This article has an associated ECR Spotlight interview with Blanca Jimeno and Paola Merino Millanes., Glucocorticoid (GC) hormones have traditionally been interpreted as indicators of stress, but the extent to which they provide information on physiological state remains debated. GCs are metabolic hormones that amongst other functions ensure increasing fuel (i.e. glucose) supply on the face of fluctuating energetic demands, a role often overlooked by ecological studies investigating the consequences of GC variation. Furthermore, because energy budget is limited, in natural contexts where multiple stimuli coexist, the organisms' ability to respond physiologically may be constrained when multiple triggers of metabolic responses overlap in time. Using free-living spotless starling (Sturnus unicolor) chicks, we experimentally tested whether two stimuli of different nature known to trigger a metabolic or GC response, respectively, cause a comparable increase in plasma GCs and glucose. We further tested whether response patterns differed when both stimuli occurred consecutively. We found that both experimental treatments caused increases in GCs and glucose of similar magnitude, suggesting that both variables fluctuate along with variation in energy expenditure, independently of the trigger. Exposure to the two stimuli occurring subsequently did not cause a difference in GC or glucose responses compared with exposure to a single stimulus, suggesting a limited capacity to respond to an additional stimulus during an ongoing acute response. Lastly, we found a positive and significant correlation between plasma GCs and glucose after the experimental treatments. Our results add to the increasing research on the role of energy expenditure on GC variation, by providing experimental evidence on the association between plasma GCs and energy metabolism., B.J. was funded by grant FJC2019-039748-I of MCIN/AEI/10.13039/501100011033 with European Union NextGenerationEU/PRTR, and by the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 101027784. This research was jointly funded by research grants from the Spanish Ministerio de Ciencia, Innovación y Universidades to D.G. (CGL2017-83843-C2-1-P, PID2021-126673NB-I00) and L.P.-R. (PGC2018-099596-B-I00, co-financed by MCIN/AEI/10.13039/501100011033 and by ERDF – A way of making Europe), and by research grant LINCG23031 from the Spanish Consejo Superior de Investigaciones Científicas (Programa CSIC LINCGLOBAL 2023) to L.P.-R., Open access funding provided by CSIC., Peer reviewed

Floaters are non-breeding individuals that lack a territory or a breeding site. In many species, they can be seen visiting the territories of conspecifics before obtaining their own breeding site. Prospecting behavior is hypothesized to benefit floaters through information acquisition, enhanced site familiarity and dominance over other floaters. Here, we used detections of PIT-tagged male floaters in a population of spotless starlings (Sturnus unicolor). We investigated how floater activity varied across breeding stages and how their visits
influenced subsequent nest site selection. We also tested whether distance, reproductive success, and phenotype and fate of the former owner influenced final settlement. We found that floater activity increased during the nestling-rearing period as nestling age increased. Floaters were more likely to breed near the area where they had been detected the previous year, suggesting that prospecting allows males to secure a foothold in their future settlement area. Although prospecting was higher in nests with a higher number of nestlings, neither breeding success, phenotype, nor provisioning rate of the last owner were related to nest choice, suggesting that public information is not used by males to decide where to settle. However, we found that floaters were more likely to breed in nest boxes where the previous owner had disappeared from the colony, suggesting that visits by male floaters in this species allow them to detect new vacancies. Our results suggest that prospecting might serve several non-mutually exclusive functions. Further studies in non-saturated colonies could shed light on the functional aspects of prospecting., This work was supported by the Spanish Ministry of Science and Universities (CGL2017-83843-C2-1-P to DG and PID2021-126673NB-I00 to LP-R) and draws on data collected during previous projects of DG (refs.: CGL2008-03501/BOS, CGL2011-26318 and CGL2017-83843-C2-1-P) and LP-R
(ref.: PGC2018-099596-B-I00, co-financed by the European Regional Development Fund-ERDF-). IR was recipient of a FPU (Formación de Profesorado Universitario; grant number FPU16/02746) grant from Spanish
Ministry of Education. JM was supported by a postdoctoral contract for scientific excellence in the development of the Plan Propio I+D+i of the UCLM, co-funded by the European Social Fund Plus. JM was also supported during later analytical and write-up phases by a Marie Skłodowska-Curie Actions Postdoctoral Fellowship (grant number 101063149). RF was funded by a fellowship from “la Caixa” Foundation (ID 100010434), under the
fellowship code LCF/BQ/DR23/12000026. EG-LL was funded by a technology doctorate grant from the Comunidad de Madrid (IND2018_AMB9317), Peer reviewed

