The use of artificial breeding chambers as a conservation measurefor cavity-nesting procellariiform seabirds: a case study of the

Madeiran storm petrel (Oceanodroma castro)

Mark Bolton*, Renata Medeiros, Becky Hothersall, Ana Campos

Department of Oceanography and Fisheries, University of Azores, 9901-862 Horta, Azores, Portugal

Received 15 October 2002; received in revised form 20 March 2003; accepted 20 March 2003

Abstract

Procellariiform seabirds are amongst the most severely threatened taxa worldwide. Whilst the specific threats vary among species,problems such as introduced predators, loss of suitable nesting habitat and inter- and intra-specific competition for nest sites arecommon to many situations. This paper reports on an experiment to increase the availability of secure nest sites at a colony of

Madeiran storm petrels breeding on an islet (free from introduced predators) in the Azores islands where there is evidence of inter-specific competition. The provision of nest boxes that were designed to exclude larger species led to around a 12% increase in thesize of the breeding population in the first year and a 28% increase over the original colony size in the second year. Over three

seasons, the breeding success of storm petrels nesting in boxes averaged 2.9 times greater than that of birds at natural sites, despitetheir likely reproductive inexperience. The techniques presented here should serve as a model for conservation management of otherburrowing procellariiform species in situations where colony size or breeding performance is believed to be constrained by nest sitequality.

# 2003 Elsevier Ltd. All rights reserved.

Keywords: Nest-box; Procellariiformes; Nest site competition; Azores

1. Introduction

In evolutionary terms, the Procellariiformes (i.e.albatrosses and petrels) represent one of the oldest andmost successful avian Orders, and yet they are nowamongst the most threatened taxa worldwide. Of the 125species currently recognised, 55 (44%) have a conservationstatus classified as Critical, Endangered or Vulnerableaccording to IUCN criteria (BirdLife International,2000). A further 14 species (11%) are considered Near-threatened and four species are insufficiently known toallow a conservation classification. The introduction ofmammalian predators to seabird breeding colonies hasbeen a consequence of human colonisation of archipelagosin every ocean, with devastating effects on procellariiformpopulations. The availability of suitable nesting sites onthe remaining predator-free islands and islets may

consequently limit the size of the breeding population inmany situations. Of the 104 species whose nesting habitsare well documented, 78 (75%) nest in burrows or cre-vices (Schreiber and Burger, 2001) and there is evidenceof intra- and inter-specific competition for adequatenesting cavities in some colonies (Harris, 1969; Ramoset al., 1997; Gardner and Wilson, 1999). In mixed spe-cies colonies of cavity-nesting Procellariiformes, thesmaller species are likely to be at considerable competi-tive disadvantage in disputes over nest ownership. Forexample, Cory’s shearwater (Calonectris diomedea) hasbeen documented killing adult Bulwer’s petrels (Bul-weria bulwerii), little shearwaters (Puffinus assimilis) andMadeiran storm petrels (Oceanodroma castro) whenenlarging previously occupied nest burrows for theirown use (Ramos et al., 1997).The Madeiran storm petrel is the smallest species

among the assemblage of Procellariiformes breeding inthe Azores islands where it is restricted to just threeknown breeding colonies, each located on small off-lying islets which have remained free of introduced

0006-3207/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0006-3207(03)00178-2

Biological Conservation 116 (2004) 73–80

www.elsevier.com/locate/biocon

* Corresponding author. Current address: Royal Society for the

Protection of Birds, The Lodge, Sandy, Bedfordshire SG19 2DL, UK.

E-mail address: mark.bolton@rspb.org.uk (M. Bolton).

mammalian predators (Monteiro et al., 1996). Each ofthese colonies also supports up to three other species ofcavity-nesting Procellariiformes (Bulwer’s petrel, littleshearwater and Cory’s shearwater), leading to potentialcompetition for nest sites. There is historical evidencethat procellariiform species were exceedingly abundantthroughout the Azores archipelago when the islandswere first settled by humans in the fifteenth century(Frutuoso, 1561; Monteiro et al., 1996), but were pro-gressively eradicated from the major islands by a com-bination of human harvesting and depredation byintroduced dogs, cats, rats and mustelids.Within the Azores, Madeiran storm petrels breed in

