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1. Nest structure affects auditory and visual detectability, but not predation risk, in a tropical songbird community

   https://doi.org/10.1111/1365-2435.13405  

   Mouton, James C. ; Martin, Thomas E. ; Galván, ed., Ismael ( August 2019 , Functional Ecology)

   Offspring mortality varies dramatically among species with critical demographic and evolutionary ramifications, yet the causes of this variation remain unclear. Nests are widely used for breeding across taxa and thought to influence offspring mortality risk. Traditionally, more complex, enclosed nest structures are thought to reduce offspring predation by reducing the visibility of nest contents and muffling offspring sounds compared to open nests. Direct tests of the functional bases for nest structure influence on predation risk are lacking.

   We used experiments and 10 years of observational data to examine how nest structure influences nest predation risk in a diverse community of tropical songbirds. First, we examined how nest size was related to nest structure and nest predation rates across species. Second, we assessed how nest structure influences the detectability of nestling begging calls both in field and in laboratory settings. Finally, we examined how the acoustic properties of different nest structures influence nest predation risk. Specifically, we experimentally broadcast begging calls from open and enclosed nests to determine how auditory cues and nest structure interact to affect predation on plasticine and quail eggs. We also tested whether nest structure was associated with differences in nest predation rates between the incubation (no begging cues) and nestling (begging cues) stages.

   We found that enclosed nests are larger than open nests after accounting for adult size, and larger nests had increased predation rates. Moreover, enclosed nests did not consistently alter nestling begging calls in ways that reduce the likelihood of predation compared to open nests. Indeed, begging cues increased predation rates for enclosed but not open‐cup nests in our playback experiment, and nest predation rates showed greater increases after hatching in enclosed than open‐cup nests.

   Ultimately, enclosed nests do not necessarily provide greater predation benefits than open nests in contrast to long‐standing theory.

   A freeplain language summarycan be found within the Supporting Information of this article.

    

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2. Intraspecific variation in incubation behaviours along a latitudinal gradient is driven by nest microclimate and selection on neonate quality

   https://doi.org/10.1111/1365-2435.13772  

   Lundblad, Carl G. ; Conway, Courtney J. ; Sandercock, ed., Brett ( March 2021 , Functional Ecology)

   The strategies by which animals allocate reproductive effort across their lifetimes vary, and the causes of variation in those strategies are actively debated. In birds, most research has focused heavily on variation in clutch size and fecundity, but incubation behaviour and other functionally related traits have received less attention. Variation in incubation period duration is notable because time‐dependent sources of clutch mortality should impose strong directional selection to minimize the incubation period. However, life‐history theory predicts multiple mechanisms by which inter‐ and intraspecific variation in incubation behaviours may be adaptive.

   We conducted one of the first studies of intraspecific latitudinal variation in avian incubation behaviours across a large portion of a single species’ range. We placed motion‐activated nest cameras inside burrowing owl nests at five study sites to quantify variation in daily nest attentiveness, cumulative nest attendance and incubation period duration. We tested predictions of two alterative hypotheses that have been proposed to explain variation in incubation periods: theparental risk tolerance hypothesisand theneonate quality hypothesis.

   Daily nest attentiveness, cumulative nest attendance and incubation period duration in burrowing owls were all positively correlated with latitude. Burrowing owls reduced their daily nest attentiveness at low latitudes and on days when the average nest temperature was within the range that is optimal for embryo development. Further, longer incubation periods were most strongly associated with greater cumulative nest attendance instead of reduced daily nest attentiveness.

   These results support predictions of theneonate quality hypothesis:longer incubation periods result from stronger selection on neonate quality rather than selection to reduce reproductive effort in response to low extrinsic mortality risk. However, some owls facultatively reduced their daily nest attentiveness, and this result supports the general hypothesis that incubation decisions reflect a trade‐off between reproduction and self‐maintenance, and that the optimal solution to that trade‐off varies systematically in response to latitudinal gradients in adult mortality.

   A freePlain Language Summarycan be found within the Supporting Information of this article.

