Wednesday, September 16, 2015

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

1 comment:

Bird Removal New Jersey said...

Seeing packs of dead birds is so scary.