Research Article |
Corresponding author: Innes M.W. Sim ( innes.sim@rspb.org.uk ) Academic editor: Klaus Henle
© 2015 Innes M.W. Sim, Nicholas I. Wilkinson, Davide Scridel, David Anderson, Staffan Roos.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Sim IMW, Wilkinson NI, Scridel D, Anderson D, Roos S (2015) Food supplementation does not increase demographic rates in a passerine species of conservation concern. Nature Conservation 10: 25-43. https://doi.org/10.3897/natureconservation.10.4556
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Numerous studies have examined the effects of the provision of supplementary food on aspects of avian reproductive success, but far fewer have gone on to examine the potential positive effects of food supplementation on the demographic rates which are key for population growth rate. Testing for potential effects of food shortage on vital rates is likely to be particularly important in species of high conservation concern, where populations are particularly small, isolated or decreasing rapidly. Here we test the effects of the provision of supplementary food on reproductive success, body condition at fledging and post-fledging survival of ring ouzels (Turdus torquatus), a species of high conservation concern in the UK. However, food supplementation had no detectable effect on any of these parameters. There was no significant difference in return rates of fed and unfed fledglings in the year following hatching, and most post-fledging mortality was apparently caused by predation by raptors and mustelids. We conclude that the supply of invertebrate food sources for nestlings was not a major limiting factor in our study area, at least during this two-year study. Further studies are required to quantify the precise mix of habitats used by ring ouzels, at the appropriate scale, which provide concealment from predators and access to food supplies throughout the spring and summer months.
Food supplementation, demographic rates, passerine species, conservation
The provision of supplementary food has been trialled as a method for increasing reproductive success and/or survival in a range of avian species, especially those of conservation concern where populations are particularly small, isolated or decreasing rapidly (e.g.
Since providing supplementary food can be a costly and labour-intensive conservation management action, it is important that there is clear evidence for food shortage before feeding begins. For example, in the New Zealand hihi, providing supplementary food increased abundance and survival of translocated birds on Kapiti Island, following evidence that food was limiting on previous island translocations (
The ring ouzel, hereafter ‘ouzel’, is a species of high conservation concern in the UK (
A recent demographic analysis of one ouzel study population indicated that λ was most sensitive to apparent first-year survival (fledging to age one year), closely followed by re-nesting rate and early-season reproductive success, and that first-year survival contributed most to observed variation in λ (
In this paper we report the results of a two-year field experiment in which we provided supplementary food to a declining population of ouzels, in an attempt to improve their reproductive success and post-fledging survival. We hypothesised that adult provisioning rates to nestlings, fledging success, brood size at fledging, nestling body condition at fledging and post-fledging survival would all be higher, and within-brood variation in nestling body condition would be lower, in fed than in control territories, where no supplementary food was provided.
The ring ouzel is a medium-sized migratory thrush that breeds in north-west and central Europe and winters in southern Spain and North Africa (
In each year we aimed to locate all ouzel breeding pairs and nesting attempts. The study area was systematically surveyed, by walking all ground to within 200 m of observers, every one to two weeks between mid-April and mid-July (
To allow individual identification, 263 nestlings (145 from early and 118 from late nests), comprising 90% of individuals known to fledge in the study area, were ringed with BTO metal rings and individual combinations of three plastic colour rings. In addition, 17 (2011) and 29 (2012) individually colour ringed adults marked previously in the study area as either breeding adults or nestlings returned to breed, and a further 12 (2011) and 24 (2012) adults were caught and colour ringed.
In 2011, we randomly allocated 50% of the known territories (i.e. those occupied at least once between 1998 and 2010; Ntotal = 86) to receive supplementary food for both early and late breeding attempts (‘fed’ territories). The remaining 50% of the known territories were ‘control’ territories, where no supplementary feeding occurred. In 2012, we reversed treatments, so that control territories from 2011 became fed territories, and vice versa. In both years our aim was to have approximately equal numbers of fed and control territories. However, this ‘ideal’ experimental design was not possible in territories that were occupied only in either 2011 or 2012, in those territories where ouzels did not find or utilise the supplementary food, or where feeding had to be abandoned before nestlings fledged due to other species taking the supplementary food (see Results for details). When feeding was not possible in a planned fed territory, the next occupied territory on the ‘fed’ random list was selected to receive supplementary food. Thus, despite our ambitions, we ended up with an unbalanced experimental design in terms of numbers of fed and control territories across the two years.
