Breeding ground temperature rises,more than habitat change,are associated with spatially variable population trends in two species of migratory bird

Habitat loss and climate change are key drivers of global biodiversity declines but their relative importance has rarely been examined. We attempted to attribute spatially divergent population trends of two Afro-Palaearctic migrant warbler species, Willow Warbler Phylloscopus trochilus and Common Chiffchaff Phylloscopus collybita, to changes in breeding grounds climate or habitat. We used bird counts from over 4000 sites across the UK between 1994 and 2017, monitored as part of the BTO/JNCC/RSPB Breeding Bird Survey. We modelled Willow Warbler and Common Chiffchaff population size and growth in relation to habitat, climate and weather. We then used the abundance model coefficients and observed environmental changes to determine the extent to which spatially varying population trends in England and Scotland were consistent with attribution to climate and habitat changes. Both species' population size and growth correlated with habitat, climate and weather on their breeding grounds. Changes in habitat, in particular woodland expansion, could be linked to small population increases for both species in England and Scotland. Both species' populations correlated more strongly with climate than weather, and both had an optimum breeding season temperature: 11°C for Willow Warbler and around 13.5°C for Common Chiffchaff (with marginally different predictions from population size and growth models). Breeding ground temperature increases, therefore, had the potential to have caused some of the observed Willow Warbler declines in England (where the mean breeding season temperature was 12.7°C) and increases in Scotland (mean breeding season temperature was 10.2°C), and some of the differential rates of increase for Common Chiffchaff. However, much of the variation in species' population abundance and trends were not well predicted by our models and could be due to other factors, such as species interactions, habitat and climate change in their wintering grounds and on migration. This study provides evidence that the effect of climate change on a species may vary spatially and may switch from being beneficial to being detrimental if a temperature threshold is exceeded.     

Wing shape variation of Finnish and Central European Willow Warblers Phylloscopus trochilus and Chiffchaffs P. collybita

Wing shape variation of European Willow Warblers Phylloscopus trochilus and Chiffchaffs P. collybita was studied using indices calculated from wing formulae. Our data were from free-living local populations of P. t. acredula and P. c. abietinus from southern Finland, and P. t. trochilus and P. c. collybita from southwestern Germany.
There were no significant shape differences between the subspecies of the Willow Warbler in which sexual dimorphism was pronounced. The subspecies of the Chiffchaff were significantly different while the sexes were not. There were also significant differences between adult and immature individuals in the autumn. The intrapopulation variation must be considered in attempts to recognize different subspecies or populations in data on birds caught during migration.
In the Willow Warbler at least, the difference between age-groups was larger in males than in females. If the change in wing formula due to the complete pre-nuptial moult is similar in both sexes, juvenile males intermediate between adult females and adult males suffer disproportionately high mortality. It was earlier suggested that sexual selection increases body size in Willow Warbler males, but the present results imply additional selection pressures for increasing sexual dimorphism.     

Woodland area,species turnover and the conservation of bird assemblages in lowland England

A study over 4 years into the number of breeding bird species and species turnover (extinctions and colonisations) in relation to area was conducted in 35 woodlands, set in an intensively farmed landscape, in north-east Essex, UK. A total of 46 species was recorded. The number of species breeding increased with woodland area; the slope of the species–area relationship did not differ between years. Habitat diversity was the only other measured variable to influence species richness. Absolute species turnover was independent of woodland area but relative turnover declined with increase in woodland area. The numbers of territories of nine species were determined. For four summer visitors the number of woods occupied increased as the overall populations increased but, for the other species, changes in overall population size led to changes in numbers in occupied woods. Chaffinch Fringilla coelebs and Song Thrush Turdus philomelos were more associated with woodland edges, Nightingale Luscinia megarhynchos, Garden Warbler Sylvia borin, Chiffchaff Phylloscopus collybita and Willow Warbler P. trochilus with interiors. Several species showed an inverse relationship between population density and woodland area. Collections of small woods hold similar species richness to single large woods. While the acquisition of large woods for conservation purposes should be a priority, the extension of smaller woods to a size of about 10 ha would be highly beneficial to both the species richness and population stability of regional woodland bird assemblages.     

