Data from: Thick eggshells of brood parasitic cowbirds protect their eggs and damage host eggs during laying

Brood parasites lay thick-shelled eggs and numerous hypotheses have been proposed to explain the significance of this trait. We examined whether thick eggshells protect the parasite egg during laying events. We used eggs of the parasitic shiny cowbird (Molothrus bonariensis) and its hosts, the house wren (Troglodytes aedon) and chalk-browed mockingbird (Mimus saturninus) in South America and the eggs of the parasitic brown-headed cowbird (M. ater) and its hosts the house wren and red-winged blackbird (Agelaius phoeniceus) in North America. We experimentally dropped parasite eggs onto host eggs to simulate laying by the parasite, parasite eggs onto parasite eggs to simulate multiple parasitism, host eggs onto parasite eggs to simulate hosts laying from the height cowbirds lay, and stirred eggs to simulate jostling that may occur when cowbirds and hosts interact during laying events. We found that cowbird eggs were significantly less likely to be damaged than host eggs when they were laid onto a host egg and when host and cowbird eggs were laid onto them. There was minimal damage to eggs during jostling experiments, thereby failing to support the hypothesis that thick eggshells provide protection when eggs are jostled. These findings support the hypotheses that thick eggshells resist damage when laid from an elevated position, when additional cowbird eggs are laid onto them in multiply parasitized nests, and these eggs also damage host eggs when laid.

育雏寄生鸟类（brood parasites）会产下厚壳卵，学界已提出诸多假说以阐释这一性状的演化意义。本研究旨在探究厚卵壳是否能在产卵过程中保护寄生卵。本研究分别以南美的寄生鸟类辉牛鹂（Molothrus bonariensis）及其宿主家鹪鹩（Troglodytes aedon）、白眉嘲鸫（Mimus saturninus），以及北美的寄生鸟类褐头牛鹂（M. ater）及其宿主家鹪鹩、红翅黑鹂（Agelaius phoeniceus）的卵为实验材料。实验设置包括：将寄生卵滴落在宿主卵上以模拟寄生鸟的产卵行为，将寄生卵叠置于其他寄生卵之上以模拟多重寄生现象，将宿主卵滴落在寄生卵上以模拟宿主从牛鹂产卵的高度进行产卵，以及搅拌卵群以模拟产卵过程中牛鹂与宿主互动时可能产生的挤碰情况。研究结果显示，当寄生卵被放置在宿主卵上，以及宿主卵与寄生卵被放置在寄生卵上时，牛鹂卵的破损概率显著低于宿主卵。挤碰实验中卵的破损程度极低，因此无法支持“厚卵壳可在卵群挤碰时提供保护”这一假说。本研究结果支持以下三项假说：厚卵壳可抵御从高处产下时所受的损伤；在多重寄生的巢中叠放额外牛鹂卵时，厚卵壳也能避免卵体破损；且牛鹂卵在被产下时亦可对宿主卵造成损伤。