Oscar Gordo

The relative importance of conditions in wintering and passage areas on spring arrival dates: the case of long-distance Iberian migrants

Remote sensing data have been used in previous studies to assess the effects of winter ecological conditions in Africa on biological parameters recorded in bird populations during the following breeding season in Europe. Based on the results of these studies, we hypothesized that a high productivity of vegetation during the winter and, thus, high resource availability, should advance the arrival of long-distance migrants to the European breeding areas due to enhanced ecological conditions. To test this hypothesis, between 1982 and 2000 we examined the first arrival date to the Iberian Peninsula of five species (White Stork, Cuckoo, Common Swift, Barn Swallow and Nightingale) in relation to several explanatory variables: ecological conditions in their African wintering grounds and passage areas, as reflected by the normalized difference vegetation index (NDVI), temperature and precipitation in their passage areas and the winter North Atlantic Oscillation (NAO). Ecological conditions in the wintering areas were important for White Stork, Cuckoo and Barn Swallow phenology, while both NDVI in passage areas and NAO did not have an effect on any species. Migratory birds arrived earlier after winters with high vegetation productivity in Africa. Temperature in passage areas was important for the later species (i.e. Cuckoo, Common Swift and Nightingale), although in all cases the true relevance of this factor was scarce due to the poor explanatory capacity of the models. These species were recorded in the Iberian Peninsula earlier in the spring of those years with warmer temperatures in passage areas. The nexus between African NDVI and arrival phenology is hypothesized through increases in wintering survival rates and/or the faster acquisition of pre-migratory body condition and progression through sub-Saharan areas.

Ecological Impacts of the North Atlantic Oscillation (NAO) in Mediterranean Ecosystems

Large-scale climate indices have received much attention in recent years in ecology-climate research due to the advantages they have over typically used local weather variables, such as temperature or rainfall. In the Mediterranean, the North Atlantic Oscillation (NAO) is a major forcing of climate patterns, especially precipitation. More than 60 studies to date have demonstrated the effects of the NAO on both terrestrial and aquatic Mediterranean ecosystems. In terrestrial ecosystems, the NAO affects the phenology and growth of plants and crop yields. It also affects the condition and diet of mammals and disease-related mortality in amphibians. The effects of the NAO are probably better known in marine ecosystems, where the impact on the hydrodynamics of the water column and currents is felt in the dynamics of populations from plankton to fishes in both pelagic and benthonic environments. Additionally, birds are an especially well studied taxon when it comes to effects of the NAO. The NAO has been shown to affect the population dynamics of water birds by impacting the availability and extent of their habitat and by influencing dispersal decisions of individuals. The NAO plays an essential role in the migration of birds throughout the Mediterranean basin, and it is probably a reason for the observed advance of arrival dates during the spring in Europe. In spite of the notable number of studies carried out to date, we are far from knowing accurately the ecological impacts of the NAO on Mediterranean ecosystems. More efforts are needed to understand regional differences in the NAO effects within the Mediterranean basin and how they compare with more northern latitudes of Europe.

Determining the environmental factors underlying the spatial variability of insect appearance phenology for the honey bee, Apis mellifera, and the small white, Pieris rapae.

Abstract The spatial patterns of the variability of the appearance dates of the honey bee Apis mellifera L. (Hymenoptera: Apidea) and the small white Pieris rapae (L.) (Lepidoptera: Pieridae) were investigated in Spain. A database of more than 7,000 records of the dates of the first spring sightings of each species in more than 700 localities from 1952–2004 was used. Phenological data were related to spatial, topographical, climate, land use, and vegetation productivity explanatory variables by means of multiple regression models in order to search for the environmental mechanisms underlying the observable phenological variability. Temperature and altitudinal spatial gradients accounted for most of the spatial variability in the phenology of the studied species, while vegetation productivity and land use had low relevance. In both species, the first individuals were recorded at those sites with warmer springs and dry summers, at low altitudes, and not covered with dry farming (i.e., cereal crops). The identity and magnitude of the effect of the variables were almost identical for both species and closely mirrored spatial temperature gradients. The best explanatory models accounted for up to half of the variability of appearance dates. Residuals did not show a spatial autocorrelation, meaning that no other spatially structured variable at our working resolution could have improved the results. Differences in the spatial patterns of phenology with regard to other taxa, such as arrival dates of migratory birds, suggest that spatial constraints may play an essential role in the phenological matching between trophic levels.

Wintering forest birds roost in areas of higher sun radiation

We analyze environmental determinants of roost site selection by tree gleaning passerines wintering in a Mediterranean montane oakwood at a craggy area of high variation in altitude and hill-shading pattern. We hypothesize that in temperate latitudes of cold winter climate, birds should spend the night in areas of low altitudes, higher temperatures, and higher solar radiation in order to minimize thermoregulation costs during resting time and to improve foraging conditions just before and after roosting. We study night occupation of woodland locations by the presence of feces in 159 wooden nest boxes (i.e., under identical controlled roosting situations). We employ GIS methods to quantify solar radiation at each location surrounding the nest boxes and data loggers to measure air temperature in the field. Birds prefer to roost in forest patches with higher solar radiation, where the period of light available for foraging is extended and thermoregulation costs during daytime are minimized. They also selected woodland patches with taller trees, a pattern consistent with their foraging preferences for trunks and branches. Other environmental variables played a negligible role in determining the selection of roost sites. Here, we show, for the first time, the importance of sun radiation determining where to spend the night in wintering birds and call attention on considering the thermal space in forest management. Forest management should preserve woodland patches with taller trees more exposed to solar radiation to enhance winter habitat suitability for birds in these Mediterranean oakwoods.