Supplementary food was provided during the ouzel nestling-rearing stage in black plastic seed trays (38 cm × 24 cm × 6 cm) placed on prominent knolls, boulders or in short grass-rich areas on the ground, between 20 m and 50 m from ouzel nests. These locations were chosen to make the food as obvious as possible to the ouzels, while reducing the risk of predators locating the nest. We observed these feeding trays to determine (a) if the ouzels and/or other species fed from them and (b) what food source [live earthworms (Dendrobaena spp.), or live mealworms i.e. the larvae of the mealworm beetle (Tenebrio molitor)] was preferred. In 2011, we provided 100 g of both earthworms and mealworms in each territory daily, in order to replicate the key natural food sources of nestling ouzels. Mealworms have a relatively high protein (45–60%) and fat (30–45%) content (
We monitored adult provisioning rates to nestlings at fed and control sites using x10 binoculars or ×15–40 zoom telescopes, from hides, cars, or by observers well concealed in open moorland, at distances of 30–200 m from nests, depending on topography and the sensitivity of the adults to disturbance. Each provisioning watch lasted for 60 minutes, and we classified food brought to the nest as supplementary (earthworms or mealworms taken from the feeding trays) or natural (gathered away from the feeding trays and therefore assumed to be natural). In 2011, we carried out 1–2 (mean 1.25 ± 0.11) watches at control nests when nestlings were aged 7–12 (mean 9.37 ± 0.33) days old, and 1–8 (mean 2.60 ± 0.22) watches at fed nests when nestlings were aged 5–13 (mean 9.26 ± 0.25) days old. In 2012, we carried out 1–3 watches at control (mean 1.97 ± 0.10) and fed (mean 1.95 ± 0.10) nests when nestlings were aged 4–12 (mean; control 8.16 ± 0.30: fed 7.94 ± 0.31) days old.
To test whether within-brood variation, and individual nestling body condition, at fledging varied between fed and control nests, we measured body condition index (BCI) as the residual of a regression of body mass on wing length3 (body mass = 66.2 + 0.00001* wing length3; r2adj = 0.05;
To measure survival during the post-fledging period, we fitted nestlings with 1.8 g TW4 single-celled radio transmitters (
We tracked juveniles with transmitters and recorded their approximate locations every 3–4 days post-fledging, until the individual was found dead, shed the transmitter, or disappeared and was assumed to have dispersed from the study area. Individuals were tracked at different times on different days. Transmitters had signal ranges of approximately 10 km when in direct line of sight, but more typically 2–3 km depending on terrain, and a battery life of 3–4 months. We used Advanced Telemetry Systems (ATS) scanning receivers attached to car roof-mounted aerials to provide approximate locations. Hand-held Telonics TR-4 receivers, attached to three-element Yagi antennas, were used to visually locate each individual on foot, and record their location using a Garmin Global Positioning System (GPS) 12 Personal Navigator. In addition, we recorded the observed return rates of individually colour-ringed nestlings from fed and control nests in the years following ringing.
We examined remains of dead juveniles to determine the most likely cause of death. Individuals found in raptor nests or elsewhere with plucked feathers and bent radio-tag aerials were assumed to have been eaten and most likely killed by raptors, whereas those located underground in tunnels, under boulders, or in the open with bitten feathers and straight aerials were assumed to have been eaten and most likely killed by mammals (
We were primarily interested in the level of support for supplementary feeding on ouzel fledging success (the proportion of hatched nestlings that fledged, excluding nests which failed to fledge any young since these were almost certainly predated), brood size at fledging (again excluding nests which failed to fledge any young), nestling BCI at fledging (both for individual nestlings and within-brood variation in nestling BCI, the latter measured as the standard deviation of brood BCI) and post-fledging survival, compared to control territories (0 days feeding). However, since there was considerable variation in the number of days that nestlings received supplementary food (hereafter ‘feeding days’; see Results for details), we used feeding days as a predictor variable in all analyses, rather than the binary predictor fed/control.
We used Generalised Linear Models (GLMs; in the base package in R;
We ran juvenile survival analyses over 100 days post-fledging (25 × 4-day periods), after which no individuals fitted with radiotransmitters remained within the study area, using the known-fate model in program MARK 5.1 (
Using AICc, we first tested the relative support for models where survival was constant or varied across all 25 four-day periods. We then tested for effects on survival of the factors year and brood, and covariates BCI, brood size and feeding days on their own, when added to one another and including all possible two-way interactions.
