Agustín Mendiburu

Fuel Load and Potential Flight Ranges of Passerine Birds Migrating through the Western Edge of the Pyrenees

Abstract. The estimation of fuel load and the potential flight ranges of migrant birds are crucial to understanding the ecological and evolutionary significance of bird migration strategies. The movement of migrant birds between Iberia and the rest of Western Europe is thought to be shaped by the Pyrenees and nearby seas. Because of this areas unique geography, the routes of migrants that move to (autumn migration) or from (spring migration) Iberia tend to pass through the western and eastern edges of the Pyrenees. Fuel-load analyses and the assessment of potential flight ranges from these edges can provide insights about the extent of Iberias use as a consistent stopover and fuelling area. Using data obtained over a period of six years (2004–2009), we calculated fuel load and potential flight ranges for ten common passerines (pre-Saharan, i.e. that overwinter mainly within the circum-Mediterranean region: Robin Erithacus rubecula, Bluethroat Luscinia svecica, Chiffchaff Phylloscopus collybita, Blackcap Sylvia atricapilla; sub-Saharan, i.e. that overwinter in tropical Africa: Sedge Warbler Acrocephalus schoenobaenus, Reed Warbler Acrocephalus scirpaceus, Melodious Warbler Hippolais polyglotta, Willow Warbler Phylloscopus trochilus, Garden Warbler Sylvia borin, Common Whitethroat Sylvia communis that stopover at Txingudi marshlands, at the western edge of the Pyrenees. The fuel load for the 25% heaviest fraction of caught birds ranged from 10.6% over lean body mass to 25.5% (mean: 18.3%) in spring and from 6.7% to 39.7% (mean: 25.2%) in autumn. Thus, potential flight ranges tended to be longer during autumn than during spring, particularly for the sub-Saharan species. All pre-Saharan species except one (Bluethroat) had sufficient fuel to arrive in southern Iberia or northern Africa, and three sub-Saharan species (Sedge Warbler, Garden Warbler, Common Whitethroat) had sufficient fuel to reach the north of the Sahara Desert but not its southern border. The potential flight ranges in spring were rather similar among species (ca. 1000 km from Txingudi).

A Comparison of Stopover Behaviour of Two Subspecies of the Bluethroat Luscinia svecica in Northern Iberia During the Autumn Migration Period

Summary. Iberia offers one of the last opportunities four migrants to refuel before crossing the Sahara. It is also a destination for populations that overwinter within the circum-Mediterranean region. Our aim was to analyse whether two bluethroat subspecies that stop over in northern Iberia and face different migration distances, show different stopover behaviour in terms of periods of passage, fuel management and stopover duration. The subspecies were Luscinia svecica namnetum, which overwinters within the circum-Mediterranean region; and L. s. cyanecula, some of which migrate to tropical Africa. Overall, we obtained 265 captures, involving 208 distinct bluethroats and 56 recaptures, at the Txingudi marshlands in northern Iberia, during the autumn migration periods of 2007 and 2008. Passage periods did not differ between both subspecies, nor did fuel load (body mass: 15.0 g; fat: 1.0), mass deposition rate (0.07 g/day) and stopover duration (15.3 days). However, late L. s. cyanecula showed higher fuel loads than early individuals, so these birds could potentially cover longer distances without needing to refuel. Mean fat score was low for both subspecies which, together with the very low rate of fuel accumulation, supports the hypothesis that Iberia is crossed in consecutive short steps. Long-distance L. s. cyanecula migrants could acquire the high fuel loads needed to migrate to tropical Africa in southern Iberia or north-western Africa.