A widely accepted explanation for the reliability of offspring begging signals assumes a differential benefit model balanced by direct viability costs independent of offspring nutritional condition. However, supporting evidence for this idea is inconclusive and often hampered by methodological limitations, including differential stimulation protocols and reliance on single, potentially biased markers of nestling health. This study tested the existence of direct, intrinsic, and condition-independent allocation trade-offs between begging and body mass, immunity and oxidative stress by manipulating the begging effort of spotless starling (Sturnus unicolor) nestlings while maintaining constant food intake. We addressed potential problems of previous experimental protocols, ensuring uniform stimulation levels and evaluating multiple immune and oxidative markers. We observed no significant effects of experimentally increased begging effort in any of the 14 physiological markers analyzed, with 95% confidence intervals of effect sizes consistently including zero or one (for the lysis capacity of plasma), indicating no biologically relevant effects. Overall, our findings suggest no physiological trade-offs associated with intense begging., This research was funded by a grant from the Spanish Ministerio de Ciencia, Innovación y Universidades to LP-R and TR (PGC2018-099596-B-I00, co-financed by MCIN/AEI/10.13039/501100011033 and by ERDF—A way of making Europe) and a predoctoral contract funded by the “Fondo Social Europeo” at the University of Castilla-La Mancha to DP-P (grant number 2019-PREDUCLM-11785). During the mansucript writting, DP-P was supported by a research contract (Programa INVESTIGO) awarded through the Plan de Recuperación, Transformación y Resiliencia of the Spanish Ministerio de Trabajo y Economía Social, Junta de Comunidades de Castilla-La Mancha and Consejo Superior de Investigaciones Científicas (CSIC), co-financed by the European Union-Next Generation EU. Additional support during population monitoring was provided by the research projects CGL2017-83843-C2-1-P and PID2021-126673NB-I00 awarded by the Ministry of Science and Universities to D. Gil and LP-R., Peer reviewed

Further research should explore alternative explanations for honest begging signals, such as differential cost models or cost-free equilibria, moving beyond the scope of differential benefits models. Datasets are saved as .csv files witth comma separation. Missing data are indicated as empty cells. Codes were run using R software version 4.4.1, A widely accepted explanation for the reliability of offspring begging signals assumes a differential benefits model balanced by direct viability costs independent of offspring nutritional condition. However, supporting evidence for this idea is inconclusive and often hampered by methodological limitations, including differential stimulation protocols and reliance on single, potentially biased markers of nestling health. This study tested the existence of direct, intrinsic, and condition-independent trade-offs between begging and body mass, immunity and oxidative stress by manipulating the begging effort of spotless starling (Sturnus unicolor) nestlings while maintaining constant food intake. We addressed potential problems of previous experimental protocols, ensuring uniform stimulation levels and evaluating multiple immune and oxidative markers. We observed no significant effects of experimentally increased begging effort in any of the 14 physiological markers analysed, with 95% confidence intervals of effect sizes consistently including zero or one (for the lysis capacity of plasma), indicating no biologically relevant effects. Overall, our findings suggest no physiological trade-offs associated with intense begging., Spanish Ministerio de Ciencia, Innovación y Universidades : PGC2018-099596-B-I00
University of Castilla-La Mancha : 2019-PREDUCLM-11785. Ministry of Science and Universities : CGL2017-83843-C2-1-P. Ministry of Science and Universities : PID2021-126673NB-I00., Peer reviewed

