Migratory Bats on the Farallon Islands

During the fall, we study many species of animals as they stopover at Southeast Farallon Island to find food and shelter to help them prepare for their migrations. For example, several aquatic migrants stopover at the Farallon Islands to take advantage of the nutrient rich waters, such as white sharks, blue whales, humpback whales, gray whales, and several species of dolphin. We also monitor many species of insect, such as butterflies, moths, dragonflies, and damselflies, as they migrate out over the ocean and require land to rest before they continue on to places unknown. The most abundant and conspicuous species that we study are birds. Occasionally, when the weather is just right, we wake up to find large numbers of songbirds covering the island. These are the weather conditions that fall biologists dream of. But this weather also brings another aerial migrant to the Farallones, bats.

Since the mid-1960's, fall biologists on Southeast Farallon Island have conducted daily surveys for songbirds and bats. Both tend to congregate in or around the four trees on the island, so we spend quite a bit of our time around those trees. Most of the birds are fairly easy to find as they flit about trying to find food among the branches. The bats, though, are more difficult to find as they cling, or roost, motionless under the branches. Since bats are nocturnal, there is little reason for them to fly around during the day. Our daily bat survey consists of scanning the undersides of branches in hopes of seeing a little bat hanging face-down. We spend about 15 minutes under each tree.

The hoary bat (Lasiurus cinereous) is the most common species of bat that we encounter at the island. This species is named such because of the white tipped hairs covering their back. They differ from most of the other North American bats in that they are highly migratory, with many wintering in southern California, Mexico, and Central America. As is typical of other species in the genus Lasiurus, they mostly lead solitary lives, and they use trees for roosting. Because most other bats roost in caves, the bats in this genus are frequently referred to as "tree bats". Since bat monitoring began, we have seen hundreds of hoary bats roosting in the trees on the island, with 101 counted on a single day in 1987!

Unfortunately, the numbers of hoary bats we see each fall began plummeting around 2008. There are two possible reasons for this and both probably play a part. The first is that their population has been adversely affected as wind turbines inadvertently kill them during migration. Apparently mammal lungs are highly susceptible to collapsing when exposed to a large pressure differential, such as when a massive wind turbine blade whips past. The other reason, though, which may have had an even larger impact, was the replacement of the rotating beam on the lighthouse to a weaker pulsing flash.

Another species of bat that is closely related to the hoary bat and also migrates out to the Farallon Islands is the western red bat (Lasiurus blossevillii). This species, though, is much less common out here, and prior to this year, there were only 11 records. On 20 September, while conducting an area search for birds, we found one roosting in the "Burning Bush", a large mirror bush (Coprosma repens) native to New Zealand. The last record before this was in 2001, when one was seen flying about at dusk as it dodged the repeated passes of a peregrine falcon.

Western red bats are distributed throughout the western U.S., with the exception of the Great Basin, and their range extends southward all the way to central Argentina. Red bats in the northern part of their range and at higher elevations migrate south and towards the coast for winter. Suitable habitat for this species can be found in well-developed riparian forests, where they may roost under a dense canopy of leaves. They feed on a variety of insects, with moths being preferred. This mostly solitary species only seeks company of its own kind in the fall, when they mate. After mating, the female delays fertilization until spring so that it can give birth in the summer. Amazingly, litter size can be as high as four, which the female must carry around until the pups are old enough to fly. Unfortunately, western red bat populations are also in decline, as much of their riparian habitat has either been destroyed or degraded. Hopefully this little guy makes it back to the mainland after it has rested a while during its migratory stopover.

Posted by Jim Tietz, fall biologist

The Value of Dead Birds

Many people have come to know Southeast Farallon Island by the vast amount of life that graces its waters, shores and rocky outcroppings. Abundance of life is often the first thing visitors notice when nearing the island. Another feature that many notice is the conspicuous number of dead birds. Indeed, this second observation is quite true, and has been for some time. This poem written in the mid 1800’s by Milton S. Ray, an early ornithologist and visitor to Southeast Farallon Island, describes the appearance and construction of a seabird nest, and attests to this common observation. 

“And bits of bone were mortised in,

Forlorn remains of island kin,

Lost tribes of feather, fur, and fin,

That the ever-patient sea and sun

Had long bleached smooth and white.”