Monitoring communities of small birds: a comparison between mist-netting and counting

Capsule Counting and mist-netting provided different estimates of abundance. Aims To compare the efficiency, constraint and bias of mist-netting versus line transects for the estimation of species richness and abundance of passerines. Methods Mist-nets (126 linear metres) placed crossing a 420 m-long hedgerow line, open for four hours starting at dawn, four days per month from June 2006 to May 2007. During this same period, we conducted a transect parallel to a 1.3-km hedgerow line, four times per month. Results Estimation of species richness did not vary between methods, even when controlling for ecological groups, whereas the abundance estimations did vary. Conclusions Overall, line transects are better than mist-netting for estimating species richness and abundance since they are less costly, less invasive, and less time-consuming. However, if fine ecological analyses are required, including species abundance, mist-netting is preferred for solitary passerines that feed on insects and forage in the foliage, and line transects are better for gregarious passerines that feed on seeds or forage on the ground.

Deteriorating weather conditions predict the use of suboptimal stopover sites by Aquatic Warblers Acrocephalus paludicola

Abstract. The Aquatic Warbler Acrocephalus paludicola is a globally threatened songbird and its decline is related to habitat loss. Accordingly, most studies dealing with the stopover ecology of this species have been chiefly focused on the habitat use and the availability of suitable habitats along its route of migration. In contrast, much less attention has been paid to other environmental causes potentially explaining the use of stopover sites. Our aim here was to investigate whether the Aquatic Warbler at an apparently suboptimal stopover site with small area of suitable habitats stops over only during adverse weather conditions. We used data obtained at a suboptimal (Jaizubia marshland, northern Iberia) and another optimal (Villefranque, southwestern France) stopover sites during the autumn migration over four seasons (2007–2010). The Aquatic Warbler tended to stop over at Jaizubia in days with rain, a fact that was not so evident at Villefranque, supporting the hypothesis that they used the suboptimal site only, or mostly, when adverse weather conditions forced them to land. In contrast, the optimal habitat was used independently of weather conditions. To properly identify key stopover localities for the Aquatic Warbler, we should consider the potential influence of adverse weather in occurrence of individuals, especially in small areas without preferred habitat.

Wintering survival of insect-eating passerines in southern Europe

Capsule Intra-year survival during the winter is high for some insect-eating passerines in northern Iberia. Aims To estimate intra-year survival of insect-eating passerines wintering in northern Iberia. Methods Capture–recapture data from four passerines (European Robins Erithacus rubecula; Common Chiffchaffs Phylloscopus collybita; Firecrests Regulus ignicapillus; and Dunnocks Prunella modularis) captured during four consecutive winters were analyzed using Cormack–Jolly–Seber models. Results Intra-year survival during the winter differed between species, possibly revealing different survival or wintering strategies. Thus, Dunnocks and Firecrests showed survival rates close to 1, whereas Chiffchaffs and Robins had lower survival rates (0.4 and 0.3–0.6, respectively). Transients were detected only in Dunnocks (60%). This strategy in which some birds are resident and others are vagrant is already known for other species, particularly those that feed on plants or change to a more vegetarian diet during the winter. Conclusions Northern Iberia, and particularly its coast, is a good wintering area for several insect-eating species.

Common Kingfishers Alcedo atthis along the Coast of Northern Iberia during the Autumn Migration Period

Iberia receives many dispersing, migrating and wintering Common Kingfishers Alcedo atthis. The majority of these individuals occur in coastal marshes. Between August and mid-October 2007 and 2008, Kingfishers were mist-netted at a tidal marsh in Txingudi, northern Iberia. They were especially abundant from August to September, indicating that Txingudi receives an influx of migrants from northern Europe or local birds dispersing from nearby natal areas. The population mainly consisted of juveniles (only 5.8% adults), the sex ratio was 1:1 and the timing of passage was similar between the sexes. Body mass controlled for body size was constant with time. The stopover duration was affected by sex, date (month of first capture) and a body mass/size ratio, here used as a proxy of fuel load. Females with a lower body mass/size ratio were more likely to remain for longer than those with higher ratios, but the opposite was found in males. Kingfishers captured in August were more likely to be already settled in the area than those first captured in September. Moreover, birds with short stopovers systematically lost mass, while birds that remained in the area for longer did not. These findings are discussed in the light of possible competition between newly arriving and already settled birds.