Memoria presentada por Daniel Parejo Pulido para optar al grado de Doctor con mención internacional por la Universidad de Castilla – La Mancha., Instituto de Investigación en Recursos Cinegéticos (CSIC-UCLM-JCCM) Doctorado en Ciencias Agrarias y Ambientales: Departamento de Ciencia y Tecnología Agroforestal y Genética Universidad de Castilla – La Mancha., [EN] The current understanding of begging signals and parent-offspring conflict over parental investment is largely shaped by influential models of scramble competition and signalling of need in avian broods. Godfray’s model of differential benefits, in particular, has become the dominant paradigm, inspiring extensive theoretical and empirical research. This model posits that offspring signal their need honestly, with those in greater need signalling more intensely due to higher benefits from food, allowing parents to allocate resources efficiently through an active choice mechanism. The system stabilizes via a signalling cost, creating a highly informative separating equilibrium. However, this model has faced significant criticism regarding its applicability to natural systems. While much of this criticism is theoretical, few empirical studies rigorously test the model’s assumptions, with many This thesis aims to experimentally test several assumptions and predictions of Godfray’s model that have been subject to criticism.researchers instead assuming its validity without question., [ES] La comprensión actual de las señales de petición y el conflicto entre padres e hijos por la transferencia de recursos está en gran medida influenciada por los modelos de competencia igualitaria y de señalización de la necesidad en familias de aves altriciales. En particular, el modelo de Godfray de beneficios diferenciales se ha convertido en el paradigma dominante, inspirando una amplia investigación teórica y empírica. Este modelo propone que las crías indican de forma sincera su nivel de necesidad, siendo las más necesitadas las que solicitan con mayor intensidad debido a los mayores beneficios que reciben por el alimento, permitiendo que los padres asignen recursos de manera eficiente a través de un mecanismo de elección activa. El sistema se estabiliza mediante un coste de señalización, lo que conlleva a un equilibrio de separación de señales bien diferenciadas muy informativo. Sin embargo, este modelo se ha enfrentado a numerosas críticas relacionadas con su aplicabilidad en los sistemas naturales. Aunque gran parte de estas críticas son teóricas, pocos estudios empíricos han ensayado rigurosamente los supuestos del modelo, y muchos investigadores asumen su validez sin cuestionarla.
Esta tesis tiene como objetivo principal ensayar experimentalmente ciertos supuestos y predicciones controvertidas del modelo de Godfray que han sido objeto de críticas., Los objetivos de esta tesis se llevaron a cabo gracias a la financiación proveniente de los siguientes proyectos y ayudas: - Proyecto ref. CGL2014-55969-P del Ministerio de Economía y Competitividad. - Proyecto ref. CGL2017-83843-C2-1-P del Ministerio de Ciencia y Universidades. - Proyecto ref. PGC2018-099596-B-I00 cofinanciado por el Ministerio de Ciencia, Innovación y Universidades, la Agencia Española de Investigación (10.13039/501100011033) y por FEDER – Una manera de hacer Europa. - Proyecto ref. PID2021-126673NB-I00 del Ministerio de Ciencia e Innovación. - Microproyectos EBD, Programa Severo Ochoa para Centros de Excelencia; Ref. SEV-2012-0262. Daniel Parejo Pulido fue beneficiario durante la tesis de un contrato predoctoral del Plan Propio de la Universidad de Castilla – La Mancha (referencia: 2019-PREDUCLM-11785) cofinanciado por el “Fondo Social Europeo”, seguido de un contrato asociado al Programa INVESTIGO asociado al Plan de Recuperación, Transformación y Resiliencia del Ministerio de Trabajo y Economía Social, la Junta de Comunidades de Castilla – La Mancha y el Consejo Superior de Investigaciones Científicas (CSIC), cofinanciado por los fondos NextGenerationEU., Peer reviewed

The activation of the immune system in response to a pathogen infection can impose significant costs on growth and oxidative stress. Developing individuals are particularly vulnerable to this challenge, as their physiological systems are still maturing and their rapid growth to achieve an optimal size is resource demanding. Here, we investigated the costs in terms of growth and oxidative stress of an experimentally induced immune challenge in wild spotless starling (Sturnus unicolor) nestlings. To this aim, individuals were injected with lipopolysaccharide (LPS), an antigen that triggers an immune response, or a control substance (PBS) in a within-brood design. Although the immune activation impaired the rate of body mass gain within the first 6 h after the challenge, nestlings subsequently showed an accelerated increase in mass within the following 18 h, reaching a similar body mass to the controls by the next day. This represents a compensation ('catch-up growth') occurring within as little as 6-24 h after the challenge. Despite this compensation, initially smaller challenged nestlings showed reduced body mass and survival rates by 8 days after treatment compared with smaller control nestlings. The induced immune challenge also led to increased plasma levels of oxidative damage (reactive oxygen metabolites) and antioxidant capacity, supporting the oxidative cost of immune system activation. These findings highlight the costs of mounting an immune response during early life, characterized by an altered growth dynamic and increased oxidative damage, and the condition dependence of these effects., This research was funded a research grant from the Ministerio de Ciencia, Innovación y Universidades (PGC2018-099596-B-I00) co-financed by Ministerio de Ciencia e Innovación (MCIN/AEI/10.13039/501100011033) and by European Regional Development Fund (ERDF) – A way of making Europe – awarded to L.P.-R. by the Ministerio de Ciencia, Innovación y Universidades. Additional support during population monitoring was provided by the research projects CGL2017-83843-C2-1-P and PID2021-126673NB-I00 awarded by the Ministerio de Ciencia, Innovación y Universidades to D. Gil and L.P.-R. D.P.-P. was supported by a predoctoral contract the University of Castilla-La Mancha funded by the European Social Fund Plus (‘Fondo Social Europeo’ grant number 2019-PREDUCLM-11785) followed by a research contract belonging to the INVESTIGO Program, funded by the ‘Plan de Recuperación, Transformación y Resiliencia’ of the Ministerio de Trabajo y Economía Social, Junta de Castilla–La Mancha, and the Consejo Superior de Investigaciones Científicas (CSIC), co-financed by NextGenerationEU funds. S.C. was supported by a predoctoral research grant (PRE2019-090031) from the Ministerio de Ciencia, Innovación y Universidades. A.A.R.-H. was supported by a ‘María Zambrano’ postdoctoral contract at Universidad de Castilla–La Mancha belonging to the ‘Programa de Recualificación del Sistema Universitario Español’ funded by the Ministerio de Universidades, financed by the European Union – NextGenerationEU (grant agreement MZ2021:2022-POST-21023) and a CAM Talent Attraction grant (2024-T1/ECO-31271)., Open Access funding provided by Consejo Superior de Investigaciones Científicas., Peer reviewed

