Chronic lack of breeding by Galápagos Blue-footed Boobies and associated population decline - PDF

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VOLUME 9, ISSUE 1, ARTICLE 6 Anchundia, D., K. P. Huyvaert, and D. J. Anderson Chronic lack of breeding by Galápagos Blue-footed Boobies and associated population decline. Avian Conservation and

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VOLUME 9, ISSUE 1, ARTICLE 6 Anchundia, D., K. P. Huyvaert, and D. J. Anderson Chronic lack of breeding by Galápagos Blue-footed Boobies and associated population decline. Avian Conservation and Ecology 9(1): 6. Copyright 2014 by the author(s). Published here under license by the Resilience Alliance. Research Paper Chronic lack of breeding by Galápagos Blue-footed Boobies and associated population decline David Anchundia 1, Kathryn P. Huyvaert 2 and David J. Anderson 1 1 Wake Forest University, 2 Colorado State University ABSTRACT. A survey of Blue-footed Boobies (Sula nebouxii excisa) throughout the taxon s range in Galápagos, Ecuador found ~6400 adults, compared to a rough estimate of 20,000 in the 1960s. Few pairs bred in and almost no birds in juvenile plumage were seen. Long-term data suggest that poor breeding began in Lack of recruitment over this period would mean that the current population is mostly elderly and experiencing senescent decline in performance. Anthropogenic effects such as introduced predators are unlikely to explain this decline because islands with and without such factors exhibited the same low breeding. The poor reproduction seems to be linked to diet. Previous work indicated that sardine and herring (Clupeidae) supported successful breeding, but these fish were mostly absent from the diet during this study, except in the central part of Galápagos, where most breeding attempts during this study occurred. Elsewhere in the eastern Pacific sardine abundance has decreased dramatically by natural processes in the last 15 years, as part of a well-documented and apparently natural cycle. This cyclic change in abundance provides a possible explanation for the recent demographic changes in Blue-footed Boobies in Galápagos. Whether natural or anthropogenic in origin, the implications of senescent decline in breeding ability and survival are dramatic for this genetically distinct icon of biodiversity and ecotourism. Absence chronique de reproduction chez le Fou à pieds bleus des Galápagos et déclin inhérent de la population RÉSUMÉ. Un inventaire du Fou à pieds bleus (Sula nebouxii excisa) dans l ensemble de l aire occupée aux Galápagos, en Équateur, fait état d ~6 400 adultes, comparativement aux ~ estimés grossièrement dans les années Seulement quelques couples se sont reproduits en et presque aucun oiseau en plumage juvénile n a été observé. Les données à long terme indiquent que la reproduction est mauvaise depuis Le manque de recrutement durant la période laisse supposer que la population actuelle serait surtout constituée de vieux individus aux performances reproductrices déclinantes en raison du phénomène de sénescence. Les causes de source anthropique, comme les prédateurs introduits, n expliquent vraisemblablement pas ce déclin, car toutes les îles, avec ou sans prédateurs, présentent le même faible taux de reproduction. Ce faible taux semble plutôt lié à l alimentation. Ainsi, des études antérieures ont révélé que les sardines et les harengs (Clupeidae) constituaient la base de l alimentation d individus s étant reproduits avec succès dans le passé; or, ces poissons étaient pratiquement absents de l alimentation des fous lors de notre étude, hormis dans le centre des Galápagos où la plupart des tentatives de reproduction se sont produites. Ailleurs dans l est du Pacifique, le nombre de sardines a diminué dramatiquement durant les quinze dernières années, selon un cycle bien documenté et apparemment naturel. Il est donc possible que ce changement cyclique de l abondance des poissons soit responsable des changements démographiques récents observés chez le Fou à pieds bleus des Galápagos. Qu elles soient de source naturelle ou anthropique, les répercussions du déclin des capacités de reproduction et de la survie des individus attribuables à la sénescence sont désastreuses pour cette icône de la biodiversité et de l écotourisme génétiquement distincte. Key Words: Galápagos; population cycle; population decline; sardine-anchovy cycle; seabird conservation; Sulid INTRODUCTION The abundance of seabirds across the vast Pacific Ocean basin is thought to have declined by at least 99% over the past 3000 years, coincident with the spread of Polynesian humans (Steadman 2006). Human settlements on islands lead to habitat loss, hunting, and indirect effects of predatory and other invasive animals accompanying humans (Szabo et al. 2012), and these effects are thought to explain the local extinction of most seabird species on colonized islands (Steadman 2006). The seabird populations of the Galápagos Islands, in the far east of the basin and distant from source populations of Polynesians, depart