Age-related improvements in reproductive performance in seabirds have been well documented, and may be explained by improvements in foraging efficiency or increased experience and reproductive effort with age. The interactive effects of parental age and food supply on reproductive performance, however, remain poorly understood. A widespread mass mortality of pilchards Sardinops sagax in southern Australian waters in 1998 provided a unique opportunity to investigate the effects of a sudden reduction in the availability of a major prey species on Australasian gannets Morus serrator, an important local marine predator. Age-related differences in the breeding performance of gannets were evident in 1 year of reduced pilchard availability; when food was not limited, both young and experienced parents were equally capable of rearing chicks and had similar levels of breeding success. These data clearly demonstrate the interactive effects of parental age and food supply on breeding performance and suggest that such differences only become apparent when conditions become more stressful.
Recreational use of beaches may threaten some beach-nesting shorebirds in southern Australia. Temporary Beach
Closures, comprized of a 50 x 25 m exclusion zone around a shorebird nest, represent a promising technique for
altering human behaviour by reducing both disturbance to birds and inadvertent crushing of eggs by beach visitors.
We assessed whether three commonly employed configurations of Temporary Beach Closures (sign, fence, and warden)
were effective at: (1) achieving compliance among beach visitors, and (2) reducing egg-crushing rates. Overall, 93.7%
of beach visitors complied with all Temporary Beach Closures, resulting in a reduction in egg-crushing rates within, as
opposed to adjacent to, Temporary Beach Closures. Levels of compliance were high in all Temporary Beach Closure
configurations (88.0–99.4%), and similar levels of compliance were achieved within the three configurations. Human
compliance was highest for females and when the density of beach-users was higher, while individuals aged <20 and
>61 years were less likely to comply with Temporary Beach Closures. Despite an increased probability of compliance
on high density beaches, this did not translate into a reduction in egg crushing rates on such beaches, because the
overall number of noncompliant individuals remained higher. We conclude any Temporary Beach Closure configuration
is meritorious, and that their use on high and low-use recreational beaches will benefit breeding shorebirds by reducing
the rate of egg-crushing. Targeting demographics that display lower levels of compliance, such as men, young people
(i.e. <21), and older people (i.e. >60), may further improve the effectiveness of Temporary Beach Closures in enhancing
the conservation of shorebirds.
Seabirds that forage by plunge diving dive less frequently than those that dive from the water surface, and spend less time in flight than more generalist foragers. We hypothesised that this is due to foraging by plunge diving entailing a high energetic cost, which in turn is due to high energetic costs of take-off and flight. Using heart rate as a proxy for metabolic rate, we evaluated the energetic costs of foraging by plunge diving in the Australasian gannet Morus serrator. As expected, flight entailed a high energetic cost, and energy expenditure during foraging was equivalent to that during flight and significantly higher than that when animals were resting during foraging trips or were inactive on land. These values represent the highest costs of foraging yet recorded in a seabird, and the low frequency of plunge diving can be attributed to these high costs. On average, Australasian gannets perform 2.6 dives h -1 when foraging, with a mean dive duration of 3.4 s. As a result, they spend < 0.25% of the duration of each foraging trip submerged. We combined this information with previously obtained data on diet to calculate an estimated rate of prey capture of ~287 g (min submerged) -1 . This rate is at least 7 times greater than rates recorded in other diving birds. For plunge divers, therefore, the high costs of foraging are offset by high rates of energy gain.
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