ecology of Brandt’s cormorants as seabirds that do not mate for life.
Comparisons with Related Cormorant Species
Introduction
Brandt’s cormorants belong to the Phalacrocoracidae family which includes about 40 species of cormorants and shags. How do pair bonding patterns seen in Brandt’s cormorants compare to mating systems in related cormorant species? Examining species with similar life histories and ecologies can provide insight into the evolution of serial monogamy in Brandt’s cormorants. This section summarizes pair bonding duration and fidelity in other cormorant species that also lack lifelong pair bonds.
Blue-Eyed Shag
The blue-eyed shag (Phalacrocorax atriceps) is a cormorant found along the coast of southern South America. They show a mating pattern very similar to Brandt’s cormorants. Blue-eyed shags form seasonal pair bonds during breeding but regularly switch mates between years. One 12-year study of banded blue-eyed shags found they changed partners on average every 1.3 years. No pair remained together for more than 3 seasons. Like Brandt’s cormorants, they are serially monogamous across breeding attempts.
Pelagic Cormorant
Pelagic cormorants (Phalacrocorax pelagicus) occur along the North Pacific coast. Banded birds in Alaska were observed from 1 to 7 years. The average pair duration was 2 years, with only 11% of pairs lasting more than 3 years. So again serial monogamy predominates, with most birds alternating mates annually. This species also exhibits active courtship with mate choice each season.
European Shag
In European shags (Phalacrocorax aristotelis), an 11-year study showed the average pair duration was 2.55 years. No pair remained together for more than 6 consecutive years. Survival and mate fidelity was slightly higher for European shags but most still changed partners regularly. Females were more likely to switch mates after breeding failure.
Great Cormorant
The great cormorant (Phalacrocorax carbo) has variable mate fidelity across populations but no evidence of lifelong pair bonds. In Germany and the Czech Republic, males paired with new females on average every 2 years. A Polish population had greater mate fidelity with some pairs together 4-5 years. Great cormorants show more nest site fidelity than mate fidelity, similar to Brandt’s cormorants. Overall, serial monogamy on an annual or semi-annual cycle prevails.
Conclusion
The examples above demonstrate that serial monogamy is common across cormorant species with similar marine ecologies to Brandt’s cormorants. All exhibit seasonal pair formation with high mate switching between breeding attempts. Cormorants appear designed for annual optimization of reproductive partners rather than lifelong pair bonds with a single mate. The general pattern aligns with the trade-offs faced by seabirds with unpredictable environments favoring serial monogamy as the adaptive mating strategy.
Summary and Conclusions
Summary
Brandt’s cormorants demonstrate serial monogamy with mate fidelity within breeding seasons but frequent partner switching between years.
Evidence shows:
- Average pair duration is only 1-2 seasons
- Less than 15% of pairs stay together over multiple years
- Maximum observed duration of a pair bond is 15 years
- Mate switching is driven by mortality, divorce, and breeding dispersal
- Brandt’s cormorants show active mate choice for compatible pairs
- They prioritize nest site tenacity over mate fidelity
- Related cormorant species have similar serial monogamy mating patterns
Overall, Brandt’s cormorants lack lifelong pair bonding and the available evidence clearly points to serial monogamy as their breeding strategy.
Conclusion
In conclusion, Brandt’s cormorants do not mate for life. Field studies of banded birds firmly establish serial monogamy as their predominant mating system. The prevalence of annual mate switching aligns with the ecology and life history of a marine seabird that contends with fluctuating food supplies, high adult mortality, and unstable nesting habitat. Lifelong pair bonding does not provide sufficient benefits under these conditions relative to annual mate selection. Brandt’s cormorants exemplify adaptive reproductive strategies shaped by ecological pressures. The evidence shows seasonal monogamy, not lifelong fidelity, has evolved as the mating pattern that maximizes fitness in this fascinating seabird species.
