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Prey selection in Blue Tits Parus caeruleus as a response to food levels

100%

Grieco F.

Acta Ornithologica

1999

tom 34

nr 2

199-203

Some models of central place foraging relate the time taken to deliver a prey to the selectivity of the forager. If birds have to travel longer, they are expected to take only larger prey. Prey selectivity may also increase if birds have more time available for search activities. I tested this last hypothesis by experimentally increasing the food available to Blue Tits during young rearing. Insect larvae were offered to breeding adults from egg hatching to fledging of the young. Provisioning rates and prey size were studied by videotaping. In experimentally overfed broods, part of the additional food was delivered to their nestlings by the adults, while the rate of feeding with natural prey decreased compared to unmanipulated pairs. Overfed males delivered larger insect larvae than control males, while females showed no response to the treatment. Overfed males could deliver larger prey at late chick ages (when presumably the chicks' demand is higher) than controls, suggesting that control males were time- (tor energy) constrained. Moreover, overfed parents appeared to choose relatively more prey that may be important in the chicks' diet. The results show that prey selectivity is related to time (or energy) budgets.

Mate choice in the Great Cormorant Phalacrocorax carbo: a zoo study

63%

Grieco F., Kortlandt A.

Acta Ornithologica

2000

tom 35

nr 1

73-78

A study (1982-1997) in the small cormorant colony of the Amsterdam Zoo addressed the question of what factors determine between-season divorce. The proportion of pairs splitting up the next year was 0.46, i.e. less than in wild populations. Low divorce rate could not be explained by the small population size. Computer simulations showed that if pair-formation in the zoo occurred at random, divorce rate would be higher. Occurrence of divorce was unrelated to previous breeding success or age of pair mates. The only variable that could predict divorce was the frequency of interactions of pair mates after breeding. Pair mates that were not seen together after breeding were more likely to split up the next year. Thus mate retention probably involved some process of choice by the partners. This led to the suggestion that mate retention in the wild is constrained, since contact between mates is lost during migration. We also found that extra-pair copulations (EPCs) played a role in the assessment and choice of the mate. Female Cormorants pursued EPCs mostly when they were presumably not fertile. Involvement in EPCs increased the probability that the female subsequently paired with the male concerned. It is proposed that EPCs may have the additional function of assessing the quality of potential mates for future pair-formation.

Ograniczanie wyników

2 Acta Ornithologica

2 Grieco F.

1 Kortlandt A.

1 2000

1 1999

Wyniki wyszukiwania

Prey selection in Blue Tits Parus caeruleus as a response to food levels

Mate choice in the Great Cormorant Phalacrocorax carbo: a zoo study