Brandt's Cormorant nests and unlucky chicks left after a summer breeding season. Photo: Boo Curry

The seabird colonies of the Farallones brim with both life and death. Seabirds, which are very numerous on Southeast Farallon during the summer breeding season, can potentially live long lives, averaging around 10-30 years, and even lengthier for some species. Nevertheless, all birds die, and despite seabirds’ potential for lengthy life spans, most die rather early in their lives and often in very gruesome ways.

A testament to life and death on the Farallones - dead Common Murres and Brandt's Cormorant chicks at a colony site. Photo: Eva Gruber

Instead of being unnerved by this seemingly large amount of death, many researchers use it to their advantage. Here on the Farallones, information on dead birds is used extensively, mainly because there is an excess of death all around. Interpreting death as a learning opportunity is crucial to research conducted on the island. Information we collect on dead birds includes where the bird was found, how old it was, and most importantly, how it died. We use this data to study aspects of seabird biology, such as predation rates, survival rates, and seabird demographics.

Fall intern Adam Searcy clipping the wing of a dead Common Murre. Photo: Eva Gruber

We band 9 species of breeding birds on the Farallones as well as migratory songbirds and burrowing owls that visit the island. By banding breeding adult birds and the young they produce, we can keep track of living individuals as they come and go from the island, and can also document their deaths. Year round we retrieve bands from dead adults and chicks and assess how they met their end. Death can come in many forms here on the Farallones. Starvation, predation, gull aggression, entanglement, botulism, and oiling are all common ways seabirds lose their lives. More often than not, death comes frequently to birds in their first year of life, with chicks dying before they ever leave the island. Some birds also die as they return to the island to breed for the first time. Being unaware of the impending dangers, like predators, is costly for young birds. Adult birds returning to the island have a greater sense of danger awareness, which gives them a much better chance at survival.



The measurement of how many young birds fledged from a colony and return to breed in that colony is called ‘recruitment’, and it can only be gauged by banding individuals. Recruitment is an important colony metric because it determines whether the colony is self-sustaining or requires immigration from other more successful colonies. By retrieving bands from dead chicks and recording bands from living birds we can not only evaluate recruitment in these seabirds, we begin to understand seabird survival in all life stages. After determining the cause and timing of death, we can assess how climate, food availability, predation, and other outside influences affect these populations as well. For example: if a large proportion of chicks die one season before fledging, it could indicate poor breeding conditions such as a shortage of food near the island. However, if a majority of chicks successfully fledge, but fail to return to the colony the following year, this could indicate poor conditions at sea where they attempted to overwinter. In this regard, population health is much easier to determine by considering dead birds along with the living.

 

Collected bands and associated records from dead Brant's Cormorants. Photo: Eva Gruber

To assess population health, there are two sides to the equation: reproductive rate and survival rates. Survival estimates, and with that population health, have been calculated for many seabird species, including species banded here on the Farallones. An important metric for analyzing survival is mortality, and for many of these seabirds a major contributor to overall mortality is predation. 

 

Peregrine Falcon chasing a Tufted Puffin down from Lighthouse Hill. The Puffin persisted. Photo: Adam Searcy

Peregrine Falcon attempting to attack a Western Gull. Photo: Adam Searcy

The Peregrine Falcon is a formidable predator to both seabirds and landbirds here on the Farallones. Although a very rare breeder on the island for most of its history, Peregrines are present throughout much of the year. During spring and summer their diet mainly consists of adult seabirds, like Rhinoceros Auklets. Recently in 2014, Peregrines killed 72 Rhino Auklets within one breeding season. At Año Nuevo, the Peregrine death toll is higher, at least based on previous analysis, with an estimated 1-7% annual predation of the population resulting from Peregrine Falcons, making falcon predation the largest contributor to mortality. In a healthy ecological system, mortality due to Peregrines would not have a great effect on the prey species’ population. However with other compounding factors, such as unpredictable variation in food abundance and climate caused by our warming planet, seabird populations may not be healthy enough in any given year to balance the effects of predation. Although adult survival in Rhino Auklets is estimated to be reasonably high here on the Farallones, there is variation between seasons in this survival rate, Again, this survival information would not be possible without banding and retrieving bands from dead, known individuals. 

