Tomas Pärt

Age and reproduction in birds — hypotheses and tests

It is well known that reproductive performance improves with age in birds. Many hypotheses, involving factors such as differential survival, delayed breeding, breeding experience, foraging ability and reproductive effort, have been proposed to explain this pattern. Although these hypotheses are not mutually exclusive, they can be classified in three major groups relating to progressive appearance or disappearance of phenotypes, age-related improvements of competence, and optimization of reproductive effort. However, a closer examination of the literature reveals that only few studies have rigorously tested the hypotheses. Future work should focus on carefully designed tests that critically investigate the hypotheses.

BREEDING DISPERSAL IN THE COLLARED FLYCATCHER (FICEDULA ALBICOLLIS): POSSIBLE CAUSES AND REPRODUCTIVE CONSEQUENCES

SUMMARY (1) Correlates of breeding dispersal (between years) in the collared flycatcher were analysed using data from a population on the Swedish island of Gotland in the Baltic. (2) Females on average moved further than males. Only 8% of the males and 2% of the females reoccupied their previous nest box. Individual size or body condition were not related to breeding dispersal. (3) Among females both total failures and reduced number of fledged young were associated with long subsequent movements in yearlings, whereas dispersal of older individuals was not related to previous breeding success. The pattern of breeding dispersal was similar in males but the difference was not significant. (4) Males that remained unpaired throughout the breeding season moved further in the next year than did mated males. There were no differences in breeding dispersal patterns between polygynous and monogamous males, nor between monogamous and secondary females. (5) Occupation of previous nest boxes by breeding tits were significantly correlated with dispersal distances in 2-year-old males, but not in older males. Female breeding dispersal was not related to this factor. (6) Males breeding for the first time in their natal area subsequently moved shorter distances than males breeding in another area. Females showed no significant differences in dispersal with respect to their birth place. (7) Breeding dispersal distance was not correlated with age when breeding success, occupation of previous nest boxes by breeding tits and distance from natal area were controlled for. (8) The reproductive correlates of movements were different for different categories of individuals. There was a negative correlation between distance moved and subsequent fitness (measured as survival and number of recruits produced) among previously successful females, whereas the trend was the opposite among previously unsuccessful females. These relationships were most prominent among young females. There were no significant correlations of dispersal and subsequent reproductive performance in males. (9) The hypothesis is advanced that breeding dispersal distances (to some extent) reflect individual choices among available territories, in that individuals with little and/or bad local experience disperse more because they benefit from exploring more sites before

Badge size in collared flycatchers predicts outcome of male competition over territories

The evolution of conspicuous coloration is often hypothesized to be driven by sexual selection, where colour traits may function as honest signals of individual abilities in male contest competition and female choice. However, game theory models suggest that colourful badges (i.e. energetically cheap signals) may have no function in sexually selected contests, because the value of the contested resource is too high relative to the costs of fighting. We investigated this assertion by experimentally staging male contests over nest sites (a crucial resource for attracting females) in old (>/=2 years) male collared flycatchers, Ficedula albicollisMales with a relatively large white forehead patch (i.e. a condition-dependent plumage trait displayed in male contests) enjoyed a competitive advantage in disputes over experimentally vacated territories. No other measured morphological variable predicted the outcome of such a dispute. Furthermore, the winners of the disputes acquired a female more quickly than did the losers. Thus, our results suggest that the white forehead patch of male collared flycatchers may function as a badge of status that is also used in sexually selected contests over resources. We suggest that this is because the value of the contested territory may be relatively low compared with the cost of fighting when alternative vacant sites exist in the neighbourhood.Copyright 1997 The Association for the Study of Animal Behaviour1997The Association for the Study of Animal Behaviour

Different nest predator faunas and nest predation risk on ground and shrub nests at forest ecotones: an experiment and a review

Abstract This study examined predator faunas of artificial ground and shrub nests and whether nest predation risk was influenced by nest site, proximity to forest edge, and habitat structure in 38 grassland plots in south-central Sweden. There was a clear separation of predator faunas between shrub and ground nests as identified from marks in plasticine eggs. Corvids accounted for almost all predation on shrub nests whereas mammals mainly depredated ground nests. Nest predation risk was significantly greater for shrub than for ground nests at all distances (i.e. 0, 15 and 30 m) from the forest edge. However, nest predation risk was not significantly related to distance to forest edge, but significantly increased with decreasing distance to the nearest tree. Different corvid species robbed nests at different distances from the forest edge, with jays robbing nests closest to edges. We conclude that the relationship between the predation risk of grassland bird nests and distance to the forest edge mainly depends on the relative importance of different nest predator species and on the structure of the forest edge zone. A review of published articles on artificial shrub and ground nest predation in the temperate zone corroborated the results of our own study, namely that shrub nests experienced higher rates of depredation in open habitats close to the forest edge and that avian predators predominantly robbed shrub nests. Furthermore, the review results showed that predation rates on nests in general are highest <50 m inside the forest and lower in open as well as forest interior habitats (≥50 m from the edge).

