Viviana Massoni

Egg puncture allows shiny cowbirds to assess host egg development and suitability for parasitism

Parasitic cowbirds and cuckoos generally reduce the clutch size of the hosts they parasitize by removing or destroying some of their eggs. Shiny cowbirds (Molothrus bonariensis) puncture their hosts eggs both when parasitizing the nests and also when they do not parasitize them. We propose that, by puncturing the hosts eggs, shiny cowbirds gain an informational benefit. They assess the degree of development of the hosts embryos and so avoid laying in nests that would not provide enough incubation time for the parasitic eggs to hatch. Two predictions follow: (i) punctures should occur in advance or immediately before parasitic events, and (ii) the occurrence of parasitism should depend on the degree of development of the hosts embryos when punctures occurred, i.e. on the stage of incubation. Both predictions are supported by our data of shiny cowbirds parasitizing yellow–winged blackbirds (Agelaius thilius). Egg punctures are not used to reset the hosts nesting attempt when shiny cowbirds do not parasitize the nests. We discuss the potential mechanisms implicated in egg development assessment and propose a critical experiment to test this hypothesis.

Clock gene variation in Tachycineta swallows.

Many animals use photoperiod cues to synchronize reproduction with environmental conditions and thereby improve their reproductive success. The circadian clock, which creates endogenous behavioral and physiological rhythms typically entrained to photoperiod, is well characterized at the molecular level. Recent work provided evidence for an association between Clock poly-Q length polymorphism and latitude and, within a population, an association with the date of laying and the length of the incubation period. Despite relatively high overall breeding synchrony, the timing of clutch initiation has a large impact on the fitness of swallows in the genus Tachycineta. We compared length polymorphism in the Clock poly-Q region among five populations from five different Tachycineta species that breed across a hemisphere-wide latitudinal gradient (Fig. 1). Clock poly-Q variation was not associated with latitude; however, there was an association between Clock poly-Q allele diversity and the degree of clutch size decline within breeding seasons. We did not find evidence for an association between Clock poly-Q variation and date of clutch initiation in for any of the five Tachycineta species, nor did we found a relationship between incubation duration and Clock genotype. Thus, there is no general association between latitude, breeding phenology, and Clock polymorphism in this clade of closely related birds. Figure 1 Photos of Tachycineta swallows that were used in this study: A) T. bicolor from Ithaca, New York, B) T. leucorrhoa from Chascomús, Argentina, C) T. albilinea from Hill Bank, Belize, D) T. meyeni from Puerto Varas, Chile, and E) T. thalassina from Mono Lake, California, Photographers: B: Valentina Ferretti; A, C-E: David Winkler.

A NEGLECTED COST OF BROOD PARASITISM: EGG PUNCTURES BY SHINY COWBIRDS DURING INSPECTION OF POTENTIAL HOST NESTS

Abstract Parasitized hosts of the Shiny Cowbird (Molothrus bonariensis) suffer several costs, and among the most important is the loss of eggs through egg punctures inflicted by the parasite. Unparasitized nests also have eggs damaged by cowbirds, but researchers usually ignore these losses. To quantify this cost we compared three groups of nests of the Yellow-winged Blackbird (Agelaius thilius): parasitized and unparasitized nests from an area used by Shiny Cowbirds, and unparasitized nests from an area not used by the parasite. Because cowbirds puncture eggs as soon as the first host eggs are laid, we calculated the clutch size only for those nests found during construction. Unparasitized nests in the area used by cowbirds had lower egg survival rate and hatching success and higher probability of nest desertion than unparasitized nests in the cowbird-free area. Our results indicate that one must consider egg punctures at unparasitized nests to avoid underestimating the impact of parasitism. Un Costo Ignorado del Parasitismo de Cría: Perforación de Huevos por Molothrus bonariensis durante la Inspección de Potenciales Nidos de Hospedadores Resumen. Los hospedadores de Molothrus bonariensis sufren varios costos, entre los que se destaca la pérdida de huevos debida a perforaciónes hechas por el parásito. Los nidos no parasitados también tienen huevos perforados por M. bonariensis, pero estas pérdidas son normalmente ignoradas. Para cuantificar este costo comparamos tres grupos de nidos de Agelaius thilius: nidos parasitados y no parasitados de un área usada por M. bonariensis, y nidos no parasitados de un área no utilizada por el parásito. Como los parásitos perforan los huevos del hospedador tan pronto como éstos son puestos, el tamaño de puesta se calculó utilizando únicamente los nidos encontrados durante su construcción. Los nidos no parasitados del área visitada por los parásitos tuvieron menor supervivencia de sus huevos durante la incubación, menor éxito de eclosión y mayor probabilidad de ser abandonados que los nidos no parasitados del área libre de parásitos. Los resultados obtenidos indican que deben considerarse las perforaciones de huevos en nidos no parasitados para evitar subestimar el impacto del parasitismo.

