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Foraging ecology of masked boobies: the importance of intrinsic and extrinsic factors
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Abstract
The foraging ecology of marine top predators is intricately linked to intrinsic characteristics
of the individual, as well as extrinsic, environmental factors. Analysing both the effects of
these intrinsic and extrinsic factors is important to better understand the foraging ecology
of animals that are constrained in their foraging time due to the need to return to a specific
location during reproduction.
In this thesis, foraging movements and activity patterns of chick-rearing Masked Boobies
Sula dactylatra breeding on Phillip Island (Norfolk Island Group), Australia (South Pacific)
were investigated over two breeding seasons using GPS and depth-acceleration loggers, to
determine (I) how intrinsic factors, such as body mass and size, influence individual foraging
behaviour, (II) if adults show differences in diving behaviour and foraging movements
throughout the study period, and whether this may be related to changes in prey availability
and distribution in a tropical marine environment, and (III) the accuracy of first-passage time
analysis (FPT) to determine area-restricted search behaviour in a central-place forager.
(I) Masked Boobies exhibit reversed sexual dimorphism. In this study, alternations between
short and long trips were related to body mass in females, but not in males, and were
presumably used to provision the chick and for self-maintenance, respectively. In this
species, females are the parent in charge of provisioning the chick. During short trips,
heavier females spent higher proportions of their flight time flapping and less time resting at
the water surface than lighter females during long trips. Conversely, heavier males spent
less time flapping, potentially to save energy during locomotion, and relatively more time
resting. These findings underline the importance of accounting for the effects of body mass
differences within the same sex, as well as different parental roles, when the influence of
extrinsic, environmental factors are investigated.
(II) Marine predators often forage in bouts and both bout length and distances between
dives within a bout have been related to the quality and density of prey patches, whereas
distances between bouts is thought to reflect prey encounter rates. This study compared
adult diving behaviour and foraging movements over three study periods (early (E1) and late
(L1) phase of the first breeding season and the following early season (E2)) under varying oceanographic conditions through depth-acceleration (in L1 and E2) and GPS loggers. Adults showed high foraging site fidelity over the nearby shallow shelf (‘local foraging area’) but not over distant, deeper waters (‘distant foraging area’) between trips and study periods. In E1, although chlorophyll a concentration (CHL) was relatively low, adults concentrated their
foraging activities within the local area, whereas in L1 (similar CHL as in E1, highest seasurface
temperature (SST)) and E2 (highest CHL, similar SST as in E1), numerous long
displacements into the distant foraging area were observed, indicating that prey was scarce
near the colony. Local trips presumably ensure that chicks are fed more frequently, but the
yield of these trips appears to be insufficient (except in E1) for adults to maintain their own
body reserves in addition to providing regular meals for the chicks. This forces individuals to
forage in more distant waters for self-feeding. Prey patch quality was higher in distant
waters, as indicated by increased dive rate and bout size. Prey encounter rates, although
similar between both foraging areas, were more variable, indicating less predictable prey
encounters. Stable isotope analysis of whole blood indicated that birds were feeding on
higher trophic level prey items in E1, but not in L1 and E2. This study revealed that, when
food was scarce, birds experienced a trade-off between local foraging trips, where prey
encounters were predictable but of lower quality, in order to deliver more energy to the
chick, and presumably self-feeding distant trips, where prey patch quality was higher, but
prey encounters less predictable.
(III) Area-restricted search (ARS) behaviour is used to better understand animal foraging
behaviour. However, track-based descriptive analyses used to detect ARS behaviour may be
biased by factors such as foraging trip duration as well as non-foraging behaviours (i.e.
resting on the water). This potential bias was investigated in Masked Boobies using FPT
analysis in combination with detailed foraging behaviour provided by depth-acceleration
logger. In 19 % of 57 detected ARS zones, birds spent more than 70 % of total ARS duration
resting on the water, suggesting that these ARS zones have been falsely detected (i.e. that
the birds were not primarily foraging during that time). False ARS detection occurred mostly
during short trips in close proximity to the colony, with very low or no diving activity. This
clearly demonstrates the need to account for resting on the water positions when
determining ARS behaviour based on foraging locations in marine animals. However, the adoption of ARS behaviour at greater distances from the colony was linked to enhanced
foraging activity, suggesting that ARS behaviour may be used to identify important feeding
areas in this species.
These results reveal that the foraging ecology of Masked Boobies is influenced by adult body
mass, as well as by changes in the distribution and availability of prey. This indicates that
both intrinsic and extrinsic factors should be considered in conjunction when analysing the
foraging ecology of central-place foragers. In addition, inference from track-based analyses
on animals foraging strategies should be drawn carefully, as specific non-foraging
behaviours may lead to an inaccurate detection and interpretation of ARS behaviour.
| Item Type: | Thesis - PhD |
|---|---|
| Authors/Creators: | Sommerfeld, J |
| Keywords: | foraging ecology, marine predators, bio-logging, Sula dactylatra |
| Copyright Holders: | The Author |
| Copyright Information: | Copyright 2013 the Author |
| Additional Information: | Chapter 2 appears to be the equivalent of the peer reviewed version of the following article: Sommerfeld, J., Kato, A., Ropert-Coudert, Y., Garthe, S. and Hindell, M. A. 2013. The individual counts: within sex differences in foraging strategies are as important as sexspecific differences in masked boobies Sula dactylatra, Journal of Avian Biology, 44(6), 531-540 which has been published in final form at doi:10.1111/j.1600-048X.2013.00135.x This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving. Chapter 4 appears to be the equivalent of the peer reviewed version of the following article: Sommerfeld, J., Kato, A., Ropert-Coudert, Y., Garthe, S. And Hindell, M. A. 2013. Foraging |
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