Rhinoceros Auklet killed by a Peregrine Falcon. The auklet's breast has been picked clean to reveal the keel; a typical sign of Peregrine predation. Photo: Dan Maxwell

Tufted Puffin also eaten by a Peregrine. Photo: Boo Curry

Other formidable and regular predators on S.E. Farallon are Barn Owls and Burrowing Owls. Burrowing Owls have been visiting and overwintering on the Farallones since the early 1900’s (Barn Owls were first observed much later, around the 1970’s), and they too need to eat while here. When arriving in fall, the owls begin feasting on house mice, which are abundant during this time of year. However, as house mouse populations decline, Burrowing Owls start to supplement their diets with Ashy Storm-Petrels. Throughout the winter and spring, Ashy wings are collected and recorded whenever found on the island, which allows us to estimate how many birds are killed in any given season. Wings of these birds usually pile up in known Burrowing Owl roosts, one of which was found with 20 wings! In the past few decades, Ashy Storm-Petrels have been experiencing population declines, especially here on the Farallones, which is their largest known colony. These small, long-lived seabirds have low reproductive rates and removing even a single adult from the population (i.e. death) can have a serious negative impact on overall population numbers.

Burrowing Owl spying out of a burrow on the terrace. Photo: Boo Curry

Both Ashy Storm-Petrels and Burrowing Owls are species of Special Concern in California, and understanding their predator-prey relationship is essential for their management on the island. By keeping track of these species through banding and retrieving wings, we can better assess how many Burrowing Owls are overwintering on the Farallones, and how many Ashy Storm-Petrels they are eating. With this information we can attempt to mitigate this loss.  

Ashy Storm-Petrel wings and feathers found in a Burrowing Owl roosting site. Photo: Point Blue Conservation Science

Both the early Costanoan and Coast Miwok Natives of California referred to Southeast Farallon as “The Island of the Dead”. After death, they believed their souls would be received on the island by the dead souls of others. Some believed this to be a welcoming, happy land, while others thought it to be a dreadful place. Maybe it was those who valued death that held these islands with veneration, while those that did not, feared it. 

Many people that have never before stepped onto an island seabird colony often voice concern about the number of dead birds present. Death in large quantities has always been a part of life for some species; but turning these events into valuable data for conservation is a high priority, and cannot be overlooked. One way or another, there cannot be life without death, reproduction without mortality, survival without passing. The Farallones would not have this abundance of death if it were not for the amazing abundance of life that precedes it.

Brandt's and Pelagic Cormorants perched on the rocky shores of North Landing. Photo: Boo Curry

-Posted by Boo Curry, Fall Intern

Brandt's Cormorant Diet Studies

My internship at the Farallones involved many different fascinating studies, but one of my favorite studies were the seabird diets, as they really tie in the oceanographic aspect of marine ornithology. We are lucky to be able to live on this incredible, rugged island surrounded by the Pacific ocean and work with the birds that call it home, but sometimes it can be easy to take for granted just how strong the connection is that these birds have to the marine environment. By incorporating the feeding ecology of the seabirds, we are also considering vast topics like oceanic health, fisheries ecology, and climate change, much of which is still poorly understood. Taking part in studies that delve into this mysterious, watery realm is pretty exciting. Brandt’s cormorants also happen to be one of my favorite birds on the island - how could you not love the silky black birds that look like Muppets with long necks and giant feet, and whose eyes are a vibrant, deep turquoise?

One of the last seabird studies to take place on the Farallones is Brandt’s cormorant diet sampling.  The diet study is based on the contents of cormorant pellets, which are regurgitated by the birds at the colony. This is one of the longest running diet studies in the world, with data ranging back 40 years.  And it has proven incredibly valuable in disclosing what we can’t see that goes on beneath the surface of the ocean. The cormorants, through their foraging activity, can be used as a method of sampling the most abundant prey species available to Farallon seabirds, which can in turn be used as a measure for the health of the surrounding marine ecosystem and the changes that it goes through over time. 

gawky Brandt's cormorant chick

Seabird diet studies are conducted in many different ways. Some diet studies are strictly observational, as in the case of Common Murre and Pigeon Guillemot whose prey items are observed from a blind as they bring them back in to feed their chicks at their nest sites. Others, such as the nocturnal Rhinocerous Auklet, involve capturing the birds at night as they bring fish back to the nest, and when captured drop their prey whole. Whole prey items are very useful as the weight, exact size, and species (and therefore energetic value) can be accurately determined. Rhinocerous Auklet diet information is also used to supplement information from cormorant diet studies, especially in terms of rockfish diversity.

