Why does parental effort of the common tern (Sterna hirundo) decrease after a late-season flooding event? A multi-hypothesis approach
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The production of two clutches within the same breeding season is uncommon amongst many shorebirds species, and even more rare is the act of double-brooding. In late June of 2006, heavy rains flooded over 120 common tern (Sterna hirundo) nests, causing a re-nesting effort of over 140 pairs, and perhaps one case of double-brooding. To evaluate the level of parental effort after a late-season flooding event, I weighed and then compared the eggs of the second clutches to corresponding egg data from the first clutches of 1983 and 1984 from the same colony on Oneida Lake, NY. The mean egg weights from the second clutch of 2006 all weighed significantly less than the corresponding mean egg weights of the first clutches of 1983 and 1984. In both first and second clutches, there was a linear decrease in egg weight with laying order. Three hypotheses have been postulated to explain the intraclutch variation of egg weights, the ?brood-survival?, ?brood-reduction?, and ?insurance? hypotheses attempt. Given the circumstances of 2006, the brood-reduction hypothesis fits best at explaining the intraclutch variation in egg weights. The observed decline in parental effort late in the season can be explained by the ?timing? hypothesis, which in the case of this study included a decreased food supply and reduced breeding synchrony later in the season. Certain aspects of the life history of common terns such as molt, which overlapped with the production of the second clutch, as well as the allocation of energy between parents in a bi-parental care system, also decreased the success of many second clutches. Many adults associated with second clutch nests abandoned their eggs and/or chicks in order to enhance the probability of their own survival.
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