two distinct seasons (Monteiro and Furness, 1998),termed ‘‘hot’’ (April–September in Azores) and ‘‘cool’’(August–April). Elsewhere in Europe the Madeiranstorm petrel breeds in the Madeiran archipelago, theSalvages, on the Canary Islands and on FarilhaoGrande off continental Portugal and has a conservationstatus classified as Vulnerable (Tucker and Heath,1994).Artificial nest chambers have been successfully

employed for a small number of procellariiform species(e.g. Pterodroma cahow Wingate, 1977, Puffinus pacifi-cus Byrd et al., 1983, Oceanodroma leucorhoa Podolskyand Kress, 1989, Hydrobates pelagicus Bolton, 1995,Pterodroma leucoptera Priddel and Carlile, 1995, andCalonectris diomedea Ramos et al., 1997), but the con-servation and research potential of nest boxes remainslargely unexploited. This paper describes the use ofartificial nesting chambers as a conservation measure toincrease the availability of secure nest sites forMadeiran storm petrels nesting in the Azores archipe-lago, and compares the breeding success of pairs nestingin artificial boxes and natural nest cavities.

2. Methods

2.1. Study site and species

The study was carried out on Praia Islet (37�570N39�030W), an uninhabited 12 ha volcanic island, lying 1km east of Graciosa in the Azores archipelago. Theisland rises to a summit of 59 m and the vegetation isdominated by annual graminaceous species with scat-tered Tamarix africana (maximum height ca. 8 m). PraiaIslet supports an estimated 100 breeding pairs of hotseason birds and 200 pairs of cool season birds, whichrepresent up to 40 and 30% of the Azores population ofeach form respectively (Monteiro et al., 1999).

2.2. Artificial nest chambers

On Praia Islet, as at other breeding colonies, Madeiranstorm petrels nest in rock crevices and burrows dug in

earth banks. Artificial chambers were installed in amanner to simulate these two nesting habitats. Due tologistic constraints and financial considerations, weavoided a highly machined, complex nest box designsuch as that employed in previous studies (e.g. Bolton,1995; Priddel and Carlile, 1995). Instead we opted forthe simplest design possible making use of readilyavailable local materials. The chambers (Fig. 1) wereconstructed from PVC for durability. Space constraintswere also relevant since the chambers had to be trans-ported by air and inflatable dinghy between islands. Welocally sourced two types of material which could pro-vide the required design: PVC tubing as used by Bolton(1995), and plant containers. The latter offered anextremely lightweight, economical solution, and sincecontainers were tapered they could be stacked insideeach other, facilitating transport. Nest chambers werefilled with drainage material (small stones) and nestmaterial (dried grass) to leave a nesting chamber 10 cmhigh.Since daily inspection of the nest boxes was required,

flexible plastic lids were cut from black plastic sheeting0.2 mm thick, with a 3 cm lip to fit easily over the top ofthe nest chamber. These lids provided a watertight coverto the nest chamber, but could easily be partiallyremoved by peeling back one edge, to inspect the nestcontents, or remove the adult or chick for measurement.Given the spatial constraints of the holes into whichnest boxes were installed, rigid PVC lids which neededto be completely removed to access the nest, were muchless practical in the field.A total of 115 nest boxes were installed on Praia Islet

in sites previously lacking suitable storm petrel nesting

Fig. 1. Schematic view of artificial nesting chamber for Maderian

storm petrels. The internal dimensions of the nest chamber were ca. 16

cm diameter and ca. 10 cm height, with an entrance hole of 6 cm dia-

meter. Small stones were packed between the outer wall of the nesting

chamber and the soil cavity in which the chamber was housed, for

stability, and to facilitate drainage of surface water away from the

nest. The entrance tunnel was typically 30 cm long, curved to reduce

the amount of light reaching the nest, and sloping downwards to the

exterior to prevent rainwater draining into the nest.

