Coping with shifting nest predation refuges by European reed Warblers Acrocephalus scirpaceus

The prenatal assimilation of minerals and metals in nestlings of a small passerine bird

Although certain metals can reach their highest concentrations in tissues of newly hatched nestlings, their sources have yet to be fully elucidated. Evidence for the differentiation of body reserves, including some chemical elements, during avian embryonic life and early post-hatching period is limited mostly to studies on poultry. Here, we present novel findings on the bioaccumulation and egg-to-nestling transfer of 10 minerals and metals, including non-essential potentially toxic chemical elements (Cu, Ni Cd, Pb, Zn, Ag, Mg, Fe, Co and Ca) in the tissues of the gastrointestinal tract (GT), whole body (WB) and carcass (CA) of nestlings of a small passerine bird, the Eurasian Reed Warbler Acrocephalus scirpaceus, breeding in an intensively fertilized fishpond habitat. Our findings show that concentrations of metals that are relevant to contaminated systems (Cu, Ni, Cd, Pb, Zn, Fe and Co) are the highest in GT tissues across all the nestling age classes examined (from hatching to day 9 of life). The important question emerged from our findings: why the hatchlings were characterized by higher metal concentrations in GT, WB and CA compared to older nestlings. We suggest that the prenatal accumulation of chemical elements assimilated from egg contents, and their subsequent utilization for building various tissues/internal organs by the growing embryo, is a reasonable explanation for this finding. More specifically, the potential functional mechanism explaining the highest concentrations of some trace metals in hatchlings seems to be associated with an extremely high rate of utilization of residual yolk metal resources that survived from the embryonic until the post-natal period. On the basis of our own work and literature data, we identified a presumable significant discrepancy in the actual rate of egg-to-nestling and egg-to-embryo metal transfer due to the eggshell being ignored (a commonly occurrence), and only the egg contents being treated as the basic repository of chemical elements for developing embryos. Much more research is needed on the prenatal assimilation and subsequent accumulation of chemical elements in a developing avian embryo, representing both the transition from near-to-hatching embryos to hatchlings, and then to older nestlings.

Reed Warbler Hosts Do Not Fine-Tune Mobbing Defenses During the Breeding Season, Even When Cuckoos Are Rare

Hosts of brood parasitic cuckoos often employ mobbing attacks to defend their nests and, when mobbing is costly, hosts are predicted to adjust their mobbing to match parasitism risk. While evidence exists for fine-tuned plasticity, it remains unclear why mobbing does not track larger seasonal changes in parasitism risk. Here we test a possible explanation from parental investment theory: parents should defend their current brood more intensively as the opportunity to replace it declines (re-nesting potential), and therefore “counteract” any apparent seasonal decline to match parasitism risk. We take advantage of mobbing experiments conducted at two sites where reed warblers (Acrocephalus scirpaceus) experience (in Italy), or do not experience (in Finland), brood parasitism. We predicted that mobbing of cuckoos should be higher overall in Italy, but remain constant over the season as in other parasitised sites, whereas in Finland where cuckoos do not pose a local threat, we predicted that mobbing should be low at the beginning of the season but increase as re-nesting potential declined. However, while cuckoos were more likely to be mobbed in Italy, we found little evidence that mobbing changed over the season at either the parasitized or non-parasitized sites. This suggests that re-nesting potential has either little influence on mobbing behavior, or that its effects are obscured by other seasonal differences in ecology or experience of hosts.

Fitness consequences of longer breeding seasons of a migratory passerine under changing climatic conditions

Numerous studies have shown that climate change affects the timing of migration and bird laying dates, but the resulting changes in the duration of breeding seasons and their fitness consequences remain largely unknown. We compared breeding parameters of 343 individually marked female Eurasian reed warblers, a multi-brooded migratory passerine, studied in the same area in 1980-1983 and 2005-2012. The latter period was warmer, with mean temperatures during the breeding season higher by 1.5°C on average. As, in recent years, birds arrive earlier from wintering areas and the breeding season of the population is longer, we hypothesized it should result in the increased re-nesting opportunities of individual females. We found that breeding periods of individual females (from building of the first nest till the end of caring for last fledglings/last nest failure) in the current century have extended by 2 weeks compared to the 1980s. In the 2000s, females produced 75% more fledglings annually than females in the 1980s (2.8 vs. 1.6, respectively). The proportion of females raising second broods increased from 2.7% to 23.6% between the first and the second study period while the share of females that did not produce any young annually decreased from 48.1% to 15.5%. The higher offspring production in recent years was related to more successfully fledged broods and an earlier start of breeding, which secured more time to re-nest. Higher female parental effort in recent years was not manifested in the reduction of the female apparent survival: it was almost identical in the two study periods (0.30 vs. 0.31). We conclude that prolonged reproductive seasons might be beneficial for some species. Identifying causes and consequences of changes in the duration of breeding seasons may be essential to predict demography of populations under changing climatic conditions.

Interspecific information on predation risk affects nest site choice in a passerine bird

Background

Breeding site choice constitutes an important part of the species niche. Nest predation affects breeding site choice, and has been suggested to drive niche segregation and local coexistence of species. Interspecific social information use may, in turn, result in copying or rejection of heterospecific niche characteristics and thus affect realized niche overlap between species. We tested experimentally whether a migratory bird, the pied flycatcher Ficedula hypoleuca, collects information about nest predation risk from indirect cues of predators visiting nests of heterospecific birds. Furthermore, we investigated whether the migratory birds can associate such information with a specific nest site characteristic and generalize the information to their own nest site choice.

Results

Our results demonstrate that flycatchers can use the fate of heterospecific nesting attempts in their own nest site choice, but do so selectively. Young flycatcher females, when making the decision quickly, associated the fate of an artificial nest with nest-site characteristics and avoided the characteristic associated with higher nest predation risk.

Conclusions

Copying nest site choices of successful heterospecifics, and avoiding choices which led to failed attempts, may amplify or counter effects of nest predation on niche overlap, with important consequences for between-species niche divergence-convergence dynamics, species coexistence and predator-prey interactions.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1301-3) contains supplementary material, which is available to authorized users.

Survival during the Breeding Season: Nest Stage, Parental Sex, and Season Advancement Affect Reed Warbler Survival

Avian annual survival has received much attention, yet little is known about seasonal patterns in survival, especially of migratory passerines. In order to evaluate survival rates and timing of mortality within the breeding season of adult reed warblers (Acrocephalus scirpaceus), mark-recapture data were collected in southwest Poland, between 2006 and 2012. A total of 612 individuals (304 females and 308 males) were monitored throughout the entire breeding season, and their capture-recapture histories were used to model survival rates. Males showed higher survival during the breeding season (0.985, 95% CI: 0.941–0.996) than females (0.869, 95% CI: 0.727–0.937). Survival rates of females declined with the progression of the breeding season (from May to August), while males showed constant survival during this period. We also found a clear pattern within the female (but not male) nesting cycle: survival was significantly lower during the laying, incubation, and nestling periods (0.934, 95% CI: 0.898–0.958), when birds spent much time on the nest, compared to the nest building and fledgling periods (1.000, 95% CI: 1.00–1.000), when we did not record any female mortality. These data (coupled with some direct evidence, like bird corpses or blood remains found next to/on the nest) may suggest that the main cause of adult mortality was on-nest predation. The calculated survival rates for both sexes during the breeding season were high compared to annual rates reported for this species, suggesting that a majority of mortality occurs at other times of the year, during migration or wintering. These results have implications for understanding survival variation within the reproductive period as well as general trends of avian mortality.