ALBERT

Description: In territorial species, at least 1 of the 2 individuals involved in an extrapair mating has to leave its territory in order to copulate with the extrapair partner. However, data on extraterritorial forays are difficult and time-consuming to acquire, and extensive datasets on foraying behavior are rare. Here, we use an automated recording system to investigate forays of individual blue tits ( Cyanistes caeruleus ) to other pair’s nest-boxes during early spring, when extrapair matings occur. The study has 3 aims. 1) To describe daily and seasonal variation in the frequency of foraying behavior. 2) To investigate which individuals perform forays. 3) To investigate whether male and female visits to other pair’s nest-boxes predict the occurrence of extrapair paternity. We find that males visited foreign nest-boxes more frequently than females, and that the majority of visits occurred in the morning hours, but not around dawn. Male visits to a foreign nest-box were unrelated to the social and the visited females’ fertile period. Male body size, but not age predicted the occurrence of forays. Males that forayed were more likely to sire extrapair offspring with the female they visited compared with nonvisiting males. Whether males copulate during such visits or whether females seek extrapair copulations from visiting males remains unknown. However, male forays to a foreign nest-box coincided with the presence of the resident female at the same nest-box more often than expected by chance. This indicates that visits to foreign nest-boxes could be an opportunity for extrapair mate assessment or extrapair copulations.

Description: Incubation is energetically demanding, but it is debated whether these demands constrain incubation-scheduling (i.e., the length, constancy, and timing of incubation bouts) in cases where both parents incubate. Using 2 methods, we experimentally reduced the energetic demands of incubation in the semipalmated sandpiper, a biparental shorebird breeding in the harsh conditions of the high Arctic. First, we decreased the demands of incubation for 1 parent only by exchanging 1 of the 4 eggs for an artificial egg that heated up when the focal bird incubated. Second, we reanalyzed the data from the only published experimental study that has explicitly tested energetic constraints on incubation-scheduling in a biparentally incubating species ( Cresswell et al. 2003 ). In this experiment, the energetic demands of incubation were decreased for both parents by insulating the nest cup. We expected that the treated birds, in both experiments, would change the length of their incubation bouts, if biparental incubation-scheduling is energetically constrained. However, we found no evidence that heating or insulation of the nest affected the length of incubation bouts: the combined effect of both experiments was an increase in bout length of 3.6min (95% CI: –33 to 40), which is equivalent to a 0.5% increase in the length of the average incubation bout. These results demonstrate that the observed biparental incubation-scheduling in semipalmated sandpipers is not primarily driven by energetic constraints and therefore by the state of the incubating bird, implying that we still do not understand the factors driving biparental incubation-scheduling.

Description: In biparental species, parents may be in conflict over how much they invest into their offspring. To understand this conflict, parental care needs to be accurately measured, something rarely done. Here, we quantitatively describe the outcome of parental conflict in terms of quality, amount, and timing of incubation throughout the 21-day incubation period in a population of semipalmated sandpipers ( Calidris pusilla ) breeding under continuous daylight in the high Arctic. Incubation quality, measured by egg temperature and incubation constancy, showed no marked difference between the sexes. The amount of incubation, measured as length of incubation bouts, was on average 51min longer per bout for females (11.5h) than for males (10.7h), at first glance suggesting that females invested more than males. However, this difference may have been offset by sex differences in the timing of incubation; females were more often off nest during the warmer period of the day, when foraging conditions were presumably better. Overall, the daily timing of incubation shifted over the incubation period (e.g., for female incubation from evening–night to night–morning) and over the season, but varied considerably among pairs. At one extreme, pairs shared the amount of incubation equally, but one parent always incubated during the colder part of the day; at the other extreme, pairs shifted the start of incubation bouts between days so that each parent experienced similar conditions across the incubation period. Our results highlight how the simultaneous consideration of different aspects of care across time allows sex-specific investment to be more accurately quantified.

Description: It is well established that artificial night lighting can influence animal orientation, but there is less information about its effects on other behaviors. Previous work suggested that light pollution can affect both seasonal and daily patterns of behavior. The aim of our study was to investigate the effects of artificial night lighting and daytime traffic noise on the timing of dawn and dusk singing in 6 common songbirds. We recorded singing behavior in 11 nonurban plots: 2 plots with light, but no noise, 3 with light and noise pollution, 3 with noise, but no light, and 3 undisturbed forests. Our results show that artificial night lighting, but not noise, leads to an earlier start of dawn singing in 5 out of 6 species, ranging on average from 10min for the song thrush to 20min for the robin and the great tit. This effect was strongest at higher light intensities. We further show that dusk song is also affected: 3 species continued dusk singing for longer in lighted areas, but the effect was smaller than that observed for dawn song (from about 8min for the blackbird to 14min for the great tit). For all species, onset and cessation of singing changed relative to sunrise and sunset with the progress of the season. Rain delayed the onset of singing at dawn and advanced the cessation at dusk. We discuss the implications of our findings in the context of sexual selection.

Description: In several bird species, the period around dawn seems important for extrapair behavior. For example, a study on great tits ( Parus major ) showed that females that emerged earlier from their roosting place during the peak of their fertile period were more likely to have extrapair young in their brood. We investigated the potential effect of female emergence times on extrapair behavior in the blue tit ( Cyanistes caeruleus ). First, we tested the relationship between natural female emergence times from the nest-box and the presence or frequency of extrapair offspring in the brood, using 4 years of data. Females progressively emerged earlier from the nest-box as egg laying approached, with the earliest emergence 2 days before the start of laying. However, we found no relationship between female emergence time and the occurrence of extrapair young in the brood. Secondly, in 2 breeding seasons, we experimentally advanced female emergence times by supplying the roosting females with additional light in the early morning. Although the experiment had inconsistent effects on the occurrence of extrapair young in the brood, we found no evidence that female emergence time during peak fertility is directly linked to extrapair paternity. Interestingly, females exposed to artificial light were more likely to return to breed in the next year.