Summary
The short answer is yes, although there is considerable debate over whether the threat is significant enough to lead to appreciable declines in bird populations. Currently, it seems that predation on nests/chicks may have a signficant impact in habitats where ground nesting species are already in decline or where hedgehogs have been introduced.
The details
European hedgehogs (Erinaceus europaeus) are well known to be opportunistic feeders, and this will include taking chicks if they encounter a nest. In captivity, hedgehogs will readily accept meat and, in his 1964 book Hedgehogs, Konrad Herter wrote of a lactating hedgehog that ate 120 grams (4.2 oz.) of chicken, a sparrow (weighing 24 g. / 0.8 oz.) and 85 g. (3 oz.) of milk in a single night, representing about a third of her weight. Another captive hedgehog seemed quite happy being fed nothing but sparrows for ten days, so it seems that birds aren’t overlooked if the opportunity arises. Indeed, in his 2007 The New Hedgehog Book, Pat Morris notes how chicks are eagerly attacked and eaten, in what he refers to as “a gruesome manner” and I recently saw a trailcam video showing this very behaviour when a hedgehog stumbled across a fledgling blackbird. In addition, an article to The Shooting Gazette published in February 1990 notes how:
"In very dry conditions, hedgehogs can be a problem. With their other food source of worms being scarce they may turn to eggs. In prolonged dry spells some keepers leave a drink out in a saucer for hedgehogs which curbs their egg eating instinct. In very dry conditions, hedgehogs have been known to take small poults. They apparently crack the top off the skull and suck out the brains! It must be stressed however that hedgehogs are not normally a problem and that they are protected animals."
The idea that hedgehogs will eat chicks is also supported by stomach content analysis and, in a study of a gull colony in Cumbria, Hans Kruuk identified gull-chick down in 30% of hedgehog faeces, while a study of hedgehogs in New Zealand found avian remains (largely feathers) in 10% of guts analysed. Unfortunately, it is difficult to separate active predation from scavenging when dealing with stomach and faecal analyses which makes assessing impact of these findings almost impossible.
We also know that hedgehogs will readily take eggs, although the size and shell thickness makes some eggs more vulnerable than others. There have been many attempts to discern the palatability of different eggs to hedgehogs and to get a handle on the potential level of nest predation. Before we look at the data, it is worth taking a moment to consider why hedgehogs would want to eat eggs in the first place.
Go to work on an egg
The eggs of most vertebrates are telolecithal; this means that the yolk mass is separate from the developing embryo, as opposed to most invertebrates, where the yolk is incorporated into the dividing cells. So, when you crack open a chicken’s egg, for example, you’ll find a yellow (depending on the bird’s diet) yolk surrounded by a ‘gloopy’ clear liquid called the ‘white’, or albumen. The egg white is cytoplasm, protein dissolved in water, that protects the valuable yolk and provides an additional source of nutrients for the developing chick. Proteins in the white are anti-bacterial, block digestive enzymes, repair holes in the shell, thicken the albumen to inhibit viruses and bind various vitamins and minerals – all serving to protect the yolk. Consequently, most of the egg’s nutritional value (by which we mean calories, fats, cholesterol, folate, vitamins, calcium, iron, etc.) are found in the yolk. Indeed, the yolk is the main source of nutrition for the developing embryo; it contains all the fat and cholesterol as well as all the fat-soluble vitamins and various saturated and unsaturated fatty acids. Overall, The Diet Channel sum up succinctly when they describe eggs as “one of Nature’s great nutrition powerhouses.”
Eggs have long been identified from the stomach analyses of hedgehogs, although early reports may be biased because eggs were commonly used as bait in hedgehog traps. In his 1987 book, The Complete Hedgehog, Les Stocker wrote that it is a physical impossibility for a hedgehog to break into a duck’s egg and unlikely that it could do much damage to an unbroken tern’s egg; its jaws are just too small and weak to be able to crack the shell. Stocker also noted how, even when given no other food for a week, a hedgehog in his care was still uninterested in eggs. Stocker’s view is echoed by other authors. In their book, The Natural Hedgehog, Lenni Sykes and Jane Durrant mention that, during their experiments on captive hogs at the Welsh Hedgehog Hospital, they found these animals were uninterested in hen and quail eggs unless the shell was broken for them – the authors suggest that the hedgehogs were unable to open the eggs themselves and showed no interest in even trying. Similarly, in Hedgehogs, Herter noted that they probably do eat the eggs of ground-nesting birds like quail, larks and partridges; but that larger eggs, or eggs with thicker shells, would be largely invulnerable to them. Herter continued:
“Eggs of the corncrake (Crex crex) which average 36 millimetres by 26 [1.4 x 1 in.] are eaten by hedgehogs but not those of the common snipe (Capella gallinago) which measure 39 by 28 [1.5 x 1.1 in.].… Whether the much larger (45 by 35 [1.8 x 1.4 in.]) but relatively thin-shelled, eggs of the pheasant are eaten as is often asserted, appears to me questionable. The contents of broken hen and pigeon eggs are licked up, but most hedgehogs seem to have no particular preference for them.”