The proportion of ouzel pairs in fed territories that we observed feeding the supplementary food to their nestlings did not differ between 2011 (13/19, 68%) and 2012 (16/18, 89%: χ2 = 2.29, p = 0.13). However, nestlings received supplementary food for a longer period in 2012 (mean 10.69 ± 0.42, range 8–12 days) than in 2011 (mean 6.23 ± 0.51, range 4–9 days; t = 6.54, df = 25, p < 0.0001). Wheatears (Oenanthe oenanthe) and meadow pipits (Anthus pratensis) occasionally ‘stole’ the supplied food, but were quickly chased off by ouzels and were considered to have a negligible impact upon the amount available to ouzels. Common gulls (Larus canus) located the food at two territories after 3–6 days of feeding in 2011, and at six territories after 3–8 days of feeding in 2012, and rapidly emptied the trays. We then ceased the food supplementation in these territories, since no food was available for ouzels and because of the increased risk of predation of ouzel nestlings by common gulls. Such territories were subsequently removed from the experiment, since they could not be reliably categorised as either fed or control. We re-classified the six (2011) and two (2012) territories where we provided food, but never observed it being taken by ouzels, as controls. Thus, during 2011–12, we successfully provided supplementary food at 21 territories, with a further 34 territories classed as controls.
We carried out provisioning rate observations, by adults to nestlings, at 5 of 13 (38%) early, and 9 of 16 (56%) late, control territories, and at 7 of 11 (64%) early, and 8 of 9 (89%) late, fed territories in 2011. In 2012, observations were carried out at all early (n = 16) and late (n = 9) control territories, and at 12/13 (92%) early, and at all 11 late, fed territories. Supplementary food was supplied in 538/740 (73%) of deliveries to nestlings by adults at fed nests. In addition, adults were observed eating the supplementary food at 9/12 (75%) of fed territories in 2011, and at 15/17 (88%) of fed territories in 2012.
The best supported model for provisioning rate included the added positive effects of nestling age and year (Suppl. material
Reproductive success results are summarised in Table
Ring ouzel reproductive success parameters measured for early and late broods at all fed and control nests during 2011–12. Figures presented are means ± s.e., with sample size given in parentheses. Data for mean clutch size in late nests comes only from individually identifiable colour ringed females.
Variable | 2011 fed | 2011 control | 2012 fed | 2012 control |
---|---|---|---|---|
Number of early territories | 9 | 14 | 10 | 17 |
Number of late territories | 7 | 16 | 8 | 10 |
Mean clutch size in early nests | 4.11 ± 0.11 (9) | 3.85 ± 0.10 (13) | 4.08 ± 0.08 (12) | 3.86 ± 0.10 (14) |
Mean clutch size in late nests | 5.00 ± 0.00 (2) | 4.00 ± 0.31 (5) | 4.00 ± 0.26 (6) | 4.00 ± 0.32 (5) |
Early brood fledging success | 0.74 (9) | 0.84 (14) | 0.89 (10) | 0.85 (17) |
Late brood fledging success | 0.74 (7) | 0.87 (16) | 0.87 (8) | 0.70 (10) |
Mean brood size at fledging in successful early nests | 3.25 ± 0.37 (8) | 3.07 ± 0.20 (14) | 3.78 ± 0.22 (9) | 3.31 ± 0.27 (16) |
Mean brood size at fledging in successful late nests | 4.00 ± 0.55 (5) | 3.86 ± 0.21 (14) | 3.71 ± 0.36 (7) | 3.71 ± 0.18 (7) |
Since the null model predicting individual nestling BCI at fledging received almost as much support as the two top-ranked models, we conclude that none of the models successfully predicted nestling BCI at fledging (Suppl. material
The model with the highest support regarding juvenile survival for up to 100 days post-fledging indicated a positive association with BCI at fledging (Suppl. material
Of the 143 juveniles fitted with transmitters, 60 (42.0%) could no longer be tracked 20–84 days after fledging and were assumed to have dispersed outside the study area, 40 (28.0%) were found dead, 35 (24.5%) shed their transmitters within the study area, and a single (0.7%) transmitter was known to have stopped working prematurely (the individual could still be identified by individual colour rings). A further seven (4.8%) transmitters were assumed to have stopped working within 16 days of fledging, since their signals became increasingly weak and intermittent in the days preceding their loss.
Of the 60 juveniles that were considered to have dispersed outside the study area, 41 (68%) had moved unusually long distances (1.5–6.0 km) from their nest sites during the 10 days preceding the estimated date at which they left the study area (nine were subsequently located outside the study area). Of the remaining 19, three were also subsequently located outside the study area. This suggests that the majority of juveniles that we lost track of did in fact disperse, rather than experience transmitter failure. Of the 40 found dead, 16 (40.0%) and 11 (27.5%) were eaten and presumably killed by raptors and mammals, respectively. A further 12 (30.0%) apparently died of starvation/exposure (six deaths occurred during a 2-day period of exceptionally cold, wet and windy weather in late May 2011), and a single (2.5%) bird was apparently killed by a car.