This package contains three Excel workbooks plus six accompanying R scripts documenting field and laboratory data and analyses on multicomponent nestling begging displays in spotless starlings.

The core raw datasets are (i) "Natural_broods.xlsx", a parental visit-level table from natural nests (4 188 rows) in which each row represents a nestling’s behaviour and feeding outcome during a parental visit; and (ii) "Laboratory_nestlings.xlsx", which includes a bout-level postural dataset (300 rows; one row per chick × deprivation treatment × stimulus) and a call-level acoustic dataset (1 904 rows; one row per call with within-bout sequencing and bout identifiers).The file "Data_figures.xlsx" provides processed, figure-ready tables used to plot the manuscript figures and is intended for direct reuse in visualisation or verification.

The six R scripts correspond to (1) the three analysis streams: (a) mixed-model effect sizes (R²-based)(scripts "R2_Models_/), (b) information-theoretic measures of coding effectiveness (scripts "Information_/"), and (c) receiver-based decoding via random-forest classification (script "Random_forest_laboratory_nestlings"; plus (2) One script for figure plotting.

All scripts assume that the Excel data files are available in the working directory (or that paths in the scripts are edited accordingly)., [Description of methods used for collection/generation of data] Data collected to quantify how multicomponent begging displays encode nestling hunger (food deprivation) in spotless starlings, using (i) observations of natural broods at nest boxes and (ii) a controlled laboratory “hunger experiment”.

1 Experimental efforts and data collection

1.1 Study system and timing
Fieldwork was conducted on a nest-box population of spotless starlings in central Spain. Begging behaviour was recorded when nestlings were 6 days old, a stage of rapid growth when nestlings obtain virtually all food through begging and physical interference among nestmates is still limited.
1.2 Observations at natural nests (field component)
Sample: 24 first broods containing 4–5 six-day-old chicks.
Recording context: early-morning feeding peak, using infrared video cameras.
Pre-recording standardisation: nestlings were fed to satiation with mealworms, weighed, and individually marked.
Behavioural scoring: a trained observer (blind to the study aims) reviewed the first three hours of footage after the first parental visit.
Key variables recorded in the field included: timing of parental visits, identity of fed chicks, maximum postural begging intensity, begging rank order, time since previous feeding (deprivation time), and the size of the last prey item (relative to bill length).

1.3 Laboratory hunger experiment (controlled component)
Sample: 25 first-brood nests; two intermediate-mass-ranked chicks per brood were transported to the laboratory the day before trials.
Housing: artificial nest cups under controlled temperature and humidity.
Conditioning: nestlings were habituated to a standard begging stimulus (playback of a parental feeding call from the same population) and fed on demand during multiple conditioning trials.
Feeding schedule before tests: four routine feeding trials at 45-min intervals (ration adjusted to nestling mass).
Hunger manipulation and recording: after the last meal, begging was recorded at 30, 60, and 90 min. Each trial comprised two playback stimuli, separated by sufficient time for begging to extinguish.
Recording equipment: synchronized video (posture) and close-range audio (vocalisations).
Post-trial handling: nestlings were fed ad libitum, blood-sampled for sexing, and returned to their original nests.