NUMBER OF CLOSE SPATIAL AND TEMPORAL NEIGHBORS DECREASES THE PROBABILITY OF NEST FAILURE AND SHINY COWBIRD PARASITISM IN COLONIAL YELLOW-WINGED BLACKBIRDS

Abstract We investigated whether the synchrony and proximity of nests of Yellow-winged Blackbirds (Agelaius thilius) provided protection against nest predation or brood parasitism by Shiny Cowbirds (Molothrus bonariensis). We analyzed the effect of the temporal aggregation of nests on the daily probability per nest of predation, desertion, egg punctures, and parasitism throughout the breeding season. The probabilities of nest predation and nest desertion increased through the breeding season. The temporal aggregation of nests was negatively associated with the probability of nest desertion, egg punctures, and parasitism, but there was no association with the probability of nest predation. We also analyzed the effect of the number of close neighbor nests on the daily probability per nest of predation, desertion, egg punctures, and brood parasitism. The spatial aggregation of nests was negatively associated with the probability of nest failure and brood parasitism, but there was no association with the probability of egg punctures. We discuss whether dilution effect or group defense, two mechanisms proposed to explain the antipredatory advantages of colonial nesting, are likely to apply to our system. El Número de Vecinos Espaciales y Temporales Disminuye la Probabilidad de Pérdida del Nido y de Parasitismo por Molothrus bonariensis en Colonias de Agelaius thilius Resumen. Se investigó si la sincronía y la proximidad de nidos del Agelaius thilius proveyó protección ante la depredación del nido y el parasitismo de cría por Molothrus bonariensis. Se analizó el efecto de la agregación temporal de nidos sobre la probabilidad diaria por nido de depredación, abandono, picaduras y parasitismo a lo largo de la temporada reproductiva. Las probabilidades de predación y abandono del nido aumentaron a medida que avanzó la temporada reproductiva. La agregación temporal de nidos estuvo negativamente asociada con la probabilidad de abandono, picaduras y parasitismo pero no se observó asociación de ésta con la probabilidad de predación. También se analizó el efecto del número de nidos vecinos cercanos sobre la probabilidad de pérdida del nido y de parasitismo de cría. La agregación espacial de nidos estuvo asociada negativamente con la probabilidad de pérdida del nido y de parasitismo de cría pero no se observó una asociación de ésta con la probabilidad de picadura de huevos. Se discute si el efecto de dilución o la defensa grupal, dos mecanismos propuestos para explicar los beneficios antipredatorios de la nidificación colonial, son probables de ocurrir en este sistema.