74 M. Bolton et al. / Biological Conservation 116 (2004) 73–80

cavities, during February and March 2000. At this timeof the year cool season storm petrels are tending chicks.Forty-nine nest boxes were built into a low (ca. 30 cm)‘‘wall’’ extending for some 16 m close to the island’ssummit and within 10 m of the cliff edge. Since theisland receives a large number of visitors on occasionduring the summer months, care was taken to ensurethat no box was visible but birds could access each boxvia a network of entrance tunnels. Observer access toeach nest box was obtained by removing a small num-ber of stones (in many cases, just a single large stone),covering the lid. The remaining 66 boxes were installedin earth banks, ca. 30 cm from a vertical erosion face.Due to a lack of sufficient large stone slabs to cover thenest box cavities, we used expanding polyurethane foam(incorporating local stones for an inconspicuous result).Since polyurethane foam expands to fill the availablecavity before hardening, the resulting covers perfectly fit-ted each nest box cavity, and were extremely lightweight.Nest boxes were generally installed in lines along

naturally occurring erosion faces, containing from threeto 10 boxes per line, according to the length of the ero-sion face available. To maximise the available banks,boxes were located as densely as possible, usually about0.5 m from adjacent boxes. Where the bank was foundto be in danger of eroding further, or where other pro-cellariiform species (especially Cory’s shearwater)attempted to enlarge the entrance burrows, rocks wereplaced along the erosion face to protect the entranceburrows. All boxes were installed into banks within 20m of the cliff edge, and generally within 50 m of thenearest known natural nest site. All boxes were sitedaway from areas occupied by breeding terns (Sternaspp.) to avoid disturbance to other breeding seabirdsduring the subsequent daily inspection of the stormpetrel boxes. At the close of the hot season breedingseason, in September 2000, a further 32 nest boxes wereinstalled in a bank close to the cliff summit, within 20 mof the cliff edge.

2.3. Attraction of storm petrels to artificial nest chambers

In order to attract prospecting storm petrels to theareas in which the nest boxes were located, recordings ofthe nest song were broadcast at night using an auto-mated system powered by a solar panel and wind gen-erator. An auto-reverse tape recorder was activated bya light sensitive switch, to function from sundown untilsunrise. We used recordings appropriate for each popu-lation, which were made at the study site. Since audiorecordings will attract storm petrels to the precisesource of the sound, a total of 14 loud speakers wereplaced alongside the burrow entrances (approximatelyone speaker per eight nests). Results from the first (hot)season indicated that the attraction effect of the loudspeakers was extremely local and so additional speakers

were installed before the start of the second (cool) sea-son. For the hot season population, audio recordingswere played nightly from mid April 2000 until deacti-vated in early July (when further nest box occupationwas considered improbable). For the cool season popu-lation audio recordings were broadcast nightly from thebeginning of September 2000. During the hot season2001, the audio system functioned only intermittently,from mid April. However, it is probable that the returnof birds which had bred successfully in nest boxes in theprevious year served to attract prospecting birds.

2.4. Data collection and analysis

The study encompassed two hot-season breeding per-iods (2000 and 2001) and one cool-season period (2000–2001). During the hot-season, nest box sites wereinspected daily from mid-April until the end of thebreeding season in September and for the cool-season,from August until April, to determine nest occupancyand the fate of all eggs laid and chicks hatched. Theonly extended periods for which no data were obtaineddirectly were from 6 October to 14 October 2000, from10 December 2000 to 16 January 2001 and from 12March to 12 April 2001 when inclement weather pre-vented landing on the Islet. However, the outcome ofevery nesting attempt was unambiguous. All storm pet-rels present in a nest box during the day were markedwith a unique two-colour combination of non-toxicpaint on the upper-parts, for subsequent individualrecognition. Due to the extreme sensitivity of somestorm petrel species, birds found at the nest duringdaylight were not handled during the pre-laying phase.We obtained a sample of nests in natural sites by

locating birds singing from nests at night, and byinspecting potential nest crevices using a torch, bothduring daylight and at night. We selected nests wherethe nest chamber was visible from the entrance. Due tothe high egg failure rates of natural nests, to increasesample sizes to estimate fledging success, we includednests found during the late stages of incubation (whichwere not included in the estimates of hatching success).For this reason sample sizes are not equal for all ana-lyses. In practice, estimates of fledging success werealmost identical whether based on the original sample ofnests, or incorporating additional nests located duringincubation. For simplicity we base statistical compar-isons on the original sample but comparisons of nestproductivity between natural and artificial nest sites gaveidentical results using either sample. Once occupied nat-ural sites had been located, they were inspected daily.Non-parametric tests were used for statistical analysis

due to parameter distributions departing significantlyfrom normality. Non-parametric procedures followRoscoe and Byars (1971) and Zar (1999). All prob-ability values are two-tailed.