1.4 Behavioural and acoustic measurements

Postural begging was scored on an ordinal 0–4 scale:
0 = no begging
1 = gaping, neck flexed, body horizontal
2 = gaping, neck extended, body horizontal
3 = gaping, neck extended, body raised, back reclining
4 = fully stretched, vertical back (sometimes with wing flapping)

Laboratory videos were additionally used to quantify display fine structure (e.g., time spent at each postural level), from which total begging duration, mean postural intensity, maximum postural intensity, time at maximum intensity, and latency to beg were derived.

Begging vocalisations were analysed from spectrograms to obtain temporal measures (e.g., bout duration, number of calls, calling latency) and call-level spectral/entropy measures. Some spectral traits were summarised via PCA to produce single composite variables.

Files and variables

Data
Three Excel workbooks (.xlsx). Each workbook includes a sheet named Metadata containing the authoritative variable definitions (and, where relevant, abbreviations and units). Below, I summarise the contents of each file and list all variables used., [Methods for processing the data] Accompanying R code., This package contains three Excel workbooks plus six accompanying R scripts documenting field and laboratory data on multicomponent nestling begging displays in spotless starlings. The core raw datasets are (i) Natural_broods.xlsx, a visit-level table from natural nests (4 188 rows) in which each row represents a nestling’s behaviour and feeding outcome during a parental visit; and (ii) Laboratory_nestlings.xlsx, which includes a bout-level postural dataset (300 rows; one row per chick × deprivation treatment × stimulus) and a call-level acoustic dataset (1 904 rows; one row per call with within-bout sequencing and bout identifiers). Variables include identifiers (nest, chick, parent), deprivation (time since last feeding or controlled deprivation), ordinal maximum postural intensity (0–4), timing traits (latency, bout duration), feeding outcome (0/1), and multiple acoustic descriptors (temporal measures and frequency/entropy metrics, including PCA-derived composites). Data_figures.xlsx provides processed, figure-ready tables used to plot the manuscript figures and is intended for direct reuse in visualisation or verification. The R code for statistical analyses focuses on (a) mixed-model effect sizes (R²-based), (b) information-theoretic measures of coding effectiveness, and (c) receiver-based decoding via random-forest classification., Ministerio de Ciencia, Innovación y Universidades, PGC2018-099596-B-I00
Agencia Estatal de Investigación, MCIN/ AEI/10.13039/501100011033
Ministerio de Ciencia y Universidades, CGL2017-83843-C2-1-P
Ministerio de Ciencia y Universidades, PID2021-126673NB-I00, File List: #Data All data are provided as three Excel workbooks (.xlsx): "Natural_broods.xlsx" "Laboratory_nestlings.xlsx" "Data_figures.xlsx" #Code Code is provided as six R scripts (.R): "R2_Models_laboratory_nestlings.R" "R2_Models_natural_broods.R"" "Information_laboratory_nestlings.R" "Information_natural_broods.R" "Random_forest_laboratory_nestlings.R" "Figures.R", Peer reviewed

Conspecific brood parasitism is an alternative reproductive tactic in birds. It occurs when some females lay their eggs in conspecific nests and rely on the hosts to raise their offspring. Parasitic females lack control over the outcome of their reproductive event beyond their investment in egg production. However, they might select when and where to lay their eggs to increase their chances of success. Here, we used a breeding colony of spotless starlings (Sturnus unicolor) in Spain to explore the patterns of host nest selection by parasitic females. We used data from eight breeding seasons, during which we recorded all parasitism events. We hypothesized that both conspecific brood parasitism and parasitic nestling survival are influenced by host characteristics and brood timing, but not necessarily in the same direction. We found that most parasitic events happened during the first clutch wave of the season, at the beginning of host clutch laying, and that parasitic females laid their eggs synchronously with the rest of the colony. Furthermore, parasitic eggs were more often found in nests containing eggs of similar size, probably due to the ejection of dissimilar eggs by hosts. Parasitic eggs laid early in the host laying sequence were more likely to hatch, and those that hatched were more likely to fledge when laid in synchrony with the wave peak of the colony. Whether parasitic females use public information to select those host nests and time windows that offer higher chances of offspring survival is a question that deserves further research., European Commission; Université Sorbonne Paris Nord; Comunidad de Madrid; Ministerio de Ciencia, Innovación y Universidades (España); Agencia Estatal de Investigación (España); Unión Europea; Fondo Europeo de Desarrollo Regional; CSIC - Museo Nacional de Ciencias Naturales (MNCN)