    

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3. Factors affecting the nesting success of Swainson's Thrush ( Catharus ustulatus ) along an elevational gradient

   https://doi.org/10.1002/ece3.10738  

   Deckel, Sarah C. ; DeLuca, William V. ; Gerson, Alexander R. ; King, David I. ( January 2024 , Ecology and Evolution)

   Montane birds experience a range of challenges that may limit their breeding success, including nest predation and severe climactic conditions. The continuing effects of climate change are causing shifts in biotic and abiotic factors that may compound these threats to montane bird species. In northeastern montane forests, many bird species are shifting downslope, potentially as the result of increased precipitation and temperature at higher elevations. Although lower elevations might be more favorable in terms of climactic conditions, nest predation is higher at lower elevations. Thus, montane birds might be faced with the opposing pressures of adverse climactic conditions at higher elevations and increased predation at lower elevations. We monitored nests of Swainson's Thrush (Catharus ustulatus) along an elevation gradient in the White Mountain National Forest in New Hampshire in 2016, 2018, 2019, and 2021 to examine the effect of biotic and abiotic factors on daily nest survival rate (DSR). Linear time explained the most variation of DSR in AICc model comparison, indicating that DSR decreases across the breeding season. Rain intensity (mm/h) had a weak negative effect on DSR, indicating that heavier rain per hour decreases Swainson's Thrush DSR. Moreover, we found some support for a negative interaction effect of elevation in conjunction with minimum daily temperature: DSR of Swainson's Thrush nests at low elevations (281 m) increased with increasing minimum daily temperatures and decreased at high elevations with increasing minimum daily temperatures. Our results suggest nesting survival of montane breeding birds may be at risk as heavier precipitation events become more frequent and intense due to the changing climate and raises the possibility that other passerine species could be at risk in this system.

    

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4. Post‐fledging spatial dispersion and movement behaviour differs between cavity‐nesting and open‐cup nesting passerines

   https://doi.org/10.1111/ibi.13027  

   Şahin Arslan, Necmiye ; Martin, Thomas E. ( November 2021 , Ibis)

   Spatial dispersion and movement behaviour of siblings vary across animal taxa and can affect fitness. The factors underlying why species differ in movement behaviour of young and why some species form family groups while other species have dispersed young have rarely been addressed. We tested a hypothesis that siblings should be more dispersed in space and move less in species that have offspring with poor locomotor abilities to reduce the chance of the loss of the whole brood. In contrast, siblings should aggregate and move more when their mobility and ability to avoid predators is strong. We tested this hypothesis using songbirds because cavity‐ versus open‐cup‐nesting species strongly contrast in nesting strategy. Offspring of species that nest in cavities leave the nest with well‐developed wings and initially are more mobile and incur lower predation rates than fledglings of species that nest in open‐cups. We used radiotelemetry to track fledglings of three cavity‐ and three open‐cup‐nesting songbird species to test whether young differ in spatial dispersion and movement behaviour between nest types. We found that mobile young of cavity‐nesting species moved farther on a daily basis but remained more aggregated as a family than the less mobile young of open‐cup‐nesting species. Movement distances increased with mobility as young aged, especially in open‐cup‐nesting species, but families of open‐cup‐nesting species remained dispersed throughout the first week after fledging. This variation in spatial dispersion and movement behaviour of young can have important implications for parental care strategies and juvenile survival.

    

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5. I need some space: solitary nesting Adélie penguins demonstrate an alternative breeding strategy at Cape Crozier

   https://doi.org/10.1007/s00300-024-03246-9  

   Cox, A. M. ; Khor, D. J. ; Elrod, M. L. ; Lescroël, A. ; Ballard, G. ; Schmidt, A. E. ( April 2024 , Polar Biology)

   According to the ‘selfish herd’ hypothesis, most seabird species breed colonially so that individuals can decrease their risk of predation by forming compact groups. However, costs and benefits associated with colonial breeding may not be evenly distributed among individuals within a colony. At Adélie penguin colonies, individuals nesting on the periphery of subcolonies (distinct groups of nests) may experience higher rates of nest predation by south polar skuas, and thus the optimal aggregation pattern for Adélie penguins may be within groups that minimize the proportion of edge nests. Nevertheless, some penguins choose to nest solitarily, at significant distances from conspecifics. We tracked 50 of these “solitary-nesting” Adélie penguins at Cape Crozier, a large colony on Ross Island, during the 2021 nesting season and compared their breeding success to individuals nesting within subcolony boundaries. We found that both solitary and subcolony nests successfully raised chicks large enough to join crèches and left unattended by adults. However, chicks from solitary nests exhibited a rate of mortality more than six times higher during the transition from nest brooding/guarding to crèche stage. In the 2022 nesting season, we found that solitary nests which had previously hosted actively breeding penguins were more likely to be re-occupied. Solitary nesting therefore appears to be a less-successful alternative to breeding within subcolonies, but enough individuals could be successful with this approach to maintain the apparently disadvantageous behavior and effectively pioneer previously unused locations, possibly including eventual new colony locations.

    

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