M. Bolton et al. / Biological Conservation 116 (2004) 73–80 75

Table1

DaytimeoccupancyandbreedingsuccessofMadeiranstormpetrelsnestinginartificialandnaturalnestsites

Season

(year)

Daylight

occupancy

rate

Laying

rate

Hatchingsuccess

(eggshatched/eggslaid)

Fledgingsuccess

(chicksfledged/chickshatched)

Productivity

(chicksfledged/breedingattempts)

Artificial

Natural

Sig.

Artificial

Natural

Sig.

Artificial

Natural

Sig.

Hot

(2000)

47/115

(40.9%)

12a/115

(10.4%)

6/13

(46.2%)

6/13

(46.2%)b

Fisher’sExact

TestP=1.000

5/5

(100.0%)c

5/15

(33.3%)

Fisher’sExact

TestP=0.033

5/12

(41.7%)

2/13

(15.4%)

Fisher’sExact

TestP=0.20

Cool

(2000–01)

40/147

(27.2%)

22/147

(15.0%)

16/22

(72.7%)

7/21

(33.3%)

X2 1=5.212

P=0.022

14/16

(87.5%)

16/21

(76.2%)

X2 1=0.199

P=0.66

14/22

(63.6%)

6/21

(28.5%)

X2 1=3.994

P=0.046

Hot

(2001)

49/147

(33.3%)

28/147

(19.0%)

22/28

(78.6%)

7/18

(38.9%

X2 1=5.780

P=0.016

18/22

(81.8%)

3/7

(42.9%)

Fisher’sExact

TestP=0.067

18/28

(64.3%)

3/18

(16.7%)

X2 1=8.186

P=0.004

aAsecondeggwaslaidinoneboxfollowingfailureofthefirstegg.

bMaximumestimate:ninenestsnotfollowedfromthestartofincubation.

cExcludingonecasewherethedeathofthechickwasdirectlyattributabletohumaninterferenceandnotnaturalcauses.

3. Results

3.1. Nest box occupancy

The occupancy of nest boxes by Madeiran storm pet-rels during daylight varied according to season, from 27to 41%, and breeding took place (i.e. an egg was laid) in10 to 19% of boxes (Table 1). The proximity of the nestbox entrance to a loudspeaker broadcasting storm pet-rel song was important in determining daytime occu-pancy of boxes. Those boxes which were occupied indaylight were significantly closer to the nearest loudspeaker, than those boxes in which birds were neverpresent during daylight (median distances (cm)�IQR(n), hot season 2000: 70�107 (23) and 120�145 (35)respectively, Mann–Whitney Z=1.9647 P=0.0495; coolseason: 35�20 (30) and 65�85 (68) respectively,Mann–Whitney Z=3.0046 P=0.0027, analysis limitedto boxes sited in banks, since boxes sites in the wall hadmultiple entrances).

3.2. Breeding performance of Madeiran storm petrels inartificial chambers and natural sites

The proportion of eggs that hatched and the propor-tion of hatched chicks which survived to fledging arepresented in Table 1, for each of the three seasons,according to nest type (natural site or nest box). In eachof the three seasons there were significant differences ineither hatching success or fledging success according tonest type, which resulted in storm petrels breeding inartificial nest sites rearing a higher proportion ofyoung to fledging, despite their presumed reproductiveinexperience.

3.2.1. Causes of hatching failureThe major cause of hatching failure at natural sites

(pooled across seasons) was egg damage, caused bysmall stones in the nest floor puncturing the egg shell.Such damage was responsible for hatching failure at21.2% of all nests in natural sites (n=52). In many casesthe damage, though fatal to the egg, was relativelyminor, and the egg was incubated for the usual period.At nest box sites in contrast, only 1.6% of eggs weredamaged (n=63, X21=9.670 P=0.002). The major causeof hatching failure at nest boxes, accounting for 17.5%of nests, was a lack of good incubation routines andabandonment of the egg after an initial period of incu-bation. Four nesting failures (6.3% of nests) were due toa complete absence of incubation; eggs were laid butnever incubated. Four further failures were due toincubation being carried out by a single adult, whicheventually abandoned the egg. A further three losses(4.8%) were due to the egg being abandoned havingbeen initially incubated for a time by both adults. It isunlikely that poor incubation resulted from disturbance