Brandt's Cormorant chick reaching deep down a parent's throat for food

Brandt’s Cormorant feed their chicks straight from the gullet, therefore observational diet watches would be in vain. Instead, we rely on collecting regurgitated pellets from which prey parts can be separated and identified. These pellets can only be collected post-breeding season, after the cormorant chicks have fledged and the colony is abandoned. 

searching for pellets at the colony

 The pellets are constituted of mucous membrane encasing indigestible fish bones, which the birds will regurgitate after consuming a meal. They can be hard to find among the deserted colony, and it takes some practice figuring out the pellets from dried clods of dirt and guano! 

After collection, the pellets are sent off the island to the Point Blue lab, where lab technicians treat the pellets with a detergent (to stop enzyme activity, which could destroy the contents), soak them in water, and pass them through a fine sieve to pick out the relevant bones. Specifically, lab techs are looking for otoliths – tiny bones found in the inner ear of all vertebrates that can be used to accurately identify fish prey down to the species level. Invertebrate prey items can also be identified by their hard parts – cephalopod beaks and decapod carapaces.

Rockfish sp. otolith

Overall, population data collected on Southeast Farallon show that the number of breeding adults have declined through the 1970s and 1980s, fluctuated in the 1990s, increased in the early 2000s, and dropped precipitously in 2007. The highest count on the island was in 1974, when 23,800 adults were counted, while 2009 held the lowest population at 1,248 adults. Since 2008, the numbers of adult cormorants on the island have not exceeded 5,000. While it is not known why the population declined so dramatically in 2007, years of abnormally low productivity correspond to years with El Nino or other warm-water events. Other factors that play into the variability of available prey are related to the pressure exerted upon prey populations by other seabirds, many of which are competing for the same resources. While the diversity of seabird species present on the Farallones is supported in part by resource partitioning (such as foraging depth, habitat, etc), in years with lower forage fish diversity, many of the breeding seabirds may be left with no choice but to target the same prey species, creating much more difficult conditions for finding enough food for themselves and their chicks, leading to low productivity.

Southeast Farallon Brandt's Cormorant Diet summary

Brandt’s are pursuit-diving seabirds that propel themselves underwater using their large webbed feet. Although little direct information is available on the actual diving limit, it is thought that they can forage at up to 150 feet deep, with a max diving time of 95 seconds and a mean diving time of 51 seconds. Cormorants forage in offshore pelagic zones as well as on nearshore rocky benthic reefs, depending on where fish are available. Pellets have revealed that the diet of cormorants on SEFI is composed of mainly three groups of fish: northern anchovy (Engraulis mordax), rockfish species (genus Sebastes, family Scorpaenidae), and flatfish species (order Pleuronectiformes). Other prey items commonly found in the cormorant diet include plainfin midshipman (Porichthys notatus), a variety of sculpin species (family Cottidae), cod species (family Gadidae), and spotted cusk-eel (Chilara taylori). Trends in the fish present in cormorant diet have changed over the years, with the 1970s showing a majority of rockfish in the diet. In 1994, however, northern anchovy became more frequent, and then by 2008 this species was decreasing in the diet, being replaced by rockfish again. In the past few years, the diet has fluctuated much more. The diet switched from anchovy to rockfish from 2009-2010, then another switch to flatfish occurred from 2011-2012, and then another switch back to rockfish in the last couple of years. The shift from anchovy and offshore rockfish species to nearshore-settling rockfish and neritic flatfish is most likely one based on abundance and availability rather than preference.  Changes in oceanic conditions could be driving this change in community structure, which have become favorable to nearshore species like flatfish and certain rockfish species. This further seems to be supported by the appearance and growth of several Brandt’s cormorant colonies on the mainland, which are located closer to nearshore shallow foraging habitat.

 

By studying the feeding ecology of seabirds, attempts are made to elucidate the factors that limit population size and breeding success, and to monitor the health of the surrounding marine ecosystem. Marine systems are constantly in flux, yet we are likely to see more anomalies and fluctuations as the climate changes. By monitoring what the seabirds are foraging to feed themselves and their chicks, we gain a glimpse into what is happening with forage fish species in the ocean in relation to ocean health. This is very important information as forage fish are near the base of the marine food web and support not only other seabird species, but a great many other marine species as well. Through pellet collection, we have formed a better understanding of why in certain years Brandt’s cormorant productivity dropped precipitously, and thus, how they might respond to changes in the oceanic climate in the future. This kind of information can have profound conservation implications, and is vital in the adaptation of climate-smart, ecosystem-based management policies. 

Posted by Eva D. Gruber, SEFI Seabird Intern