Audio and video recordings of Levaillant’s cuckoo and host fledglings reared by Blackcap Babblers (Turdoides reinwardtii) and Brown Babblers (Turdoides plebejus) in The Gambia were collected or curated by Clive R. Barlow between 2006 and 2025. Recordings of one Arrow-marked Babbler cuckoo and two host fledglings associated were provided by Derek Engelbrecht and Isaac Kilusu, some previously published (Derek Engelbrecht/Macaulay Library, Cornell Lab: ML617040028, ML617040040, ML617040041, ML617040048; Isaac Kilusu: XC819163, www.xeno-canto.org/819163; XC819164, www.xeno-canto.org/819164). We also included a unique recording of a Levaillant’s cuckoo fledgling begging and being fed by a Kurrichane Thrush (Turdus libonyana) in South Africa (Tony Archer, XC418749, www.xeno-canto.org/418749). No recordings of Kurrichane Thrush fledgling calls were available, but we included fledgling begging calls of the closely related African Thrush (Turdus pelios) recorded by CRB in The Gambia., This study investigated the presence of vocal mimicry in Levaillant’s cuckoos. The dataset contains acoustic variables automatically extracted with Avisoft SASLab (see “Steps to reproduce”), including call duration, and the mean and standard deviation of frequency bandwidth, entropy (energy dispersion across frequencies—low in pure tones, high in broadband calls), and the first and third frequency quartiles of the energy distribution (robust indicators of low and high frequency limits). Acoustic analyses and classification methods revealed that cuckoos reared by their primary host, the Brown Babbler (Turdoides plebejus), produced non food-transfer begging calls closely resembling those of host fledglings. By contrast, cuckoos raised by secondary hosts, including Blackcap Babblers (T. reinwardtii), showed little or no vocal similarity., [Steps to reproduce] All original audio files were annotated by the recordists to identify call types and behavioural context. A descriptive list of recordings is provided in the “List of recordings” Excel file.
Vocalisations were analysed with Avisoft-SASLab Pro 5.3.2-36. For each recording, continuous annotated spectrograms were generated to identify callers, describe behavioural context, and classify vocal elements. Each element was defined by its distinctive amplitude envelope; where two successive elements formed a single vocalisation, they were analysed separately.
Fledgling calls were classified as food-transfer calls, given during food delivery by adults, or non-transfer calls, given in the long intervals between feedings and often accompanied by wing-quivering or crouching. Cuckoos reared by Blackcap Babblers produced two distinct types of non-transfer calls that were categorized separately. To minimise background noise, a 0.6 kHz high-pass filter was applied. Spectrograms were generated with a 512-point FFT, FlatTop window, 100 % frame size, 50 % overlap, and a bandwidth of 162 Hz, yielding 11.6 ms temporal and 43 Hz frequency resolution. For each element, non-overlapping extraneous sounds were manually removed and amplitude thresholds adjusted to optimise signal-to-noise ratio.
Nine acoustic parameters were automatically extracted for each element. As most vocalisations were harsh or multiband calls with low-frequency modulation, spectral measurements were based on the mean spectrum of the entire element as described in Avisoft-SASLab manual., Ministerio de Ciencia, Innovación y Universidades
Spain
Grant ID: PID2021-126673NB-100, Peer reviewed

Vocal mimicry in brood parasites may enhance fledgling survival by facilitating care or social acceptance by hosts. We quantified vocal similarity between fledgling Levaillant's cuckoos (Clamator levaillantii) and host fledglings using recordings from two host associations in West Africa and a small comparative set from South Africa. We distinguished two context-defined classes of begging calls: food-transfer calls produced during feeding events and non-transfer calls produced outside food transfer. Multivariate acoustic measurements combined with discriminant and classification analyses showed that non-transfer calls of cuckoos associated with the primary West African host, the Brown Babbler (Turdoides plebejus), closely matched non-transfer calls of host fledglings. In contrast, food-transfer calls were broadly similar across host species and showed little evidence of host-specific vocal similarity in any association. Cuckoos associated with a secondary West African host, the Blackcap Babbler (T. reinwardtii), showed weak or no host-specific vocal similarity overall, although one individual produced a distinctive non-transfer call type that resembled host non-transfer calls. South African material was sparse and revealed no clear host-specific vocal similarity. Because the hosts compared are closely related babblers occupying similar habitats, the restriction of vocal similarity to a single association is unlikely to reflect general ecological convergence or shared constraints on vocal production. Instead, our findings are most consistent with host-mediated selection acting after fledging, potentially favouring calls that promote tolerance or social acceptance within cooperatively breeding host groups. We also report new observations of host aggression towards fledgling cuckoos, highlighting the potential importance of vocal cues during the post-fledging period.s, This work was supported by Ministerio de Ciencia, Innovación y Universidades (PID2021-126673NB-100). Additional funds were provided by Severo Ochoa Program Grant CEX2024-001498-S funded by MICIU/AEI/10.13039/501100011033., Peer reviewed

