72.6 Friday, Jan. 6 Real-Time Modulation of Egg Size in an Altricial Bird SOCKMAN, K.W.; Univ. of North Carolina, Chapel Hill email@example.com
The size of a female's eggs can affect the size of her offspring, her survival, and her lifetime reproductive success. Female birds modulate egg size according to several factors, such as diet and reproductive experience, but the capacity for modulating egg size in real time — as the eggs are being formed on the ovary and in the oviduct — would enhance the ability of females to respond rapidly to recent environmental stimuli. Using a wild, free-living population of Lincoln's sparrows (Melospiza lincolnii), I tested the hypothesis that females use cues from the first-laid egg in modulating the size of the subsequently laid eggs in their clutches. Specifically, I exchanged the first-laid eggs between synchronous nests on the day those eggs were laid, measured the length and width of subsequently laid eggs, and regressed their calculated volumes against the volume of the substitute egg, while controlling for the volume of the first-laid egg. If the first laid egg was smaller than its substitute, then, regardless of laying order, the volume of subsequently laid eggs increased with the size of the substitute, thus minimizing within-clutch variation in egg size. However, if the first-laid egg was larger than its substitute, I found no such relationship. That is, females increased egg size in response to a large substitute egg but did not decrease egg size in response to a small substitute egg. These results show that females can use cues from the first-laid egg to modulate in real time the size of subsequent eggs, even egg two, which is already in the oviduct at the time the first is laid. In an effort to understand the adaptive significance of this capacity, I also discovered that, in unmanipulated nests, hatching delay increased as the egg deviated in volume from the clutch mean. Therefore, minimizing within-clutch variation in egg size through real-time modulation may benefit the female by reducing hatching delays.