Bird Study (2008) 55, 290–296
Selection of foraging habitat and nestling diet by Meadow Pipits Anthus pratensis breeding on intensively grazed moorland DAVID J.T. DOUGLAS1,4, DARREN M. EVANS2,3* and STEPHEN M. REDPATH2 1School
of Biological Sciences, University of Wales – Bangor, Deiniol Road, Bangor, Gwynedd, LL57 2UW, UK, 2NERC Centre for Ecology and Hydrology, Hill of Brathens, Banchory, Aberdeenshire, AB31 4BW, UK, 3School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 1UG, UK and 4BTO Scotland, Cottrell Building, University of Stirling, FK9 4LA, UK
Capsule Foraging sites with low vegetation height and density, but with high arthropod biomass, are selected. Aims To test the hypothesis that on intensively grazed moorland, breeding Meadow Pipits forage for nestling food where arthropod prey are most readily available, and therefore that foraging site choice is a function of prey abundance and vegetation structure. Methods Observations of adults provisioning nestlings were made from hides positioned close to 19 nests within grazed, 3.3-hectare experimental plots at Glen Finglas, Scotland. Vegetation height and density and arthropod abundance from mapped foraging sites were compared with control sites. Prey items fed to nestlings were quantified and compared with their relative abundance. Results Meadow Pipits selected foraging sites with significantly lower vegetation height and density, but with significantly higher arthropod biomass. Our data suggest that within foraging sites, Meadow Pipits select particular prey types to provision nestlings, in particular, Lepidoptera larvae, adult Tipulidae and Arachnida. Conclusions In intensively grazed upland systems, it appears that Meadow Pipits select foraging sites that optimize total food abundance and accessibility. In order to understand how anticipated changes to livestock farming in Europe will affect grassland birds, we recommend that future studies should investigate the foraging and vigilance behaviour, diet composition and breeding success of a variety of bird species provisioning nestlings under a range of livestock management scenarios.
The loss and degradation of grassland habitats due to intensive livestock grazing has been considered a primary cause of the severe population declines in grassland birds worldwide (Fuller & Gough 1999, Vickery et al. 1999). In the UK, severe grazing pressure, especially by sheep, has been blamed for deleteriously affecting vegetation and birds in many upland regions (Evans et al. 2006a, Fuller & Gough 1999, Thompson et al. 1995). The total number of sheep in the UK has increased by over 50% since 1976 (Vickery et al. 2001) and high grazing pressure has been implicated in widespread population declines of Black Grouse Tetrao tetrix, Willow Ptarmigan (Red Grouse) Lapogus lapogus, Eurasian Dotterel Charadrius morinellus and Rock *Correspondence author. Present address: School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK. Email:
[email protected]
© 2008 British Trust for Ornithology
Ptarmigan Lapogus muta (Baines & Hudson 1995, Thompson & Brown 1992). Livestock grazing alters habitat structure and thus the suitability of the sward for nesting and feeding birds. Vegetation structure may influence foraging site selection as it can affect predation risk and foraging efficiency (Butler & Gillings 2004, Whittingham & Evans 2004). The foraging behaviour of many grassland birds therefore is highly influenced by sward height, which modifies prey availability and detectability, and hence grazed areas are often preferred by arthropodfeeders (Vickery et al. 2001). Too much or too little grazing, on the other hand, might lead to reduced food availability and thus be detrimental to birds (Evans et al. 2005b). In many avian species, provisioning of young in the nest results in the highest energy expenditure in the annual life cycle (Bryant 1988, Drent & Daan 1980).
Foraging by breeding Meadow Pipits
The choice of food provided to nestlings by the parents can have a substantial influence on nestling growth, condition at fledging and subsequent survival (Wright et al. 1998). Moreover, stress caused by reduced food resources can significantly affect parental fitness by lowering their probability of survival or impairing their subsequent production of offspring (Partridge & Harvey 1988). Meadow Pipits Anthus pratensis are multi-brooded, ground-nesting insectivores and are a major source of food for birds of conservation concern such as Hen Harriers Circus cyaneus and Merlin Falco columbarius (Vanhinsbergh & Chamberlain 2001). High grazing pressure could partly explain why the national breeding population of Meadow Pipits declined by 34% between 1970 and 2004 (Eaton et al. 2006). Recent experimental work supports this hypothesis. High grazing pressure negatively affected egg size and abundance of Meadow Pipits in the Scottish uplands (Evans et al. 2006b). In this paper we explore how high grazing pressure influences foraging site selection and the type of food selected within foraging sites. Despite being the most common upland passerine in the UK, there have been relatively few foraging studies of this species during the nestling period. Seel & Walton (1979) found that Meadow Pipits increased their foraging range when collecting food for chicks. However, they did not investigate the factors influencing foraging site selection. For example, it is not known whether Meadow Pipits actively forage in patches where certain prey items are at a higher density compared to surrounding areas, or whether they choose to forage in patches that offer easier access to, and detection of, prey. By investigating foraging site selection and prey choice on intensively grazed moorland, we test the hypothesis that Meadow Pipits will forage for nestling food where arthropod prey are most readily available, and therefore that foraging site choice will be a function of both prey abundance and vegetation structure. METHODS Study area