Audio and video recordings of Levaillant’s cuckoo and host fledglings reared by Blackcap Babblers (Turdoides reinwardtii) and Brown Babblers (Turdoides plebejus) in The Gambia were collected or curated by Clive R. Barlow between 2006 and 2025. Recordings of one Arrow-marked Babbler cuckoo and two host fledglings associated were provided by Derek Engelbrecht and Isaac Kilusu, some previously published (Derek Engelbrecht/Macaulay Library, Cornell Lab: ML617040028, ML617040040, ML617040041, ML617040048; Isaac Kilusu: XC819163, www.xeno-canto.org/819163; XC819164, www.xeno-canto.org/819164). We also included a unique recording of a Levaillant’s cuckoo fledgling begging and being fed by a Kurrichane Thrush (Turdus libonyana) in South Africa (Tony Archer, XC418749, www.xeno-canto.org/418749). No recordings of Kurrichane Thrush fledgling calls were available, but we included fledgling begging calls of the closely related African Thrush (Turdus pelios) recorded by CRB in The Gambia., This study investigated the presence of vocal mimicry in Levaillant’s cuckoos. The dataset contains acoustic variables automatically extracted with Avisoft SASLab (see “Steps to reproduce”), including call duration, and the mean and standard deviation of frequency bandwidth, entropy (energy dispersion across frequencies—low in pure tones, high in broadband calls), and the first and third frequency quartiles of the energy distribution (robust indicators of low and high frequency limits). Acoustic analyses and classification methods revealed that cuckoos reared by their primary host, the Brown Babbler (Turdoides plebejus), produced contact and food-related calls closely resembling those of host fledglings. By contrast, cuckoos raised by secondary hosts, including Blackcap Babblers (T. reinwardtii), showed little or no vocal similarity., PID2021-126673NB-100, Peer reviewed

Audio and video recordings of Levaillant’s cuckoo and host fledglings reared by Blackcap Babblers (Turdoides reinwardtii) and Brown Babblers (Turdoides plebejus) in The Gambia were collected or curated by Clive R. Barlow between 2006 and 2025. Recordings of one Arrow-marked Babbler cuckoo and two host fledglings associated were provided by Derek Engelbrecht and Isaac Kilusu, some previously published (Derek Engelbrecht/Macaulay Library, Cornell Lab: ML617040028, ML617040040, ML617040041, ML617040048; Isaac Kilusu: XC819163, www.xeno-canto.org/819163; XC819164, www.xeno-canto.org/819164). We also included a unique recording of a Levaillant’s cuckoo fledgling begging and being fed by a Kurrichane Thrush (Turdus libonyana) in South Africa (Tony Archer, XC418749, www.xeno-canto.org/418749). No recordings of Kurrichane Thrush fledgling calls were available, but we included fledgling begging calls of the closely related African Thrush (Turdus pelios) recorded by CRB in The Gambia., This study investigated the presence of vocal mimicry in Levaillant’s cuckoos. The dataset contains acoustic variables automatically extracted with Avisoft SASLab (see “Steps to reproduce”), including call duration, and the mean and standard deviation of frequency bandwidth, entropy (energy dispersion across frequencies—low in pure tones, high in broadband calls), and the first and third frequency quartiles of the energy distribution (robust indicators of low and high frequency limits). Acoustic analyses and classification methods revealed that cuckoos reared by their primary host, the Brown Babbler (Turdoides plebejus), produced contact and food-related calls closely resembling those of host fledglings. By contrast, cuckoos raised by secondary hosts, including Blackcap Babblers (T. reinwardtii), showed little or no vocal similarity., [Steps to reproduce] All original audio files were annotated by the recordists to identify call types and behavioural context. A descriptive list of recordings is provided in the “List of recordings” Excel file.
Vocalisations were analysed with Avisoft-SASLab Pro 5.3.2-36. For each recording, continuous annotated spectrograms were generated to identify callers, describe behavioural context, and classify vocal elements. Each element was defined by its distinctive amplitude envelope; where two successive elements formed a single vocalisation, they were analysed separately.
Fledgling calls were classified as food-transfer calls, given during food delivery by adults, or begging contact calls, given in the long intervals between feedings and often accompanied by wing-quivering or crouching. Cuckoos reared by Blackcap Babblers produced two distinct types of contact calls that were categorized separately. To minimise background noise, a 0.6 kHz high-pass filter was applied. Spectrograms were generated with a 512-point FFT, FlatTop window, 100 % frame size, 50 % overlap, and a bandwidth of 162 Hz, yielding 11.6 ms temporal and 43 Hz frequency resolution. For each element, non-overlapping extraneous sounds were manually removed and amplitude thresholds adjusted to optimise signal-to-noise ratio.
Nine acoustic parameters were automatically extracted for each element. As most vocalisations were harsh or multiband calls with low-frequency modulation, spectral measurements were based on the mean spectrum of the entire element as described in Avisoft-SASLab manual., Ministerio de Ciencia, Innovación y Universidades
Grant ID: PID2021-126673NB-100, Peer reviewed