from this pattern. Paleontological data give no evidence of permanent human habitation before approximately 200 years ago (Latorre 1997), and also no evidence of local extinction of seabird species (Steadman 1986, Jiménez-Uzcátegui et al. 2006). However, some species show clear evidence of recent anthropogenic effects that reduced population size (Vargas et al. 2005, Jiménez-Uzcátegui et al. 2006, Anderson et al. 2008), while other species are too poorly studied to allow similar evaluations. With a seabird fauna with most or all of its original members present, evaluation of possible anthropogenic effects on observed population declines must be a conservation priority. Address of Correspondent: David J. Anderson, Department of Biology, Wake Forest University, Winston-Salem, NC USA, Blue-footed Boobies (S. nebouxii) breed on Galápagos and on islands and headlands on the west coast of South and Central America and México. The demography and population biology of the Galápagos subspecies (S. n. excisa) is poorly known. However, serial data from two former breeding sites in Galápagos (Daphne Major and Punta Cevallos [Española]) indicate an abrupt change in breeding activity in approximately 1997, from irregular but frequent breeding to essentially none until the present (Fig. 1). This pattern is consistent with anecdotal observations of long-term scientists and tour guides that adults are seen less frequently, and breeding sites are seldom occupied, in recent years (D. J. Anderson, unpublished data). If the chronically poor breeding affects the entire subspecies, then low recruitment should be reflected in a reduced population size. Population size has been estimated only once, by Nelson in the 1960s. He concluded that the total Galápagos population must exceed 10,000 pairs and could be substantially more (Nelson 1978:515). Our goals in this study were to evaluate, at the archipelago scale, the indication from two colonies of poor breeding, and to estimate the current size of the Galápagos population. Fig. 1. Numbers of active nests of Blue-footed Boobies (Sula nebouxii excisa) at Punta Cevallos, Española (D. J. Anderson, unpubublished data) and Daphne Major (P. R. Grant and B. R. Grant, unpubublished data). Vertical line indicates March , middle of the El Niño Southern Oscillation event, and roughly the timing of declining sardine abundance in the Peruvian upwelling. Stars indicate the peak of a rapid increase in number of nests followed by a mass breeding failure in the subsequent four weeks. Terrestrial anthropogenic factors are unlikely to explain the pattern observed at the Daphne Major and Punta Cevallos colonies, because these two islands have no introduced predators and no permanent human presence. Furthermore, Nazca Boobies (S. granti), with similar terrestrial ecology, bred successfully at both sites before and after 1997 (D. J. Anderson, unpublished data; P. R. and B. R. Grant, unpublished data). These two islands (Fig. 2) differ in presence of their main native predator (Galápagos Hawks Buteo galapagoensis; Anderson 1991, Anderson and Hodum 1993) and nesting habitat occupied by Blue-footed Boobies (Townsend et al. 2002), yet their respective Blue-footed Booby populations ceased effective breeding simultaneously, suggesting a regional, marinebased cause, such as diet. Along the continental margin, S. n. nebouxii eats primarily schooling, lipid-rich (Schew and Ricklefs 1998, Müllers et al. 2009) members of two families: Clupeidae (sardines and herrings) and Engraulidae (anchovies; Zavalaga et al. 2007, Weimerskirch et al. 2009). Before 1997, Galápagos Bluefooted Boobies showed a similar specialization on a clupeid, the South American sardine (Sardinops sagax; Anderson 1989). In the only study after 1997, the most common items were also clupeids (Galápagos thread herring [Opisthonema berlangai] and European pilchard [Sardina pilchardus, almost certainly a misidentification of Sardinops sagax]; Cruz et al. 2012; L. L. Cruz, personal communication). Engraulids have not appeared in booby diets in Galápagos and are apparently rare in Galápagos (Anderson 1989, Grove and Lavenberg 1997, Cruz et al. 2012; D. J. Anderson, unpublished data). Sardines were common in the diet of Nazca Boobies at Punta Cevallos before 1997 (Anderson 1989), but virtually absent after 1997 (D. J. Anderson, unpublished data), suggesting a food-based cause of the simultaneous regional change in Blue-footed Booby population biology. We evaluated several hypotheses regarding breeding, population size, and environmental factors influencing the Blue-footed Booby population in Galápagos: (1) breeding activity is less frequent than in the past; (2) the population of adults is smaller than in the past; and (3) diet characteristics, and consumption of clupeids in particular, influences breeding parameters including colony attendance, breeding attempts, egg and clutch size, and breeding success. METHODS Breeding From May 2011-June 2013, we monitored breeding at 3- to 5-month intervals at 4 of the 6 historically largest