A preliminary assessment of the habitat selection of two Palaearctic migrant passerine species in West Africa

We investigated the habitat selected by two Palaearctic migrants (Pied Flycatcher, Ficedula hypoleuca, Willow Warbler, Phylloscopus trochilus) in a patchy landscape in Ivory Coast and compared it with the habitat selection of Afrotropical species in the same foraging guilds. Transect counts were used to test the hypothesis that migrants use more open and more seasonal habitats and have a broader use of habitats compared with resident species. Habitats compared were, in order of decreasing tree density, gallery forest, an isolated forest and bush/tree savanna. The isolated forest had the most pronounced seasonal changes (deciduous trees) and was the one with the most diverse vegetation structure. The habitat where both migrants were most frequent was the isolated forest, and thus occurred in the habitat with the most pronounced seasonal change. Diversity of habitats selected was highest in migrants but in the Pied Flycatcher this was possibly an artefact due to subdominant individuals being excluded from the preferred habitat by territorial birds. Potential competition for habitat with Afrotropical species was found to be low.     

The presence of non-territorial males in Willow Warbler Phylloscopus trochilus populations–a removal study

A total of six (unmarked) and 24 (individually colour-ringed) male Willow Warblers Phylloscopus trochilus were removed from their territories in two study areas: subalpine birch and lowland mixed forest, respectively. The removals were made during three separate periods throughout the breeding season: (1) pair-formation and egg-laying; (2) early incubation; and (3) middle and second half of the incubation period. During Period 1 all of the males removed ( n = 10) were quickly replaced by new males, while during Period 2 eight of the 11 males removed were replaced. The new males defended basically the same territorial borders as the males they replaced. During Period 3 only one of the nine males removed was replaced. Since the replacements occurred within a few hours of the removals, it is suggested that the new settlers were non-territorial males that were already present in the respective areas before the experimental removals were made. The mean wing-lengths of the two groups of males indicated that the initial occupants of the territories were 2 years old or older in both study areas, whereas the new settlers in the subalpine birch forest (mean Willow Warbler density: 55 territories/km²) were 1-year-old birds, and two years old, or older, in the mixed forest area (mean Willow Warbler density: 133 territories/km²).     

Reaktionen von Zilpzalp und Fitis (Phylloscopus collybita, Ph. trochilus) auf verschiedene Gesangsformen des Zilpzalps

Zusammenfassung 1. Die mitteleuropäische, die spanische und die kanarische Gesangsform des Zilpzalps sind voneinander klar verschieden.2. Die Zilpzalp- von Teneriffa (kanarische Inseln) und aus Mitteleuropa reagieren auf ihre vom Tonband vorgespielten Gesänge wechselseitig wie verschiedener Arten, d. h., nur einzelne werden angelockt.3. Zilpzalp- aus Mitteleuropa verhalten sich gegenüber der spanischen Gesangsform des Zilpzalps wie gegenüber artfremdem Gesang. Spanische Zilpzalp- werden von Gesangsformen des Zilpzalps aus Mitteleuropa und den kanarischen Inseln zu einem hohen Prozentsatz angelockt (86,6 und 66,6%).4. Die Zilpzalp-Populationen in Mitteleuropa, Spanien und auf den kanarischen Inseln befinden sich vermutlich auf verschieden weit fortgeschrittenen Stufen der Artaufspaltung.5. Interspezifische Kontrastbetonung ist wahrscheinlich nicht die Ursache für die großen Unterschiede zwischen dem Gesang des Fitis und des mitteleuropäischen Zilpzalps.

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Reactions of the Chiffchaff and Willow-Warbler to different song forms of the Chiffchaff

Summary 1. The song forms of the Chiffchaff in Central Europe, in Spain and on the Canary Islands are clearly distinct from each other.2. Chiffchaff males of Tenerife (Canary Islands) and Central Europe react to the playback of each other songs in the same way, as to songs of other species, i. e. only a few males being attracted.3. Chiffchaff males of Central Europe behave in the same way to the Spanish song form of the Chiffchaff as to songs of another species. A high % of Spanish Chiffchaff males are attracted to song forms of the Chiffchaff of Central Europe (86,6) and of the Canary Islands (66,6).4. Chiffchaff populations in Central Europe, Spain and on the Canary Islands should stay split as they are different evolutionary stages.5. Interspecific contrast reinforcement is probably not the cause of the differences between the song of the Willow Warbler and the song of the Chiffchaff of Central Europe.

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Mit Unterstützung der Deutschen Forschungsgemeinschaft.     

Microhabitat use and feeding strategies of the Pied Flycatcher and the Willow Warbler in their West-African winter quarters compared with resident species

Salewski, V. 2000. Microhabitat use and feeding strategies of the Pied Flycatcher and the Willow Warbler in their West-African winter quarters compared with resident species. Ostrich 71 (1 & 2): 191–193.