Thirteen of the 16 (81%) deaths attributed to raptors could not be attributed to a specific species. However, the remains of single juveniles and/or their radio transmitters were found in, or within 100 m of, peregrine (Falco peregrinus), kestrel (F. tinnunculus) and sparrowhawk (Acccipiter nisus) nests, respectively, strongly suggesting that these were the predators. None of the 11 deaths presumed to have been caused by mammals could be attributed to a specific species. However, a minimum of eight (73%) were likely killed by mustelids (stoats and weasels), because they were found in situations inaccessible to red fox, such as small holes or deep in boulder scree.
A similar proportion of early brood (14/76, 18%) and late brood (13/67, 19%) juveniles were apparently depredated (χ2 = 0.02, p = 0.88). During the first four days post-fledging, signs suggested that mammals were the main predator (5/6, 83%), with raptors being the main apparent predator thereafter (15/21, 71%). Mortality apparently from exposure occurred during the first eight days post-fledging, while the individual apparently killed by a car died approximately 3 weeks post-fledging.
During 2011–12, we successfully provided supplementary food at 21 ouzel territories, with a further 34 territories classed as controls. However, none of adult provisioning rate to nestlings, fledging success, brood size at fledging, within-brood variation and individual nestling BCI at fledging, and post-fledging survival were positively associated with the provision of supplementary food. Post-fledging survival was positively associated with BCI at fledging, but there was no apparent association between BCI and the provision of supplementary food. Post-fledging mortality was apparently mainly due to predation by raptors and mustelid mammals. We therefore conclude that, during 2011–12, food supplementation did not improve the demographic rates which had previously been identified to be crucial for improving the population growth of ouzels in the UK. However, since the current and previous studies (e.g.
The majority of ouzel pairs that were supplied with supplementary food found it relatively quickly, usually within a day, and used it to feed themselves and/or their nestlings in both years. Supplied food was mainly mealworms, which have been widely used to feed a number of passerine species due to their high protein and fat content, similarity to natural invertebrate food, and availability to receive in bulk at short notice (
Contrary to our predictions, we found no positive effect of food supplementation on ouzel fledging success, or brood size at fledging. These results are consistent with 9/19 (47.4%) of published studies on fledging success in small passerines, which found no positive effect of food supplementation (
None of our models predicting fledgling BCI successfully out-competed the intercept only model. In particular, supplementary fed individual ouzel nestlings fledged with a similar BCI to nestlings which received no supplementary food, in contrast to previous studies which indicated a positive effect of feeding on fledgling body mass (
Juvenile ouzels that fledged with a higher BCI had higher survival through the post-fledging period, a common (e.g.
There was no positive effect of food supplementation on subsequent ouzel post-fledging survival, a result consistent with the only previous passerine studies which have tested for this effect, in the New Zealand hihi (
Mortality rates of juvenile ouzels were considerably lower during 2011–12 than those during a similar study in the same study area during 2006–08, with most deaths apparently due to predation in both time periods (
Contrary to our predictions, the provision of supplementary food had no positive effect on adult provisioning rates to nestlings, fledging success, brood size, individual BCI at fledging, or subsequent post-fledging survival, and did not decrease within-brood variation in nestling BCI in ouzels. We therefore conclude that the supply of invertebrate food sources for nestlings was not a major limiting factor in our study area during 2011–12. However, it is possible that invertebrate food sources were unusually abundant during the two-year study period, in which case the provision of supplementary food would not be expected to create a positive effect on reproductive success or juvenile survival. Most feeding experiments, including this one, are short-term in nature and may therefore fail to detect potential positive effects on population demographics of providing supplementary food in years of natural food shortage (
It remains entirely possible that factors acting on the migration routes and/or in the wintering grounds are important in driving observed declines in ouzel numbers in the UK, and further work is required to investigate these (
We thank Invercauld Estate for co-operation with access to Glen Clunie, and Graham Rebecca for assistance with fieldwork.
Supplementary Information
Data type: species data
Explanation note: Details of the models predicting adult ring ouzel provisioning rate to nestlings, fledging success, brood size at fledging in successful nests, nestling BCI at fledging, within-brood variation in nestling BCI at fledging, and juvenile survival probability for up to 100 days post-fledging.