Function of egg punctures by Shiny Cowbirds in parasitized and nonparasitized Creamy-bellied Thrush nests

Abstract

Avian brood parasites usually remove or puncture host eggs. Several hypotheses have been proposed to explain the function of these behaviors. Removing or puncturing host eggs may enhance the efficiency of incubation of cowbird eggs (incubation‐efficiency hypothesis) or reduce competition for food between cowbird and host chicks in parasitized nests (competition‐reduction hypothesis) and, in nonparasitized nests, may force hosts to renest and provide cowbirds with new opportunities for parasitism when nests are too advanced to be parasitized (nest‐predation hypothesis). Puncturing eggs may also allow cowbirds to assess the development of host eggs and use this information to decide whether to parasitize a nest (test‐incubation hypothesis). From 1999 to 2002, we tested these hypotheses using a population of Creamy‐bellied Thrushes (Turdus amaurochalinus) in Argentina that was heavily parasitized by Shiny Cowbirds (Molothrus bonariensis). We found that 56 of 94 Creamy‐bellied Thrush nests (60%) found during nest building or egg laying were parasitized by Shiny Cowbirds, and the mean number of cowbird eggs per parasitized nest was 1.6 ± 0.1 (N= 54 nests). At least one thrush egg was punctured in 71% (40/56) of parasitized nests, and 42% (16/38) of nonparasitized nests. We found that cowbird hatching success did not differ among nests where zero, one, or two thrush eggs were punctured and that the proportion of egg punctures associated with parasitism decreased as incubation progressed. Thus, our results do not support the incubation‐efficiency, nest‐predation, or test‐incubation hypotheses. However, the survival of cowbird chicks in our study was negatively associated with the number of thrush chicks. Thus, our results support the competition‐reduction hypothesis, with Shiny Cowbirds reducing competition between their young and host chicks by puncturing host eggs in parasitized nests.

Invasive birds: global trends and impacts

“…Females synchronize parasitism with host laying, but the degree of synchronization varies among hosts. In open nesters it is relatively high (70-80%; Massoni and Reboreda, 1998;Mermoz and Reboreda, 1999;Fiorini and Reboreda, 2006;Astié and Reboreda, 2009), while in cavity nesters it is lower (47-57%; Kattan, 1998;Tuero et al, 2007). Shiny Cowbird eggs have an eggshell that is thicker than expected by egg volume (Spaw and Rohwer, 1987;Mermoz and Ornelas, 2004).…”

“…Egg punctures reduce the clutch of the host and therefore the number of host chicks that hatch, thus reducing the competition for food and increasing the survival of the parasite nestlings. This is particularly advantageous when hosts are larger than the parasite (Mermoz and Reboreda, 2003;Astié and Reboreda, 2009;Fiorini et al, 2009;Gloag et al, 2012). Once a female has parasitized a nest, she does not visit it again (Gloag et al, 2014;Scardamaglia et al, 2017), thus avoiding the costs of competition between her own chicks and damaging her own previously laid eggs (Gloag et al, 2014).…”

“…Brood parasitism by Shiny Cowbirds reduces the breeding success of their hosts in several ways. First, parasite females peck and puncture host eggs during parasitic and non-parasitic visits, reducing clutch size and increasing the probability of nest abandonment (Post and Wiley, 1977b;Cruz and Andrews, 1997;Kattan, 1998;Reboreda, 1998, 2002;Mermoz and Reboreda, 1999;Sackmann and Reboreda, 2003;Cruz et al, 2005;Astié and Reboreda, 2009;Dominguez et al, 2015). Second, parasite eggs may interfere with host incubation lowering the hatching success of the remaining host eggs (Wiley, 1985;Woodworth, 1997;Reboreda, 2006, Tuero et al, 2007).…”

Reproductive Success of the Creamy-Bellied Thrush in a Southern Temperate Zone

“…Average adult mass is 55 g (Astié and Reboreda 2005). This species is heavily parasitized by the Shiny Cowbird (Molothrus bonariensis) in the study area (Astié and Reboreda 2005, 2006, 2009a occupies areas irrigated for agriculture. The study area was a 1,000-ha cultivated field with vineyards, olives (Olea europaea), and poplar (Populus nigra) groves.…”

Efecto de las características físicas del hábitat sobre el éxito reproductivo de cuatro especies de aves que utilizan los talares bonaerenses como sitio de nidificación

“…Presentan un comportamiento característico de picar y perforar huevos durante las visitas a los nidos (Llambías et al 2006). Esto implica un costo en el éxito reproductivo de los hospedadores tanto en nidos parasitados como no parasitados (Reboreda et al 2003, Astié y Reboreda 2006) y podría servirle al parásito para evaluar el grado de incubación de los huevos del hospedador (Massoni y Reboreda 1999), y/o reducir la competencia para sus pichones (Astié y Reboreda 2009). Los pichones de Tordo Renegrido nacen luego de un periodo de incubación de 12 días y permanecen en el nido de 12 a 14 días (Fraga 1985).…”