breeding colonies of Bluefooted Boobies (Daphne Major, Cabo Douglas [Fernandina], Punta Vicente Roca [Isabela], and Seymour Norte), and one additional recently established colony (Playa de los Perros [Santa Cruz]; Fig. 2). A successful breeding attempt takes 5 to 6 months (42 d. of incubation, ~100 d. of nestling rearing, and at least 28 d. of post-fledging feeding at the colony [Nelson 1978, Harris 1982]), so we were unlikely to have missed successful breeding. We visited each of these five focal colonies at night, when attendance is highest, recording the number of adults present (birds in juvenile plumage were never present), band numbers if visible, and the number of active nests with eggs and the number with nestlings. A sixth regularly large and active colony, Punta Suárez (Española), and two others (Punta Pitt [San Cristobal] and Punta Cormorant [Floreana]; Fig. 2) were selected as nonfocal colonies and were visited less frequently (three or four times each) than were focal colonies. Breeding was monitored when possible during these visits, with time of day varying (Table 1). The seventh historically large colony at Punta Cevallos, was known to be essentially unattended through our group s other research activities there. An additional, Fig. 2. Location of focal and nonfocal colonies, islands, and sections scanned per day during the coastal survey of June apparently newly established, nonfocal colony on Baltra was discovered in the second year of the study and entered the study in August 2012 as a nonfocal colony. If diet explains any depression in breeding, this may be reflected in females egg formation (clutch size and egg volume), as has been observed in this species (Dentressangle et al. 2008) and a congener (Anderson 1990, Clifford and Anderson 2001). Few eggs were available to us during this study, but in June 2012 we measured the length and breadth of 46 eggs from 26 clutches in the Playa de los Perros colony with calipers (0.1 mm precision) and calculated egg volume as V = 0.51*LB² (Hoyt 1979), where L is the length and B the breadth of the egg. We compared these volumes with data collected during years of high attendance and successful breeding from Punta Cevallos (136 eggs in 1984 and 69 eggs in 1985). We used an information-theoretic approach to compare a cumulative logit model incorporating no effect of year (null model) on the response variable, clutch size, with a model incorporating the effect of year. We applied a similar approach to egg volume for eggs from 1 or 2 egg clutches, comparing a general linear model incorporating no effect of year on egg volume (null model) with a model incorporating the effect of year (year model), and we compared these models with a third model incorporating the effect of clutch size (clutch size model). These analyses were conducted using ProcLOGISTIC and ProcGLM, respectively, as implemented in SAS v. 9.3 (SAS Institute, Cary, North Carolina, USA). Population size We intended to use mark-resight methods (McClintock and White 2009) to estimate the sizes of the breeding and nonbreeding components of the Blue-footed Booby population and associated demographic parameters. We marked 879 adults with two leg bands (one numbered stainless steel band and one field-readable plastic band) at the five focal colonies. The majority of the birds were marked at the beginning of the study, in May We now recognize the large number of birds available for banding on this occasion as an anomaly, and attendance was dramatically lower in later visits to these colonies. The low attendance probably does not reflect a disruptive effect of our first visit, based on the reactions of Blue-footed Boobies to the same techniques at our Punta Cevallos site ( ) and at the Playa de los Perros colony, which we visited frequently in We resighted a total of only 245 banded birds in the five colonies, with these birds providing 328 total resights, during six resight sessions (i.e., resight rate averaged 6.2% per session) conducted at 3-5 month intervals until June 2013, due principally to low attendance and secondarily to some loss of plastic bands, rendering the mark-resight approach unworkable. We made two surveys of the entire coastline of the islands south of the equator, including all of Isabela, as an alternative measure of population size. Blue-footed Boobies seldom visit the tropical, less productive waters (Houvenaghel 1978, Feldman 1986, Hayes and Baker 1989) around the five islands north of the Equator (Genovesa, Marchena, Pinta, Darwin, and Wolf), both Table 1. Schedule of visits and activities done on each colony. : monitoring breeding, presence of banded adults, diet sampling, at night; : monitoring breeding, presence of banded adults, diet sampling, at day; C: count of adults at day during coastal count across survey range. Colony site May 2011 Jun 2011 Aug 2011 Dec 2011/ Jan 2012 May 2012 Jun 2012 Aug 2012 Dec 2012/ Jan 2013 FOCAL COLONIES Playa de los Perros C C - Santa Cruz Daphne Major C C Cabo Douglas C C - Fernandina Pta. Vicente Roca C C - Isabela Seymour