Habitat choice, microhabitat and foraging behaviour of the palaearctic Pied Flycatcher and Willow Warbler are described in their West-African wintering areas and are compared with those of resident species. The migrants were more flexible in habitat choice and foraging techniques, but in general did not feed in more open habitat.     

An example of character release in host selection and egg colour of cuckoos Cuculus spp. in Japan

We have studied in northern Japan the hitherto unidentified eggs of a species of cuckoo in the nests of the Bush Warbler Cettia diphone . The cuckoo in question appeared to be the Himalayan Cuckoo Cuculus saturatus which parasitizes mainly the Willow Warbler Phylloscopus occipitalis in southern Japan. The egg colour in this northern Cuckoo was chocolate-brown or orange-brown, similar to that of the Bush Warbler but unlike that of the southern Himalayan Cuckoo. Egg size was significantly larger than that of the southern Himalayan Cuckoo and instead similar to that of the Little Cuckoo C. poliocephalus which uses the same host species in southern Japan, to which the Little Cuckoo is confined. The shift in host species and egg colour in the northern Himalayan Cuckoo seems to be a case of character release in the absence of the Little Cuckoo.     

Population limitation in Palaearctic-African migrant passerines

We report the results of key factor analyses for Sedge Warbler Acrocephalus schoenobaenus, Blackcap Sylvia atricapilla, Whitethroat Sylvia communis and Willow Warbler Pftylloscopus Irochilus based on extensive data from the United Kingdom gathered by the British Trust for Ornithology. These results are compared with those from published key factor analyses based on intensive studies of Swallows Hirundo ruslica, Redstarts Phoenicurus phoenicums and three populations of Pied Flycatchers Fiiedula hypoleuca. Variation in losses between fledging and the following breeding season lk₄ ) accounted for most of the variation in total losses (K_T) for all populations. Breeding performance was poorly correlated with population changes. Density-dependence was found mainly in 6₄, its strength decreasing as average rate of population change increased. There was also evidence of weak density-dependence in the clutch size of Swallows and in Willow Warbler egg losses. Fluctuations in k₄ were correlated with conditions on the wintering grounds in Sedge Warblers, Whitethroats and Swallows, for the first two species these relationships being confirmed by analyses of independent survival estimates based on mark-recapture data. Populations of Sedge Warblers and Whitethroats appear to be limited by competition for resources on the wintering grounds. Removal experiments suggest that Willow Warblers, Pied Flycatchers and perhaps Blackcaps are limited through recruitment to the breeding population, in the case of Pied Flycatchers this limitation being in relation to the availability of nest sites.     

Willow Warbler Phylloscopus trochilus habitat in woods with different structure and management in southern England

Capsule Use of Light Detection and Ranging (LiDAR) data identified suitable Willow Warbler habitat based on mean vegetation height. This habitat model provided maps of distribution and occupation of suitable habitat.

Aims To identify habitat associations in woods with different vegetation structure and management systems during a period of low Willow Warbler populations.

Methods Locations of all Willow Warblers were mapped during the breeding season in three woods of contrasting management; recent low intervention, actively coppiced woodland and high forest with clear‐fells. Height profile models of each wood were derived from airborne LiDAR. The mean vegetation height at locations with Willow Warblers and a sample from the rest of the wood were used to produce models of optimum habitat and breadth of habitat occupied in each wood. The habitat model was then used to produce maps of suitable habitat.

Results The habitat models did not differ between woods, with highest probability of Willow Warbler occurrence in mean vegetation heights of 3.7–5.3 m. Habitat of heights 6–11 m appeared less suitable, being only partly occupied. Habitat maps showed that habitat of suitable height was only occupied when it occurred as large patches; smaller patches (mostly <0.5 ha) and edges along rides and fields were not used.

Conclusion The use of LiDAR derived measures of vegetation height identified areas of suitable habitat for Willow Warblers. Willow Warblers occupied areas of low mean vegetation height either as early successional or open canopy woodland in all woods. Height‐based habitat maps can identify areas of suitable habitat within larger expanses of heterogeneous woodland and are a potentially useful tool in assessing changes in extent of what are often temporary patches of habitat.     