Fieldwork was carried out from 27 April to 19 July 2003 at Glen Finglas (56°16′N, 4°24′W) in the Loch Lomond and Trossachs National Park, Scotland, UK. The site is a typical grazed rough grassland estate of 4039 ha and between 200 m and 500 m in altitude. The
291
dominant vegetation types are Molinia caerulea– Potentilla erecta mire, Juncus effususlacutiflorus–Galium palustre rush pasture and Festuca oviana–Agrostis capillaries–Galium saxatile grassland communities (Rodwell 1991, 1992). Parts of the estate are currently being used for an experimental study using four different stocking densities of sheep and cattle (Evans et al. 2005b). These areas are fenced off into individual plots (each of 3.3 ha), with each treatment having six replicate plots. For this study, we used nests located within the ‘commercially grazed’ plots (i.e. plots with a commercial stocking rate of approximately 3 ewes/ha). Nest location and sample size
Within each plot, Meadow Pipit nests were located either by flushing incubating females or by watching adults with food returning to their nest. Nest position was recorded using a global positioning system (GPS, accurate to ±5 m) and the site marked with a cane. Nineteen nests were selected that encompassed the full duration of the breeding season (watched between 30 May 2003 and 19 July 2003). Nest watches
Observations of adult foraging behaviour were made in dry weather between 09:00 and 12:00 hours from within a hide positioned 10 m from the nest and erected the day before the watch. Watches were conducted when nestlings were between four and 11 days. On arrival at the hide, observations commenced once the adults had ceased anxiety calling. Adults were observed using a pair of 8 × 40 binoculars and the location of each foraging site was marked on a sketch map of the surrounding area. Research by other scientists at Glen Finglas meant the majority of females were individually marked with colour rings, facilitating observations of individuals. Birds generally foraged by landing at a site and then walking along the ground for some distance, before returning to the nest with food. When this occurred, the spot where they were last observed was recorded as the foraging site. Each time an adult returned to the nest, any items of food in the bill were identified using a telescope (×30 magnification). The length of each item was estimated to the nearest 5 mm (using bill length as a yardstick; mean bill length for adults trapped at the study site was 11.5 ± 0.4 mm, n = 30, D.M. Evans, unpubl. data) to allow the biomass of each item to be calculated (see below). © 2008 British Trust for Ornithology, Bird Study,
55, 290–296
292
D.J.T. Douglas, D.M. Evans and S.M. Redpath
It was critical to match the food items to the site where they had been collected, therefore watches were conducted until four foraging sites had been mapped, with the type and size of each food item correctly assigned to these sites. This normally took 1–1.5 hours. Due to time constraints, only one watch was conducted on each nest. At the end of each observation, the position of each foraging site was marked as a 2 × 2 m square. Each foraging site was paired with a control site (where adults were not observed to forage). Control sites were selected at the same distance from the nest as their paired foraging site and at a random distance up to 100 m (determined from a computer-generated list) in an arc away from the foraging site. Vegetation and arthropod sampling
After each nest observation, three procedures were carried out at each foraging and control site: sweepnetting (for foliar and flying arthropods), turf-stripping (for ground- and soil-dwelling arthropods such as Tipulid larvae) and vegetation measurements. To minimize the resulting bias, sweep-netting was conducted first, then vegetation measurements, and then turf-stripping. Each foraging site was sweepsampled in a standard pattern. Eight evenly spaced sweeps were made through the vegetation whilst walking through each half of a foraging site. This procedure was then repeated to give a total of 32 sweeps for each foraging site. Samples were frozen as soon as possible the same day, for processing at a later date. Seven measurements of sward height (to the nearest 5 cm) were made at each site using a measurement stick marked with bands at 5 cm intervals. Sward density was also recorded at the same seven points, by looking down the length of the measuring stick into the vegetation and recording the lowest 5 cm band that was visible. Because of the importance of soil-dwelling Tipulid larvae in Meadow Pipit diet (Evans et al. 2005a), three turf samples were taken from within each foraging site. A spade was used to cut a standard sized square of turf, 2 cm deep, from within the foraging site. The depth of soil sampled was primarily assumed to reflect arthropods potentially available to Meadow Pipits, but also took into account possible variation in depth across the turf sample. The turf was then turned vegetation-side down onto a tray and the soil tapped several times with the spade to encourage break-up of the soil and bring any arthropods to the surface. The soil and vegetation was then broken up by hand, and any arthropods © 2008 British Trust for Ornithology, Bird Study,
55, 290–296