Norte C C Jun 2013 Punta Cormorant, Cuevas - Floreana Punta Pitt - San Cristóbal Punta Suárez - Española Punta Cevallos - Española La Millonaria - Baltra NONFOCAL COLONIES,C,C,C,C,C historically (Nelson 1978, Harris 1982) and during this study (D. J. Anderson, personal observation); this fact justifies the exclusion of these sites from the survey range comprising 1100 km of coastline of 14 islands and 20 islets. Almost no birds were breeding at the times of the two surveys, and we reasoned that nonbreeders would spend much of their time resting on sea cliffs based on our experience with this species, justifying the choice of a boat-based coastal survey. During the 2012 survey we also recorded birds sighted on the open ocean when the boats moved between islands as an additional test of the assumption. During both surveys, observers gave special attention to detecting new colonies, and counted historical and newly detected colonies on foot. In the first survey, we made daytime counts in piecemeal fashion, covering the entire survey range in a boat at 1-m/s between 3 June and 7 August 2011, with a single observer (D. J. Anchundia) using binoculars m from the coast to count birds. Each Bluefooted Booby perched on land or flying against the direction of the boat s movement was recorded, with adult or juvenile status, time of day, and latitude and longitude measured by a hand-held GPS unit. In the second survey, on 1-3 June 2012, we used a dependent double observer protocol (Nichols et al. 2000). A primary observer indicated to a secondary observer each bird that was roosting on the cliff faces directly to the side of the boat as the boat traveled parallel to the island. Birds that were flying away from the island and toward the boat were also counted if they flew within 200 m of the boat. At the time of the detections, the secondary observer recorded these data and any additional birds that were detected only by the secondary observer. Observers travelled on a boat moving 2-8 m/s m from the coast, using binoculars to detect birds on land and flying against the direction of the boat s travel. In the single exception to this protocol, during the 2012 count in the northwest part of Santiago, the boat moved 1 km from the coast for 19 km (1.7% of the total coastline surveyed) because of hazardous navigation, so the birds on land and over water near the coast were missed. Birds sighted were counted and binned into 30 min travel intervals ( stretches ) by 2-5 pairs of observers, depending on the day, each pair on a separate boat covering a unique part of the coastline (Fig. 2). GPS locations were recorded at the start and end locations of each stretch. To minimize double-counting or missing a bird due to its movement between parts of the survey range, we conducted the survey over the smallest time period possible (3 d) given the availability of suitable boats. All pairs of observers working on a given day counted in the same area of the survey range, with the areas chosen to minimize the possibility of birds moving between the three daily count areas during the survey (Fig. 2). On 1 June, two observer pairs counted the western archipelago, reasoning that interchange between that region and the rest of the archipelago was rare. In particular, few birds cross the narrow Perry Isthmus in the middle of Isabela each day (Fig. 2; Anchundia 2013). On 2 June, 5 observer pairs counted eastern Isabela and the 8 islands and 15 islets in the central part of the survey range. On 3 June, three observer pairs counted the relatively isolated islands and islets in the east, southeast, and south of the archipelago (Fig. 2). The second survey differed from the first in time frame (compressed in 2012) and in number of simultaneous observers, enabling estimates of detection probability in 2012, and it can be expected a priori to provide a more accurate estimate than the 2011 count. Data collected during these double-observer surveys resulted in individual encounter histories of 10 for birds that were detected by the primary observer and 01 for birds detected only by the secondary observer. We tabulated the number of birds with each type of encounter history for each stretch and treated each stretch as a separate group in our analyses. To estimate the probability of detection (p), we used a Huggins closed captures model (Huggins 1989, 1991) as implemented in Program MARK (White and Burnham 1999), which uses information from the two observer types to calculate the maximum likelihood value of detection probability. We developed a set of four models reflecting four hypotheses for variation in p where p was estimated as (1) a common probability for all stretches combined (CONSTANT), (2) a common probability for all stretches counted by each of the six primary observers (OBSERVERS), (3) a common probability for all stretches around each island or geographically distinct section of very large islands (ISLANDS), and (4) a separate detection probability for all stretches (STRETCHES).
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