Clutch size limitation in waders: experimental test in redshank Tringa totanus.

Several hypotheses have been raised to explain the upper limit of clutch size at four eggs in waders (suborder Charadrii), which may play an important role in the evolution of the variety of mating and parental care systems in this group. Experimental tests of the hypotheses have produced conflicting results. It was recently suggested that the combined effects of several incubation costs of a larger clutch suffice to limit its size to four eggs in this group. Here we test the incubation-limitation hypothesis in a field experiment, in redshank Tringa totanus. We created five-egg clutches by adding one egg from another nest to a just completed four-egg clutch. Four-egg control clutches were created by replacing one of the eggs by an egg from another nest. All egg removals, additions and replacements were done before incubation started. Incubation time in five-egg clutches increased by 1 day to 24.3±0.23 days, compared to 23.3±0.32 days in four-egg clutches. Egg hatchability and nest predation rates did not differ significantly between treatments. On average five-egg clutches produced one extra chick at hatching (4.5±0.26 chicks) compared to four-egg clutches (3.5±0.27 chicks). Also when several additional costs from incubating enlarged clutches are added, redshanks by laying a fifth egg would on average increase their reproductive success at hatching by an estimated 22%. The incubation-limitation hypothesis therefore is clearly rejected in this species. Possible mechanisms behind the four-egg clutch limit in waders and ways of testing the alternatives are discussed.