76 M. Bolton et al. / Biological Conservation 116 (2004) 73–80

of the nests since, in most of these cases, one or bothadults never incubated the egg during daylight (ratherthan incubating for a time, then abandoning because ofdaily disturbance). However, taken together, the per-centage of nests failing due to poor incubation beha-viour did not differ between nest boxes (17.5%, n=63)and natural sites (15.4%, n=52 X21=0.002 P=0.96).The next most important cause of egg failure at naturalsites (9.6% of nests) was excavation by larger seabirdspecies (Cory’s and little shearwaters), which occurredin both seasonal populations (9.7% of hot season nestsand 9.5% of cool season nests). There were no egglosses attributable to inter-specific competition amongstorm petrels nesting in artificial nest boxes, a differ-ence which was statistically significant (X21=4.232P=0.04).

3.2.2. Causes of fledging failureThe most common cause of fledging failure (account-

ing for 13 of 86 chicks monitored, 15.1%) was unde-termined—the chick simply disappeared from the nestsite, often overnight, without any obvious signs of nestdisturbance. In two cases the chick was subsequentlyfound dead nearby, and chicks which abandoned thenest site are unlikely to have survived adverse weather(heavy rain and wind or hot sun, dependent on sea-son) or predators [yellow-legged gull (Larus cachin-nans) and long-eared owl (Asio otus)]. Two possiblecauses of chicks leaving the nest site prematurely werehunger and heavy nest parasite burdens. The propor-tion of chicks which disappeared from artificial nestchambers (one of 43 chicks across all seasons) waslower than for chicks reared in natural sites (12 of 43chicks, X21=9.062 P=0.003). The proportions of chickswhich were well nourished but died following extremelyheavy external parasite infestations was similar betweennest box and natural sites (three of 43 and two of 43chicks respectively, X21=0.000 P=1.00), and deathsattributed to external parasites were significantly morefrequent during the hot season (eight of 49 chicks) thanduring the winter (zero of 37 chicks, X21=4.866P=0.027).

3.2.3. ProductivityThe proportion of eggs which survived from the

beginning of incubation to fledging is given in Table 1.Estimates of nest productivity calculated by multiplyinghatching success by fledging success, the latter based ona larger sample of nests located during incubation, givesvirtually identical estimates of productivity and resultsin the same statistical conclusions.There were no differences in the productivity of

nest boxes located in earth banks and those built intoa low wall in any of the three breeding seasonsstudied (Fisher’s Exact Test: P=0.22, P=0.98 andP=0.67).

4. Discussion

4.1. Effect of nest box provision on colony size andbreeding success

Since it was not possible to monitor all natural nestsites at the colony before and after the installation ofnest boxes, we cannot exclude the possibility that pairsnesting in nest boxes had simply moved from natural toartificial nesting sites. It is therefore not possible toascertain the precise increase in breeding populationrepresented by the pairs of Madeiran storm petrelsnesting in artificial chambers. Despite this, we considerit highly probable that the majority of storm petrelsbreeding in nest boxes did indeed represent additionalpairs to the colony. Procellariiformes exhibit a highdegree of nest site fidelity and established breeders gen-erally have higher breeding success than those nestingfor the first time (Wooller et al., 1990). It is unlikely,therefore, that established pairs breeding in naturalsites, would jeopardise their breeding success by movingto novel, artificial sites. Previous studies have shownthat competition for nest sites exists among the smallerprocellariiform species in the Azores islands (Ramos etal., 1997), suggesting that all available nest sites, naturaland artificial, are likely to be occupied. Taken together,it is probable that the pairs nesting in nest boxes repre-sent an increase in colony size, which would be equiva-lent to 12% in the first year, rising to 28% in the secondyear for the hot-season population. The increase in thecool-season population would be equivalent to 11% inthe first year. Due to their higher breeding success (upto 3.8 times that of pairs in natural sites), storm petrelsnesting in artificial nest chambers made a dis-proportionately large contribution to the reproductiveoutput of the colony.The stage of the reproductive cycle at which a sig-

nificant improvement in breeding performance wasachieved varied among seasons and years (Table 1),though the power of statistical analysis is limited byrelatively small sample sizes. The most consistent effectwas an increase in hatching success (in two of the threeseasons studied), attributable principally to a largereduction in egg damage caused by small stones on thenest floor puncturing the egg shell. Although the esti-mates of hatching success estimates in the other season(hot 2000) were identical for artificial and natural nestsites, they were based on smaller sample sizes and nat-ural nest sites that were not followed from the initiationof incubation and hatching success will therefore havebeen overestimated. The finding that egg damage isresponsible for a considerable proportion of nest fail-ures in Madeiran storm petrels suggests that naturalnesting cavities could be substantially improved by theaddition of a simple lining (such as a pad of dry vege-tation). However, an important further function of