Perhaps the most famous and comprehensive study of egg palatability was conducted by Hugh Cott, an oologist (his delightful word for someone who studies eggs) at Cambridge University, and published in the Proceedings of the Zoological Society of London in 1951. Cott obtained eggs from 25 species of bird comprising 10 orders; he presented four hedgehogs with a choice of two eggs at a time (in a total of 332 experiments) to measure their preferences. Cott’s data show that kittiwake (Rissa tridactyla) eggs were most palatable, closely followed by the eggs of the common eider (Somateria mollissima) and the British razorbill (Alca torda); the eggs of the linnet (Carduelis cannabina) were least palatable.
Cott also found that, generally-speaking, eggs with longer shell lengths were more popular than shorter ones and that cryptic (camouflaged) eggs were more palatable than either immaculate (un-patterned) or distinctively-marked ones. Overall, the highest palatability correlated with ground and cliff-nesting species, and colonial species had more appealing eggs than solitary nesters. It should be noted that while Cott’s results give us a good indication of egg palatability in hedgehogs, they don’t tell us which ones they’re necessarily more likely to consume in the wild, because he cracked them and mixed the yolk and white together before offering it to the hedgehogs in a petri dish. That said, Cott does cite other observers who suggest pheasant and partridge eggs are "very frequently taken by hedgehogs".
Cott cites one particular example where a partridge nest was raided in Cambridge during June 1950 and the culprit was trapped and examined by the author later that night to find partridge egg shells in the stomach and droppings. Conservationists have long argued that hedgehogs were unable to break into the eggs of many larger bird species and were scapegoats that had happened upon an already depredated nest. Early observers of captive hedgehogs have, however, described how hedgehogs use their canines to break into eggs. In his nineteenth century volumes Curiosities of Natural History, for example, Francis Buckland described the results of his offering a "fowl's egg" to his captive hedgehog:
"He hit it sideways with his sharp canine teeth, and made a hole in it just big enough to thrust in his little black nose, and then with his tongue licked out the contents, and mightily he seemed to enjoy it, little thinking what evidence he was giving against the rest of his species."
Similarly, in a short note to the Irish Naturalist in 1899, Mr Orr described giving his pet hedgehog an egg:
"I placed one on the floor over night; next morning I found the shell with a piece the size of a shilling [about 23mm, or an inch, in diameter] broken out of one side, and the contents clean gone."
So, hedgehogs do appear able to break into eggs and will sometimes eat eggs if presented with the opportunity; they may even have preferences for the eggs of certain species. The next questions we arrive at are: how often do eggs occur in the diet of hedgehogs? and how does their oophagy (egg-eating) impact bird populations?
Impact of oophagy
In a 1964 paper to Behaviour Supplements, zoologist Hans Kruuk presented the results of his study into the predation of a colony of black-headed gulls (Lars ridibundus) at Ravenglass in Cumbria (UK). Kruuk found that hedgehogs damaged an average of 4.5 eggs and killed an average of 2.5 chicks per night, typically consuming only part of their victim. Kruuk calculated that, in one way or another, gulls provided the hedgehogs with about 30% of their food. Overall, Kruuk estimated that hedgehogs might eat 2% or 3% (maximum of 8%) of the 8,000 gull broods – he identified egg remains in 21% of faeces, adult gulls in 9% and gull-chick down in 30%. A similar figure was presented by Adrian Middleton in his 1935 paper to the Proceedings of the Zoological Society of London; hedgehogs were responsible for 16 (1.3%) of the 1,232 of losses from partridge (Perdix perdix) nests across the UK.
In his analysis of 137 hedgehog stomachs from an East Anglian estate, published in the journal Acta Theriologica during 1976, Derek Yalden found feathers in 22 (16%) and egg remains in “possibly 15” (11%) – Yalden wasn’t at all confident that the remains were actually bird eggs (see later). In a survey for Scottish Natural Heritage (SNH) completed in 1995, J. Watt found eggshell remains in 11% of scats collected on South Uist in the Hebrides, while a subsequent study found eggshell in 13% of scats collected during the wader breeding season on South Uist. From an energetic perspective, data from these introduced hedgehogs on the Uists suggest that no more than six per cent of their energy intake was obtained from bird eggs and chicks.
One study on a coastal grazing marsh at Elmley in Kent published in 1987 suggested, based on tooth puncture marks, that hedgehogs were responsible for losses from two wader clutches. A paper to Biological Conservation in 2002 implicated hedgehogs in 20% of the depredation events the authors recorded on black stilt (Himantopus novaezelandiae), black-fronted tern (Sterna albostriata) and banded dotterel (Charadrius bicintus) nests on a riverbed in New Zealand. In a 2000 paper to Biological Conservation, Digger Jackson and Rhys Green attributed 7.5% of redshank nest failures and 5% of dunlin failures to hedgehogs in the Hebrides.
It should be mentioned that low percentages in the diet does not necessarily correlate with a low impact on the population. Different populations are often subject to different stressors and a mortality of 3% might easily be managed by one population, but the tipping point for another population. Where nest predators are commonplace and the nesting species relatively rare, small percentages per hedgehog diet can have a significant additive impact. Furthermore, the number of eggs in the diet is likely to be considerably underestimated, making the true severity difficult to quantify.