Canopy manipulations of exotic Bitter Willow (Salix elaeagnos) forest for indigenous seedling recruitment: A pilot study

The invasive exotic tree species Bitter Willow (Salix elaeagnos; Salicaceae) has colonised areas of rank exotic grassland and has been found to contain indigenous seed, dispersed by frugivorous birds into the monospecific stands. This small pilot study examined whether indigenous seedlings that have germinated in the understorey of exotic Bitter Willow stands could be stimulated to establish through the creation of small‐scale canopy gaps. In Bitter Willow forest, four single Bitter Willow trees were poisoned to create canopy gaps. Light transmission and seedling regeneration of tree and shrub species were assessed beneath both the four manipulated and three comparable intact Bitter Willow canopies. Over 3 years, seedling height and density increased more beneath opened compared to intact Bitter Willow canopies. These results suggest that Bitter Willow can fill the roles of both a facilitative nurse and a perch tree. Larger‐scale canopy manipulation experiments of both Bitter Willow and other Salix species are needed to determine the full potential of canopy manipulations for forest restoration.     

Woodland structure,rather than tree identity,determines the breeding habitat of Willow Warblers Phylloscopus trochilus in the northwest of England

Capsule Woodland structure, rather than tree species, is the most important determinant of breeding habitat selection by Willow Warblers in North West England.

Aims To examine how habitat characteristics predict the occurrence of male Willow Warbler territories.

Methods Woodland structure (trunk density, trunk diameter, canopy cover and understory cover), tree species and food abundance were compared between woodland areas within and outside of male territories at a site in the UK.

Results Territories contained higher trunk numbers, had a narrow range of trunk diameters, and intermediate canopy cover. Food abundance did not differ with occupancy. Willow and alder were the most common trees within territories, in contrast to birch which has been found in previous studies. The habitat structure matches young woodlands, where birches often grow. However, at the study site the birches were large and mature, and therefore unsuitable. Moreover, woodland structure variables were better predictors of occupancy than any particular tree genera.

Conclusion The results indicate that vegetation structure, but not tree species or food availability, influence breeding habitat selection by Willow Warblers. The preferred structure is similar to coppice woodlands; therefore, the Willow Warbler decline may be linked to the loss of this traditional management across south England.     

DIFFERENTIATION OF FORAGING NICHE AMONG TITS, PARUS SPP., IN NORWAY DURING WINTER

The winter foraging behaviour of Willow Tit Parus montanuss, Coal Tit P. ater and Crested Tit P. cristatus was studied in several spruce-dominated, coniferous forest areas in which either Willow Tit alone, Willow Tit and Coal Tit, or all three species together were present. In the areas where Crested Tits were absent, Willow Tits foraged significantly more often in the outermost parts of spruce branches and less frequently on the parts of branches closer to the trunk, than in the area in which Crested Tits were present. In the area in which all three species were present, the same change in foraging pattern of Willow Tits was found in mixed flocks in which Crested Tits were lacking. The foraging pattern of Coal Tits was unaffected by the presence of members of either or both the other two tit species. It is suggested that the expansion of the feeding niche of Willow Tit in the absence of Crested Tit is a result of ecological release, due to a reduction in the degree of interspecific competition.     

Long-term trends in the arrival dates of spring migrants

Data on the spring arrival dates of 23 species of migrants in Leicestershire over a 50-year period are presented. Chiffchaff, Sand Martin, Blackcap and Sedge Warbler showed a significant trend towards earlier arrival over the period, while Tree Pipit, Cuckoo, Whinchat, Whitethroat and Garden Warbler showed a significant trend towards later arrival. Fifteen species arrived noticeably earlier in the 1940s, a period of warm springs, while several species showed earlier arrivals in the 1980s. A number of species showed later arrival dates in the 1960s and 1970s, when April temperatures were colder than average. Several species showed significant correlations between arrival date and temperature. Arrival dates of the earliest species were much more variable than those arriving later, while species arriving in the second half of April showed a generally synchronous arrival. The results are discussed in the context of global warming.     

Is earlier spring migration of Tatarstan warblers expected under climate warming?

We analysed data on the arrival dates of four species of leaf warbler (genus Phylloscopus) collected in Tatarstan between 1957 and 2004. There was no evidence over the whole period that the warblers returned to their breeding sites significantly earlier, mainly because local temperatures for April and May, months when the majority of birds arrived from their wintering grounds, did not increase significantly. However, arrival dates of two species (Chiffchaff P. collybita and Willow warbler P.trochilus) were strongly related to local temperature in April, and that of Greenish warbler P. trochiloides to that in May. As expected, arrival dates for the three species wintering in Africa (Chiffchaff, Willow and Wood warblers P. sibilatrix) correlated positively with one another (P < 0.01 in all cases), but were not correlated with arrival dates of Greenish warbler (P > 0.5 in all cases), a species wintering in the Indian sub-continent.     