Agricultural intensification and biodiversity partitioning in European landscapes comparing plants, carabids, and birds

Effects of agricultural intensification (AI) on biodiversity are often assessed on the plot scale, although processes determining diversity also operate on larger spatial scales. Here, we analyzed the diversity of vascular plants, carabid beetles, and birds in agricultural landscapes in cereal crop fields at the field (n = 1350), farm (n = 270), and European-region (n = 9) scale. We partitioned diversity into its additive components alpha, beta, and gamma, and assessed the relative contribution of beta diversity to total species richness at each spatial scale. AI was determined using pesticide and fertilizer inputs, as well as tillage operations and categorized into low, medium, and high levels. As AI was not significantly related to landscape complexity, we could disentangle potential AI effects on local vs. landscape community homogenization. AI negatively affected the species richness of plants and birds, but not carabid beetles, at all spatial scales. Hence, local AI was closely correlated to beta diversity on larger scales up to the farm and region level, and thereby was an indicator of farm- and region-wide biodiversity losses. At the scale of farms (12.83-20.52%) and regions (68.34-80.18%), beta diversity accounted for the major part of the total species richness for all three taxa, indicating great dissimilarity in environmental conditions on larger spatial scales. For plants, relative importance of alpha diversity decreased with AI, while relative importance of beta diversity on the farm scale increased with AI for carabids and birds. Hence, and in contrast to our expectations, AI does not necessarily homogenize local communities, presumably due to the heterogeneity of farming practices. In conclusion, a more detailed understanding of AI effects on diversity patterns of various taxa and at multiple spatial scales would contribute to more efficient agri-environmental schemes in agroecosystems.

Nonideal breeding habitat selection: a mismatch between preference and fitness.

The selection of breeding sites in heterogeneous habitats should ideally be based on cues closely reflecting habitat quality and thus predicting realized individual fitness. Using long-term population data and data on territory establishment of male Northern Wheatears (Oenanthe oenanthe), we examined whether territory characteristics linked to individual fitness (reproductive performance and survival) also were linked to territory preference. Breeding territories varied in their physical characteristics and their potential effects on reproductive performance, and this variation among territories was correlated from one year to the next. Of all measured territory characteristics (from the focal and the previous year) only territory field layer height predicted individual fitness, i.e., reproductive performance was higher in territories with permanently short rather than growing field layers. Territory preference, instead, was only linked to the size of territory aggregations, i.e., males settled earlier at territory sites sharing borders with several adjacent sites than at those with few or no adjacent sites. This mismatch between territory characteristics linked to fitness and those linked to territory preference was not explained by site fidelity or compensated for by the different fitness components measured. Because the results were not in agreement with an ecological trap scenario, where poor habitats are preferred over high-quality habitats, our results suggest a more general case of nonideal habitat selection. Whereas nonideal selection with respect to territory field layer height may be explained by its poor temporal predictability within the breeding season, the preference for territory aggregations is still open to alternative adaptive explanations. Our study suggests that nonideal habitat selection should be investigated by direct estimates of preferences (e.g., order of territory establishment) and their links to habitat characteristics and fitness components. Furthermore, we suggest that the probability of establishing a territory needs to be included as a factor influencing patterns of habitat selection.

Seasonal Decline in Collared Flycatcher Ficedula albicollis Reproductive Success: An Experimental Approach

To assess the relationship between parental quality, seasonal environmental changes, and seasonal reproductive success, we manipulated both hatching time and clutch size among collared flycatchers (Ficedula albicollis) breeding on the island of Gotland, Sweden. Our experimental hatching delays of one week had no significant effect on the body condition of experimental females, relative to that of controls. In the absence of simultaneous clutch size reductions, experimental delays in hatching date caused significant decreases in fledging success relative to controls. However, when coupled with a 1-egg decrease in clutch size, experimental delays had no effect on fledging success. The results suggest that both parental/territory quality and environmental changes over the season affect the seasonal decline in reproductive success of birds

Estimating fitness consequences of dispersal: a road to ‘know‐where’? Non‐random dispersal and the underestimation of dispersers’ fitness