M. Bolton et al. / Biological Conservation 116 (2004) 73–80 77

artificial nest boxes is to prevent intrusion or excavationof the nest by larger burrowing seabirds which wasshown to be important cause of nest failure in both hotand cool breeding seasons.Ectoparasite burdens are known to influence growth

and survival in seabirds, including storm petrels (Kinget al., 1977; Merino et al., 1999) and data obtained inthis study suggested that ectoparasites were responsiblefor a significant proportion of chick deaths. Over time itis likely that artificial nest boxes will harbour similardensities of nest parasites to natural crevices. Indeed,the microclimate provided by nest boxes may actuallyprove more suitable for parasite survival during thepetrel non-breeding period than that of natural cavities.It is noteworthy that, even in the first years of nest boxusage, chick deaths linked to heavy parasite infestationsoccurred at similar frequencies among nestlings in arti-ficial chambers and natural sites. Clearly, the annualremoval and replacement of nesting material during thenon-breeding season would be beneficial for any nestbox population of Procellariiformes.

4.2. Nest box construction

The materials used for the construction of artificialnest chambers in this study were lightweight, inexpen-sive and locally available. In order for artificial nestchambers to provide a long term benefit it is importantthat the construction material should also be durable,with an expected lifespan of tens of years. In the absenceof long term field-testing it is not possible to specify thelifespan of the plastics used in the present study. Thematerial selected was relatively flexible which preventeddamage by shattering or cracking and made the con-tainers very easy to cut and drill. Being buried under-ground the chambers will not suffer photo-degradationby sunlight (which renders plastics brittle), and they willbe somewhat buffered from temperature extremes. Inthe light of these considerations, it is anticipated thatthe artificial chambers should have a useful lifespan inexcess of 10 years. Clearly great care should be given tothe design and selection of construction materials ofsuch artificial chambers, and the most appropriatesolution will depend on such considerations as the pri-mary aims of the project (i.e. simply to increase nest siteavailability, or facilitate monitoring, or the installationof remote equipment); the number of nest chambersrequired; local availability of materials as well as bud-getary and logistic considerations.

4.3. Establishment of nest box populations

Podolsky and Kress (1989) have shown that thedeployment of sound lures and hand-dug nest burrowscan attract Leach’s storm petrels (Oceanodroma leu-corhoa) to breed at previously uncolonised sites. Both

their study and the data presented here demonstrate theimportance of positioning nest song sound lures veryclose to artificial burrow entrances. Podolsky and Kress(1989) found that the provision of artificial nesting bur-rows without a sound stimulus failed to result in colo-nisation of new areas. However, at sites where anappropriate sound stimulus was broadcast, 70% ofburrows located within 50 cm of a loud speaker wereoccupied, compared with only 16% of burrows locatedthree or more metres from speakers. Similarly in thisstudy, occupied burrows were significantly closer tospeakers than burrows that remained unoccupied.Observations conducted on moonlit nights also

showed that storm petrels were attracted to the verysource of the sound lure. After circling repeatedly inflight and calling, individuals frequently alighted withina few centimetres of a loud speaker, and would thenenter the nearest burrow. The relatively low burrowoccupancy rates of the Podolsky and Kress (1989)study (46 of 204 burrows with adequate sound lure)compared with the results presented here (Table 1) mayhave been due to the lower density of speakers perburrow (one speaker per ‘‘set’’ of 18–36 burrows)compared with the present study (average of onespeaker per eight burrows).Taken together, the results of both studies indicate

that the provision of artificial nests in the absence of anappropriate sound stimulus is unlikely to significantlyincrease breeding densities in the short term, althoughmight prove beneficial in the long term at establishedcolonies. The installation and maintenance of equip-ment to broadcast a sound stimulus is likely to representa considerable logistic challenge in most circumstances,but should be considered an essential component of theprovision of artificial nest sites. Both studies clearlydemonstrate that sound lures need be effectivelyemployed only for the first season, to establish a ‘‘seedcolony’’, which will then become self sustaining. Eightyears after the use of sound stimulation for the estab-lishment of a Leach’s storm petrel colony, Podolsky andKress (1989) found the number of breeding pairs hadincreased from the levels attained in the first three years.This study showed a large increase in the number ofbreeding pairs from the first to the second hot season,without effective use of sound lures in the second sea-son. This suggests the establishment of a small numberof successfully breeding petrels can serve to attract pro-specting birds in subsequent years.