Estimating fat and protein fuel from fat and muscle scores in passerines

Fat is the prime energy source for birds during prolonged exercise, but protein is also catabolized. Estimates of the amount of catabolizable fat and protein (termed fat and protein fuel) are therefore important for studying energetics of birds. As fat and protein fuel can only be measured by sacrificing individuals or by use of technically complex methods, scoring systems were invented to estimate fat and protein fuel of birds in the field. The visible subcutaneous fat deposits and the thickness of the flight muscles are each scored on an ordinal scale but these scales do not correspond linearly to fat and protein fuel within species, which is needed for analyses such as flight range estimates. We developed an anova ‐type model to estimate fat and protein fuel from fat scores (FS) and muscle scores (MS) along with total mass and a size measurement. Using data from 11 337 individuals of eight passerine species (Common Nightingale Luscinia megarhynchos, Eurasian Reed Warbler Acrocephalus scirpaceus, Melodious Warbler Hippolais polyglotta, Willow Warbler Phylloscopus trochilus, Orphean Warbler Sylvia hortensis, Garden Warbler Sylvia borin, Common Whitethroat Sylvia communis, Subalpine Warbler Sylvia cantillans) mist‐netted in Mauritania, West Africa, we tested for independence of FS and MS and for variation in the relationship between scores and associated mass in response to physiological state. FS, MS and third primary length (size) explained variation in body mass of all eight species analysed (R²: 0.56–0.77). The parameter estimates of the model showed that fat and protein fuel increased monotonically with increasing fat and muscle scores. In two species we found small differences in the estimates between physiological states (seasons). We evaluated our model by comparing the predicted body mass of birds with both FS and MS equal to 0 with the mean body mass of individuals mist‐netted with both scores equal to zero. The values were very close. The amount of fat extracted from dead Garden and Willow Warblers was within the range of predicted fat fuel derived from the model. We conclude that our model is a useful non‐invasive method to estimate simultaneously mean fat and protein fuel of small passerines and we provide recommendations on its use.     

SONG STRUCTURE AND SINGING BEHAVIOUR OF WILLOW WARBLER PHYLLOSCOPUS TROCHILUS ACREDULA IN POPULATIONS OF LOW AND HIGH DENSITY

ABSTRACT

Song structure and singing behaviour in two nesting populations of Willow Warbler Phylloscopus trochilus acredula of different density were studied in Moscow and Kostroma Regions, Russia, from 1994–1997. In total 520 songs of 9 males from the low-density population and 566 songs of 12 males from the high density population were analysed. Singing periods in the high density Willow Warbler population (11 males, 758 min.) were longer (58% of the time) than in the low density population (13 males, 866 min.) (58%). In the high density population, 51% of singing time consisted of “singing duels” (singing in gaps between the songs of a neighbouring male, and singing during the singing of a neighbouring male), while in the low density population only 7% did. The initial phrases of Willow Warbler songs were significantly longer in the high density population, with up to 9–13 elements, while there were no occurrences of this length in the low density population. Songs as a whole were longer and more variable in the high density population. Thus changes in structure, singing activity and behaviour are linked to male-male interaction and density of the nesting community.     

Temporal and spatial variation in the occurrence of Palearctic warblers around Lake Victoria

Goudswaard, K. & Wanink, J.H. 2000. Temporal and spatial variation in the occurrence of Palearctic warblers around Lake Victoria. Ostrich 71 (1 & 2): 210–212.

Only one Palearctic warbler species, the Sedge Warbler, wintered at the south-eastem shores of Lake Victoria. Willow Warbler and Garden Warbler occurred mainly on passage, with peak values in the first lunar quarter, the period in which lakeflies would normally swarm. Higher warbler densities have been reported from the north-western shores. This agrees with our idea that the prevailing eastern winds will result in a higher availability of lakeflies to warblers at the western side of the lake.     

Archaic and contemporary topographic diversification of Upper Mississippi River forests

Topographic diversity is an important component of environmental heterogeneity. Topographic diversity within the Upper Mississippi River floodplain has been degraded because of modifications for navigation improvement. Efforts aimed at restoring topographic diversity in the Upper Mississippi River floodplain have been extensive but have not focused on reversing the effects of forest loss and degradation. We investigated habitat features associated with Cerulean Warbler (Setophaga cerulea) locations both within and outside of river floodplains and hypothesized this species would select topographically diverse habitats. Both topographic diversity and the distance to the upland forest/floodplain forest interface were useful predictors of Cerulean Warbler presence. We conclude that incorporation of topographic diversity into floodplain forest restoration planning would likely benefit Cerulean Warblers and the other species with similar habitat requirements. Incorporating topographic diversity into floodplain forest conservation planning will be challenging on major rivers that serve multiple purposes.