Introduction
Seabird life history strategies are often characterised
by high adult survival, delayed maturity at breeding, low
clutch sizes and variable reproductive output (Schaffner
1990, Bowler et al. 2002, Hockey and Wilson 2003,
Ramos et al. 2005). At low latitudes seabirds have small
clutches, large eggs and long incubation and fledging
periods (Hockey and Wilson 2003), and clutch size has
been related to food availability and predation risk (Shea
and Ricklefs 1996).
Primary production in tropical marine ecosystems is
generally considered to be low and seasonal, and food
may be scarce, especially at oceanic islands (Hockey
and Wilson 2003). Avian predators on small islands are
generally few, but predators may be attracted to seabird
eggs and chicks when large numbers of seabirds are
breeding at high densities (Schaffner 1991, Hockey and
Wilson 2003). Although most tropical islands are free
of large indigenous mammalian predators, predation
by smaller terrestrial predators like crabs and skinks
occurs (Schaffner 1991, Ramos et al. 2005). Intra- and
interspecific competition between breeding birds may
also lower breeding success (Bowler et al. 2002, Ramos
et al. 2005).
Cousine Island is one of the smallest islands in the
Seychelles archipelago. Seven tropical seabirds breed at
the island; two of the least common are the White Tern
Gygis alba and the White-tailed Tropicbird Phaethon
lepturus. An estimated 1 000–1 500 pairs of White Terns
and 450–850 pairs of White-tailed Tropicbirds breed on this
island (Skerret et al. 2001). White Terns are tree-nesting
birds that lay a single egg on an exposed fork or in an
artificial structure, whereas White-tailed Tropicbirds
are ground nesters that breed in a shady and sheltered
crevice (Schaffner 1991, del Hoyo et al. 1996, Bowler
and Pillay 2000). The incubation period of White Terns is
34–36 d (five weeks) and the nestling period 68 d (seven
weeks), whereas the incubation period of the White-tailed
Tropicbird is 40–42 d (six weeks) and the nestling period
77–85 d or 11–12 weeks (del Hoyo et al. 1996, Higgins
and Davies 1996). Both species exhibit life-history traits
specific to tropical island breeding. Each species has a
clutch size of one and nesting success is generally low,
from 30–50% for White-tailed Tropicbirds (Schaffner 1991,
Ramos and Pacheco 2003) and 29–40% for White Terns
(Vanderwerf 2003).
The aim of this study was to compare the reproductive
parameters of White Terns and White-tailed Tropicbirds
on Cousine Island and investigate if egg or chick failures
were associated with different stages of the incubation and
nestling periods. The study also examined the associations
between reproductive parameters and nest types
(White Terns), the two monsoon seasons and nest reuse.
We postulate reasons for observed differences in the
measured parameters between the two species
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:tut/oai:encore.tut.ac.za:d1001721 |
| Date | 13 July 2009 |
| Creators | Malan, G, Hagens, DA, Hagens, QA |
| Publisher | Ostrich |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Text |
| Format | |
| Rights | Copyright © NISC Pty Ltd |
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