extracted were placed (using forceps) into a tube with 70% ethanol. Each individual animal from the sweep-net and soil samples was identified to the group level, and the length measured to the nearest millimetre using a dissecting microscope. We also measured and weighed a sample of arthropods (n = 331) collected randomly in the field (and throughout the season) encompassing each of the major arthropod groups identified from the sweep-net and soil samples. The animals were dried in an oven at 45°C for six days, after which we measured biomass. We then estimated the dry biomass of each item observed in the chick diet and in the samples based on length measurements using regression equations (Douglas 2003). Total soil invertebrate biomass for each 2 × 2 m square was subsequently calculated. When estimating the biomass of food items provisioned to chicks, the estimations of length obtained during the nest watches were used in the calculations. Data analysis
The biomass of each arthropod Order was converted to proportions per nest. The proportions of each Order in nestling diet and found in foraging sites were compared using paired Wilcoxon signed rank tests (Bonferroni adjusted critical value P = 0.005). Prey types provisioned to nestlings in significantly greater or lower proportions than expected relative to abundance were termed ‘selected’ or ‘not selected’ prey, respectively. Prey types provisioned proportionally were termed ‘indifferent’. Vegetation and arthropod data from individual foraging and control sites (72 paired sites) were analysed using MANOVA in S-Plus 7 (Insightful Corporation 2005). Four response variables (vegetation height, vegetation density, total arthropod biomass and ‘selected’ arthropod biomass) were tested against a twolevel factor describing the sampling locations (‘foraging’ or ‘control’ sites). ‘Selected’ prey items were a combined measure of those Orders found to be selectively provisioned to nestlings. ‘Nest’ and ‘Site pair’ nested within ‘Nest’ were entered as random factors into the model. RESULTS
Meadow Pipits flew up to 120 m from the nest in search of food. Of the foraging sites observed, 76% were situated within 60 m of the nest, and 89% within 80 m. Meadow Pipits were observed provisioning nestlings with ten different prey types throughout the breeding
Foraging by breeding Meadow Pipits
season (Fig. 1). Parent birds were multiple prey loaders (carrying one to six items). Three prey types were selected more often than expected relative to their abundance (Tipulidae adults, V = 150, P < 0.001; Lepidoptera larvae, V = 156, P = 0.002; Arachnida, V = 156, P = 0.002, Fig. 1). Two prey types were not selected (Earthworms, V = 0, P < 0.001; Molluscs, V = 2, P = 0.004, Fig. 1), with Hemiptera and Formicidae not observed to be fed to nestlings at all, despite being present in the foraging sites. The proportions of the remaining five prey types (Tipulidae larvae, other Diptera adults, Lepidoptera adults, Coleoptera adults, Coleoptera larvae) did not differ significantly between nestling diet and available food. There was a significant overall difference in vegetation and arthropod characteristics between foraging and control sites (MANOVA F1,71 = 3.171, P = 0.019, Table 1). Univariate tests showed that three variables contributed to the significant overall difference between foraging and control sites: foraging sites were characterized by significantly lower vegetation height (foraged = 26 cm ± 2 se, control = 31 cm ± 2 se, F1,71 = 4.450, P = 0.038) and density (foraged = 5.8 units ± 0.5 se, control = 7.5 units ± 0.6 se, F1,71 = 7.98, P = 0.006), and significantly higher total arthropod biomass than control sites (foraged = 5.9 g ± 0.9 se, control = 4.0 g ± 0.6 se, F1,71 = 4.65, P = 0.034, Table 1). The difference
293
in the cumulative biomass of the selected prey types (i.e. Tipulidae adults, Lepidoptera larvae and Arachnida) between foraging and control sites was not statistically significant (Table 1). The multivariate MANOVA also revealed significant differences between nests (F18,71 = 1.27, P < 0.001) and site pairs per nest observation (F53,71 = 4.541, P = 0.0477), again with vegetation height, density and total arthropod biomass contributing significantly to the overall differences (Table 1). DISCUSSION
We found evidence that Meadow Pipits selected foraging sites with low vegetation height and density, but with high arthropod biomass on intensively grazed moorland. Our data also suggested that within foraging sites, Meadow Pipits are selective in what prey they provision to nestlings. This could be either due to genuine prey preferences made by adults or because some prey were not detected (or both). Indeed, taller vegetation is known to reduce the accessibility and detectability of food items (Butler & Gillings 2004) as well as restricting forager mobility (Devereux et al. 2004). We also found that the biomass of selected prey types did not differ between foraged and non-foraged sites. It therefore appears that, in intensively grazed
Figure 1. Proportion of prey types in nestling food and total arthropods present in foraging sites, showing medians with interquartile ranges (boxes) and total range of data (whiskers). *Significant difference at Bonferroni adjusted level (P = 0.005). Two additional prey types (Hemiptera and Formicidae) were present in foraging sites but were not observed to be provisioned to nestlings.
© 2008 British Trust for Ornithology, Bird Study,
55, 290–296
294
D.J.T. Douglas, D.M. Evans and S.M. Redpath
Table 1. Results of MANOVA analysing vegetation and arthropod characteristics between paired foraging and control sites (n = 19 nests, 72 paired foraging and control sites). Multivariate test indicates overall difference in the response variables between foraging and control sites. Univariate tests indicate difference in the four individual response variables between foraging and control sites. Multivariate tests df Foraging/control Nest Site pair (nest) Residuals
1 18 53 71
Wilks λ 0.843 0.065 0.044
F 3.171 1.270 4.541
P 0.019