1. Many studies investigating fitness correlates of dispersal in vertebrates report dispersers to have lower fitness than philopatric individuals. However, if dispersers are more likely to produce dispersing young or are more likely to disperse again in the next year(s) than philopatric individuals, there is a risk that fitness estimates based on local adult survival and local recruitment will be underestimated for dispersers. 2. We review the available empirical evidence on parent-offspring resemblance and individual lifelong consistency in dispersal behaviour, and relate these studies to recent studies of fitness correlates of dispersal in vertebrates. 3. Of the 12 studies testing directly for parent-offspring resemblance in dispersal propensity, five report a significant resemblance. The average effect size (r) of parent-offspring resemblance in dispersal was 0.15 [95% confidence interval (CI) = 0.07-0.22], with no difference between the sexes (average weighted effect size of 0.12 (0.08-0.16) and 0.16 (0.11-0.20) for females and males, respectively). Only three studies report data on within-individual consistency in dispersal propensity, of which two suggest dispersers to be more likely to disperse again. 4. To assess the magnitude of fitness underestimation expected for dispersing individuals depending on the heritability of dispersal distance and study area size, we used a simulation approach. Even when study area size is 10 times the mean dispersal distance, local recruitment per breeding event may be underestimated by 4-10%, generating a potential difference of 4-60% in average lifetime production of recruits between dispersing and philopatric individuals, with larger differences in long-lived species. 5. Estimates of both fitness correlates of dispersal and parent-offspring resemblance or within-individual consistency in dispersal behaviour have been reported for 11 species. Although some comparisons suggest genuine differences in fitness components between philopatric and dispersing individuals, others, based on adult and juvenile survival, are open to the alternative explanation of biased fitness estimates. 6. We list three potential ways of reducing the risk of making wrong inferences on biased fitness estimates due to such non-random dispersal behaviour between dispersing and philopatric individuals: (a) diagnosing effects of non-random dispersal, (b) reducing the effects of spatially limited study area and (c) performing controlled experiments.

Gathering public information for habitat selection: prospecting birds cue on parental activity.

Because habitat quality strongly affects individual fitness, understanding individual habitat selection strategies is fundamental for most aspects of the evolution and conservation of species. Several studies suggest that individuals gather public information, i.e. information derived from the reproductive performance of conspecifics, to assess and select habitats. However, the behavioural mechanisms of information gathering, i.e. prospecting, are largely unknown, despite the fact that they directly constrain individual selection strategies. To test whether prospectors gather public information or other cues of habitat quality, we manipulated brood size of collared flycatchers (Ficedula albicollis) and investigated subsequent attraction of prospectors. Experimentally adding two nestlings increased the probability of attracting prospectors to the nest as a result of increased parental feeding rates. Prospectors were attracted to the most successful sites because feeding rate predicted subsequent fledgling production. In the year following prospecting, individuals selected a breeding site very close to the prospected site. These results provide the first experimental evidence, to our knowledge, of the links between information gathering behaviour and breeding habitat selection strategies based on public information.

The effects of territory quality on age-dependent reproductive performance in the northern wheatear, Oenanthe oenanthe

Abstract Age differences in access to high-quality resources and its relation to nest predation risk have generally been neglected in efforts to explain why old individuals have a higher reproductive performance than young ones. I used long-term population data to investigate age- and habitat-specific nest predation risk and reproductive success of male northern wheatears breeding in a heterogeneous agricultural landscape. Old (≥2 years) male wheatears had higher reproductive success than yearling males because their nests were less likely to be predated, they produced more fledglings from successful broods and they were more likely to start a new breeding attempt following a complete failure. Also, old males arrived and bred earlier than yearlings. Old wheatears were more likely to breed in habitats with a permanently short field layer, while yearlings mainly bred in habitats with a field layer gaining in height during the incubation–nestling period. Nest predation risk was significantly lower and fledgling production among successful breeders was higher in territories with a permanently short as compared to growing field layer. Data on order of habitat occupation suggested that yearlings were not as good as older males at identifying territories with a permanently short field layer (as reflected by a significant interaction between male age and order of occupancy). Both cross-sectional and longitudinal data suggest that an age difference in territory field layer height was a major cause of age differences in reproductive performance in wheatears. Other factors, such as differential survival, breeding time, local familiarity and increased foraging and breeding skills either had small effects or did not significantly explain the observed age differences in reproductive performance. I conclude that nest predation and an age-related access to high-quality habitats are likely to be of great importance for the observed age differences in reproductive performance in the northern wheatear and possibly many other bird species.