4.4. Nest boxes as a conservation tool

Procellariiformes are extremely k-selected and all spe-cies lay a single egg, and the production of a replacementegg in the event of egg loss is extremely rare and onlycircumstantially documented (see review in Warham,1990). Annual reproductive output of Procellariiformes

78 M. Bolton et al. / Biological Conservation 116 (2004) 73–80

in general is therefore low, with obvious conservationimplications for the recovery of critical populations.This study found the breeding success of Madeiranstorm petrels nesting in natural sites varied between 0.15and 0.29 chicks per pair, depending on season, whichare virtually identical to values obtained by Harris(1969) for O. castro breeding in the Galapagos islands(0.16 to 0.29 dependent on season) and a value of 0.26chicks per pair for the cool season population breedingon Boatswain Bird Island, Ascension (Allen, 1962). Inour study, the provision of secure nesting chambers ledto a dramatic increase in reproductive output, varyingbetween 0.42 and 0.64 chicks per pair dependent onseason (Table 1). This result is particularly noteworthy,since the pairs that bred in nest boxes were breeding forthe first time in a new site and most probably repro-ductively inexperienced individuals nesting with a newpartner. All these attributes are usually associated withlower reproductive output. The reproductive output ofstorm petrels nesting in artificial nest sites at this colonymay reasonably be expected to increase slightly in futureyears therefore.Newton (1994) has reviewed the evidence for nest site

availability limiting the densities of tree hole nestingbird species, concluding that a lack of suitable cavitiesalmost invariably limits population densities in mana-ged forests. He noted the provision of nest boxes as anobvious conservation strategy in such situations. Simi-larly, the techniques described here offer considerablepotential for managing cavity nesting seabirds in situa-tions where reproductive output is believed to be con-strained by nest site quality or competition for adequatenest sites. Given the precarious conservation status of aconsiderable proportion of the world’s burrowing pro-cellariiform species, such management should be viewedas a high priority. The artificial attraction of petrels tonew breeding locations, where introduced predatorshave been eliminated, and the provision of artificialnesting chambers to maximise reproductive output,represent valuable management tools in the conser-vation of burrowing Procellariiformes.

Acknowledgements

This work was financed by grants from the Portu-guese Fundacao de Ciencia e Tecnologia (PRAXIS/C/BIA/13194/98 and POCTI-BIA-13194/98) and a SmallProject Grant from RSPB/BirdLife to the SociedadePortuguesa para o Estudo das Aves (SPEA). Permissionto work in the Special Protection Area of Praia Islet, forthe installation of nest boxes and for the handling ofstorm petrels was granted by the Direccao Regional doAmbiente of the Azores, and special thanks are due toDra Maria Pitta Groz for the efficiency with which sheprocessed these applications. The Camera Municipal da

Santa Cruz da Graciosa and the Junta de Freguesia deSao Mateus, Praia contributed considerable logistalassistance. We acknowledge the expertise of Sr MarioMelo for regular transportation to the islet often inextremely difficult sea conditions and the Direccao daProteccao Civil for helicopter evacuation. We also thankLuıs Dias, Ana Mendes, Luıs Aguiar and Colin Bealewho assisted in various aspects of nest box construction,installation and monitoring. Plant containers were pro-vided by the Botanic Garden of Faial. Thanks are alsodue to Jane and Jack for their ongoing support. We alsothank Joel Bried, Jaime Ramos, Mike Harris and BobFurness for helpful comments on a previous draft of themanuscript.This work is dedicated to the memory of our friend

and colleague, Dr. Luis Monteiro, to whom the projectfunding was granted and who was to have coordinatedthis research. He was killed in a plane crash in theAzores on 11th December 1999.

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