In social animals, maintaining strong bonds with conspecifics can be important for reproduction and survival. However, it remains unclear how best to quantify these bonds. Existing social network techniques typically focus on building robust overall networks of co-occurrences but approaches that maximize whole-network accuracy may not be optimal for detecting the most important bonds between individuals. We used data from winter social interactions in a population of spotless starlings, Sturnus unicolor, fitted with passive integrated transponder tags and detected at feeders using radio frequency identification loggers to evaluate existing methods and develop new methods for extracting co-occurrence data from temporal streams of detections. We aimed to develop a method capable of extracting key social bonds: pairs of males and females that bred in consecutive springs. Our results show that although all approaches can differentiate most known pair bonds from other social relationships, methods that focus on capturing fine-scale associations are most effective at delineating them. We then applied this method to birds that were found breeding in spring with a new partner, revealing that in over half of these newly formed pairs, individuals within a pair were each other's strongest social associate during the preceding winter. Applying a discriminant function analysis to the best performing network method demonstrated good sensitivity for identifying true pair bonds. However, we also found substantial variation in social association patterns among pairs, including differences in the formation and dissolution of bonds across the nonbreeding season. Our method improves on previous approaches while also adding to the growing evidence that pair bonding is a complex process that can begin well before the onset of breeding., Funding was provided by research grant PID2021-126673NB-I00 awarded by the Spanish Ministerio de Ciencia e Innovación to D.G. and L.P.-R. Additional support was provided by research grant CSIC/LINCGLOBAL2023/LINCG23031 awarded to L.P.-R. by the Spanish Consejo Superior de Investigaciones Científicas (CSIC). R.F. was supported by a predoctoral fellowship from the ‘la Caixa’ Foundation (ID 100010434, fellowship code LCF/BQ/DR23/12000026), and S.W. by a fellowship from the Spanish Ministry of Science and Universities (fellowship code PRE2022-101367). D.R.F. was funded by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 850859) and an Eccellenza Professorship Grant from the Swiss National Science Foundation (grant number PCEFP3_187058)., Peer reviewed

Glucocorticoid (GC) hormones have traditionally been interpreted as indicators of stress, but the extent to which they provide information on physiological state remains debated. GCs are metabolic hormones that amongst other functions ensure increasing fuel (i.e. glucose) supply on the face of fluctuating energetic demands, a role often overlooked by ecological studies investigating the consequences of GC variation. Furthermore, because energy budget is limited, in natural contexts where multiple stimuli coexist, the organisms' ability to respond physiologically may be constrained when multiple triggers of metabolic responses overlap in time. Using free-living spotless starling (Sturnus unicolor) chicks, we experimentally tested whether two stimuli of different nature known to trigger a metabolic or GC response, respectively, cause a comparable increase in plasma GCs and glucose. We further tested whether response patterns differed when both stimuli occurred consecutively. We found that both experimental treatments caused increases in GCs and glucose of similar magnitude, suggesting that both variables fluctuate along with variation in energy expenditure, independently of the trigger. Exposure to the two stimuli occurring subsequently did not cause a difference in GC or glucose responses compared with exposure to a single stimulus, suggesting a limited capacity to respond to an additional stimulus during an ongoing acute response. Lastly, we found a positive and significant correlation between plasma GCs and glucose after the experimental treatments. Our results add to the increasing research on the role of energy expenditure on GC variation, by providing experimental evidence on the association between plasma GCs and energy metabolism.