A macroecological perspective for phenological research under climate change

Life cycles of animals and plants worldwide are shifting in response to recent climate change. Macroecology, which deals with biological patterns and processes at a large scale, is ideally suited to address this global phenomenon, thus providing a more general and generalizable understanding of the impacts of climate change on ecosystems. To date, a macroecological approach to examining phenological changes based on historical ground observations remains mostly unexplored, as the phenological studies have been conducted at local scales, while the recent new technologies, e.g., remote sensing, can extend phenological study to global scales. Because phenology is probably the most widely used bioindicator of climate change, thus allowing for studies at the macroscale. Some meta-analyses have indeed employed broad-scale, long-term datasets to estimate overall shift rates in the timing of plant and animal phenological events. However, none has provided a convincing macroecological view of current phenological shifts, and few attempts have been made to make global-scale predictions of phenological responses under future climatic scenarios. Understanding spatial variability and clines may be essential for comprehending phenological variations stemming from future climate change. Here, we discuss how ecological mechanisms discovered in macroecology may provide new insights for understanding spatial variation of sensitivity of phenology to climate change.

Inability of Biometry to Discriminate Iberian and Common Chiffchaffs During the Autumn Migration Period

Summary. The Iberian Chiffchaff Phylloscopus ibericus has recently been accepted as a full species, but is still very difficult to distinguish from the Common Chiffchaff P. collybita when molecular and acoustic evidence is lacking. Distinguishing these cryptospecies is essential for obtaining accurate population estimates for each, which is especially important in the case of the Iberian Chiffchaff due to its restricted distribution. We re-assessed the discriminant methods available in the literature and propose some alternative traits to distinguish both species. We used information from 24 morphological traits measured in c. 6,700 individuals of the two species trapped during the autumn migration period in Doñana National Park, Spain, between 2004 and 2015. Discriminatory methods available in the literature were unable to distinguish the two species efficiently. Despite some biometric differences, morphological measurements showed a high degree of overlap and so could not distinguish between Iberian and Common Chiffchaffs. The best discriminatory factor at our study site was the passage date, since 90% of Iberian Chiffchaffs have already departed when the first Common Chiffchaffs arrive. Those traits associated with migration, such as wing size and shape and fat deposits, were also able to discriminate the species, albeit weakly. This result fully concurs with the trans-Saharan migration of the Iberian Chiffchaff in contrast to the Common Chiffchaff, which chiefly winters in Mediterranean latitudes. In conclusion, biometric traits are useless for discriminating the two species and we do not recommend their use. A comprehensive examination of colouration and plumage seems to be the only reliable way of guaranteeing the correct identification of these species in the hand.

Sexing of Phylloscopus based on multivariate probability of morphological traits

ABSTRACT Sexes cannot be distinguished with certainty by human observers in many avian species. However, some apparently monomorphic species have small but measurable sexual dimorphisms in biometry, which can be used to determine sex. Here, we develop a method based on multivariate probabilities to improve sexing in three Phylloscopus species. We captured 16 124 Common Chiffchaffs (P. collybita), Iberian Chiffchaffs (P. ibericus) and Willow Warblers (P. trochilus) during postnuptial migration in the Doñana National Park (SW Spain). We estimated the probability of them being male or female based on a combination of the density distributions of wing and tarsus lengths. Density distributions were derived as two normal components of the mixture distribution in both traits. Discriminatory ability was compared under different assumptions. 95% of Common Chiffchaffs, 92% of Iberian Chiffchaffs and 87% of Willow Warblers were sexed with 95% confidence. These percentages are greater than those reported for these species using sexing methods based only on wing length. Our method was not affected by changes between years in the degree of dimorphism or mixture of populations from different geographical origins. In the Iberian Chiffchaff and the Willow Warbler, sexing was improved when our method was applied to immatures and adults separately. Sex ratio was estimated to be two females per male in the Common Chiffchaff and the Willow Warbler. In the latter species, females migrated one week later than males. Sex ratio in the Iberian Chiffchaff was 1:1. Our sexing method could be used for other species with known and apparent dimorphism or to any data set of birds with biometric measures.

Are Two Days Enough? Checking the Accuracy of the Survey Protocols Used in Common Bird Monitoring Schemes

Summary. Common bird monitoring schemes have become an important tool in conservation biology because they provide useful information for assessing spatial and temporal variations of bird populations. However, recorded data may suffer from several observational procedures that cause error. In recent years, a robust mathematical framework has been developed to control for potential confounding factors affecting the assessment of the actual spatial and temporal variability of bird populations. Surprisingly few attempts have been made to check the effectiveness of current methodology empirically and thus to determine to what extent monitoring scheme data can provide accurate estimates of actual bird abundances. To check the effect of intra-annual variability of bird counts, I conducted daily surveys of House Martin Delichon urbicum and Common Swift Apus apus populations along a 2-km transect line in Tres Cantos (Madrid, Spain) between March and September 2005. The data recorded perfectly mimicked the information collected every spring by observers participating in the common bird monitoring schemes of Spain. Computer simulations with my data showed that the probability of recording the actual annual maximum of individual numbers was less than 9% by the available protocols. Simulations of protocols with increased survey frequency demonstrated that three or four surveys per year would be enough to increase the accuracy of recorded House Martin abundance significantly. However, at least weekly surveys in spring would be required to produce acceptable annual estimates of the abundance of the Common Swift. My results suggest that two surveys per spring are not enough in the studied species, due to the huge between-day variability of records. Such variability is primarily caused by the nature of the count data and the seasonal patterns of occurrence/ detectability of individuals. Abundance data for swifts and house martins recorded in common bird monitoring schemes should thus be used cautiously due to its potential inaccurary. —Gordo, O. (2018). Are two days enough? Checking the accuracy of the survey protocols used in common bird monitoring schemes. Ardeola, 65: 41–52.