Clutch size - Clutch size(number of eggs laid at one time Why so much interest in it easy to measure presumably under genetic control gives a good

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Clutch size (number of eggs laid at one time) Why so much interest in it? easy to measure & presumably under genetic control. gives a good estimate of reproduction (fitness) huge variation can be easily manipulated – lots of experiments done
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A quick look at factors that may influence clutch size Number of parents that help to raise young. Amount of food available for young. Type of young (precocial vs. altricial) Predation (higher predation = smaller clutch sizes) Those are all factors that explain variation among species. There can also be enormous variation in clutch size within species.
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Proximate vs. Ultimate causation Proximate : explains variation within a population; immediate cause Ultimate : explains variation among populations or species; evolutionary Frequency Clutch Size Ultimate Proximate
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Proximate factors 1. Food supply - Birds adjust clutch size to match food available. - Food for young is what’s important - Parents can predict how much food will be available. Example: Great tits - When young hatch and caterpillars peak in abundance are always matched - Warmer spring weather thought to trigger earlier breeding and larger clutches
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Proximate factors 1. Population Density Clutch size Number of breeding pairs Why ? More pairs mean smaller territories and less food per pair More pairs mean higher predation
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Proximate factors 3. Age of parent Young females lay smaller clutches than older females Example: Kittiwakes 2 nd year females: average clutch size = 1.9 3 rd year females: average clutch size = 2.4 Why? Younger birds are less efficient foragers Less food available for young Younger birds have poorer quality territories Less food
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4. Habitat The same species nesting in different habitats will have different clutch sizes Likely due to differences in food availability. Alternatively, poorer quality birds get poorer
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This document was uploaded on 08/24/2011.

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Clutch size - Clutch size(number of eggs laid at one time Why so much interest in it easy to measure presumably under genetic control gives a good

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