Higher divorce risk when mates are plentiful? Evidence from Denmark

Work from social and biological sciences has shown that adult sex ratios are associated with relationship behaviours. When partners are abundant, opportunities for mate switching may increase and relationship stability decrease. To date, most of the human literature has used regional areas at various levels of aggregation to define partner markets. But, in developed countries, many individuals of reproductive age spend a considerable amount of time outside their residential areas, and other measures may better capture the opportunities to meet a (new) partner. Here, we use Danish register data to test whether the sex ratio of the occupational sector is linked to divorce. Our data cover individuals in Denmark who married during 1981–2002 and we control for age at and duration of marriage, education and parity. Results support the prediction that a higher proportion of opposite-sex individuals in one's occupational sector is associated with higher divorce risk. This holds for both men and women, but associations are somewhat stronger for men and vary by education. Our results highlight the need to study demographic behaviours of men and women simultaneously, and to consider partner markets beyond geographical areas so that differing strategies for males and females may be examined.

Nests Without Eggs: Abandonment or Cryptic Predation?

We determined whether nests that did not receive eggs was attributable to cryptic nest predation (i.e. predation of eggs laid between nest checks) or nest abandonment in Common Grackles (Quiscalus quiscula). Nest predation was extremely low (∼2%), whereas more than 44% of 427 nests found during nest building never received an egg; this indicates that nest abandonment accounted for most nests without eggs. Nest construction was completed for 32% of nests that were abandoned. Few nests known to have received eggs were abandoned. As the breeding season progressed, both nest abandonment and time from nest completion to first egg decreased. It has been proposed that the delay in egg laying early in the season allows females to optimize timing of egg laying. Nest abandonment may also serve this purpose, but seems an unnecessarily expensive mechanism. Alternatively, nest abandonment could be involved with mate switching. Understanding why nests are abandoned requires data on the associated ecological circumstances, in addition to accurate identification of instances of abandonment. The latter requires distinguishing abandonment from cryptic predation. Rates of nest abandonment can be estimated for populations by using rates of known nest predation during egg laying. For individual nests, however, distinguishing abandonment from cryptic predation requires detailed observation (e.g. video cameras), except in circumstances such as ours, where predation is extremely low.Nidos sin Huevos:?Abandono o Depredación Críptica?

Plumage color as a dynamic trait: carotenoid pigmentation of male house finches (Carpodacus mexicanus) fades during the breeding season

Evolutionary biologists studying sexually selected bird plumage generally consider this trait to be static throughout a breeding season and assign trait values to individuals on the basis of single measurements. We investigated the propensity for carotenoid-based color of feather patches in male house finches (Carpodacus mexicanus ( Muller, 1776)) to change during the breeding period. We recaptured and rescored 63 males and found that the hue of feathers faded significantly over the season. The degree of hue change was a direct function of the amount of time between plumage scores; feathers faded more as the interval between measurements increased. The magnitude of hue change was not, however, related to an individual's age or initial plumage redness, which suggests that certain birds are not more or less prone to fading. Collectively, these findings imply that researchers should more carefully track plumage color expression during the course of a year, as seasonal color shifts may have important consequences for late-season male–male competitive interactions and flexible female mating tactics (e.g., social mate switching, choice of extra-pair partners). Potential mechanisms for this seasonal plumage color shift are discussed.

Mate Guarding Conflict, Extra-Pair Courtship And Signalling in the Harlequin Duck, Histrionicus Histrionicus

Socially monogamous birds pursuing extra-pair reproductive strategies may be in conflict, both sexes seeking new mates or copulations outside the pair, while simultaneously attempting to prevent infidelity by the partner. Intra-pair conflicts are augmented by inter-pair conflicts when pairs meet, when all four individuals may be sexually attracted to one member of the other pair while seeking to prevent their mate from copulating, or deserting, with the other. We studied the dynamics, signalling and resolution of these conflicts in a field experiment with the harlequin duck, recording responses to single model birds of both sexes, and to model pairs. Both sexes mate guarded by placing themselves between the mate and a model competitor, and by signalling with the head nod display. Females were closer to model pairs than their mates. Female mate guarding responded adaptively to infidelity risk, increasing to models of paired females, single females and single females inviting copulation, respectively. Males head nodded to signal the goal of (extra-pair) copulation, pairing or mate switching. Courtship by unpaired males increased to models of paired females, single females and single females inviting copulation, respectively. Paired males courted these models equally, perhaps inhibited by the parallel increase in mate guarding of their mates. Males signalled mate guarding using the head nod more than females, but this predicted difference in mate guarding was not shown for the measure of relative proximity to the models. In spite of male extra-pair courtship the harlequin is strictly monogamous, due to female fidelity and close mate guarding. The use of models revealed mate guarding by females, and failed extra-pair reproductive attempts by males, neither of which were apparent from observational studies alone.

Effects of Paternity and Mate Availability on Mate Switching in House Wrens

We studied within-season mate switching in House Wrens (Troglodytes aedon) from 1998 to 2000. Males initiated mate switching, but mate switching was not related to the male's paternity in the first brood, his body condition, or his breeding experience. Males were more likely to switch mates when unmated females were nearby. Males that switched mates sired a similar number of young as males that stayed with the same mate, and, thus, males did not appear to benefit directly from switching mates. In contrast, half of the females that were deserted did not find a second mate during the breeding season, and, thus, incurred a reproductive cost. Efectos de la Paternidad y Disponibilidad de Pareja sobre el Cambio de Parejas en Troglodytes aedon Resumen. Estudiamos el cambio de pareja dentro de una misma estación reproductiva en Troglodytes aedon entre 1998 y 2000. Los machos iniciaron el cambio de pareja, pero este cambio no estuvo relacionado con la paternidad del macho en la primera nidada, ni con su condición corporal, ni con su experiencia de cría previa. Los machos presentaron una mayor probabilidad de cambiar de pareja cuando se encontraban hembras no apareadas en las cercanías. Los machos que cambiaron de pareja engendraron un número similar de crías al de los machos que permanecieron con la misma pareja, por lo que los machos no parecieron beneficiarse directamente con el cambio de pareja. En contraste, la mitad de las hembras que fueron abandonadas no encontraron una segunda pareja durante el periodo reproductivo, y por lo tanto incurrieron en un costo reproductivo.