Differences in rates of nest-visitation and removal of faecal sacs by male and female White-rumped Swallows

Abstract Despite being a common and widespread species, the White-rumped Swallow (Tachycineta leucorrhoa) is one of the least-studied members of its genus. We examined the rates of nest-visitation and nest-sanitation of male and female White-rumped Swallows during the nestling period, in 23 nests, and compared them with those of the extensively studied Tree Swallow (Tachycineta bicolor). White-rumped Swallow pairs increased the rate of nest-visitation as nestlings grew older, as was found in Tree Swallows. Females made significantly more visits to the nest than males, and the asymmetry was maintained irrespectively of the age of nestlings. These results are similar to those reported from Tree Swallows in the eastern United States, but differ from the equality of roles found in Ontario, Canada. Female and male White-rumped Swallows removed faecal sacs at the same rate when nestlings were young (Day 4) but by Day 12 females had quadrupled their effort whereas the rate of sanitation by males remained constant. Overall, female White-rumped Swallows made a significantly larger parental investment than males (as measured by number of visits to nests and, thus, presumably rates of feeding, and in contribution to nest-sanitation) and, in this respect, the asymmetry in parental investment is greater than that reported for Tree Swallows.

Reproductive Biology of Female Saffron Finches does not Differ by the Plumage of the Mate

Abstract. Young, immature-plumaged males of species in which plumage maturation is delayed are often unable to obtain mates or pair later in the season to lower-quality females. Their clutches are smaller and the quality of their eggs is lower, leading to fewer nestlings, whose quality and fledging success are reduced. The Saffron Finch (Sicalis flaveola) is an understudied and declining secondary-cavity nester with delayed plumage maturation; yearling males look like females. The aims of this study, of birds nesting in boxes, were to compare the morphology of immature- and mature-plumaged males of the Saffron Finch and their respective females and to compare the breeding success of females mated to each category of male. We expected that females mated to immature-plumaged males would follow the pattern described for other species, having lower reproductive investment and success. Immature- and mature-plumaged males were similar in morphology, but females paired to mature-plumaged males were bigger in bill height and weight. We found no differences, however, between females paired to each category of male in any biologically meaningful variable. Reduced competition among males for nest cavities and enough females willing to pair with immature-plumaged males might be partially responsible for these results.

YOLK MASS VARIATION IN WHITE-RUMPED SWALLOWS (TACHYCINETA LEUCORRHOA)

Abstract Egg mass production is costly, but hatching from heavier eggs could be beneficial for the nestlings phenotype and fitness. Egg mass could be influenced by proximate causes, like food abundance, female condition, environmental conditions, and/or by ultimate causes, such as females depositing resources differentially within a clutch to increase the biological fitness of some eggs. Yolk mass, although poorly studied, is the source of nutrients for the embryo, so its mass should be more influential for the nestlings than total egg mass. We used a technique that allowed us to measure yolk size without destroying the eggs. We studied yolk mass in 212 eggs of White-rumped Swallows (Tachycineta leucorrhoa) and found that yolk mass was influenced by laying order, with last laid eggs having heavier yolks than first laid eggs, and the pattern was consistent with egg mass variation. Food abundance also affected yolk mass: when insect availability was high the yolks were heavier. We conclude that embryos in the last laid eggs have more resources from which to develop, and excluding food abundance, neither environmental conditions nor females condition affected yolk mass. We encourage other researchers to study yolk mass given that multiple variables affected total egg mass.

Sexual differences in the effect of previous breeding performance on nest-box reuse and mate retention in White-rumped Swallows (Tachycineta leucorrhoa)

ABSTRACT In birds, the selection of nesting sites and mates has a profound influence on the daily interactions of individuals. Both behaviours are energetically costly and time-consuming and can strongly affect the Darwinian fitness of individuals. Using a 12-year dataset on banded returning breeders, we explored variables associated with nest-box choice and mate choice in the White-rumped Swallow, a socially monogamous migratory passerine. Females reusing nest-boxes, but not males, produced significantly more fledglings than did those that changed nest-boxes. Females and males that retained mates produced significantly more fledglings than did those that divorced. General linear mixed models revealed that the number of fledglings produced in previous seasons significantly explained the reuse of particular nest-boxes by females, but not by males. Mate choice by females was not strongly affected by either past breeding performance or nest-box reuse, but nest-box reuse alone predicted mate choice by males. Nest-box reuse by males was apparently more related to retention of female partners. This study revealed that sexual differences in the nesting and mating decisions were guided by the use of information on past breeding performance in a subtropical, secondary cavity nester.