Personal observations by Chris Jones and Mark Sanders, referenced in a 2005 paper, suggest that hedgehogs tend to avoid eating the shell and focus on the yolk and white; this is difficult to identify in scat or stomach contents and would typically require biochemical or immunological tests. Additionally, it has been suggested that the precocity of the chicks may affect vulnerability to hedgehog predation. In Hedgehogs, Nigel Reeve suggested that precocial young produced by gamebirds such as partridge and pheasant, which leave the nest soon after hatching, are likely to be less vulnerable than the more altricial species, such as pipits and larks, that develop more slowly and remain in the nest for longer.
In some locations where hedgehogs have been introduced it has become apparent that they can have a considerable impact on some local bird populations. The picture is less clear in their native range, however. In his 1951 paper, Colt wrote:
“I am recently informed by … a keen and intelligent observer who has had considerable experience as a keeper in the Midlands and Eastern Counties, that to his knowledge Pheasant and Partridge [the latter more so than the former, apparently] sittings are very frequently taken by hedgehogs.”
In The New Hedgehog Book, however, Pat Morris wrote:
“[hedgehogs] may account for two or three per cent of clutches lost, one tenth of the numbers taken by foxes, and really not enough to warrant all the fuss made by gamekeepers.”
Recent introductions of hedgehogs to island communities have nonetheless raised considerable concern over the prosperity of the local seabird colonies – two well publicised cases are those of North Ronaldsay and the Uists, which I will briefly summarise here.
North Ronaldsay is a small island making up part of the Orkneys, a small group of islands off north-east Scotland; it was hedgehog-free until 1972, when a postman reportedly brought over three animals to keep in his garden in the hope that they would control garden pests. The hedgehogs escaped and thrived to the extent that some estimates in the press put the population at 10,000 animals; later reduced to 1,000. More empirical estimates came from biologist Hugh Warwick in 1986, who put the numbers at between 400 and 600.
A quote from a local ornithologist in the Sunday Express newspaper during June of the same year read: “The hedgehogs are decimating the bird population by eating birds’ eggs.” Pat Morris provides an excellent and detailed summary of this particular case and the reader is directed to his New Hedgehog Book for further details. In his conclusion, Morris notes that hedgehogs never actually became very abundant on the island and have since suffered a sharp decline in numbers:
“The failure of breeding in the bird colonies was observed on other islands where there were no hedgehogs. It was actually due to collapse of the sand eel population upon which the birds feed, nothing to do with hedgehogs.”
More recently, hedgehogs have caused controversy on the Uists, part of the Hebridean islands off western Scotland, since their introduction to South Uist in 1974 again to control garden pests. Since the introduction, the islands have seen large declines in wader numbers and nest success. The Uists contain about 33% of the UK’s breeding population of dunlin (Calidris alpine schinzii) and 25% of ringed plover (Charadius hiaticula); under the European Union Directive on the Conservation of Wild Birds (1979), these birds are afforded protection and action must be taken in the face of threats to the population. Much of the work to assess the scale of the declines and the role that hedgehogs may play has been undertaken by RSPB biologist Digger Jackson.
In a presentation to the Third International Hedgehog Workshop in 1999, Jackson presented data suggesting that the wader decline was a result of heavy nest losses and that hedgehog predation accounted for at least half of all dunlin, redshank (Tringa totanus) and snipe (Gallinago gallinago) nest failures. The data show that on South Uist only about 15% of dunlin pairs hatched young, compared to 75% of pairs in the mid-1980s, and, as such, the population had fallen from about 1,100 to 350 pairs.
Subsequent collaborative papers in Biological Conservation (2000 and 2003) and the Journal of Zoology (2006 and 2007) have served to reinforce the idea that hedgehogs may be a major contributing factor in the decline of the breeding birds on these islands and present data suggesting a link between the population expansion of the hedgehog and decline of the birds. The most recent estimate of which I’m aware suggests the population of hedgehogs on North Uist, South Uist and Benbecula stands at about 4,000 animals spread over 400 square-kilometres (155 sq-mi.); an average density of 57 individuals per sq-km (~ 143 per sq-mi.), which is substantially higher than on the mainland. Overall, the data and models presented by Jackson and his co-workers imply that, without intervention, hedgehogs could cause the extinction of dunlin on the islands and lead to significantly reduced populations of redshank and snipe.
Pointing the finger
Telling which species is responsible for a raid on a nest is not a straightforward task. In a 1999 paper to the journal The Condor, biologist Serge Lariviere at the Université Laval in Qubec, Canada discussed the problems associated with inferring culprits from nest remains. Lariviere argued that the biggest problem is that numerous species share similar depredation patterns. For example, Lariviere described how red foxes (Vulpes vulpes), raccoons (Procyon lotor), weasels (Mustela spp.) and crows (Corvus spp.) form a small part of a long list of predators known to remove eggs from the nest, such that an empty nest could be the result of a visit from any of the above. Similarly, patterns of egg breakage by gray foxes (Urocyon cinereoargenteus), coyote (Canis latrans) and bobcat (Lynx rufus) are very similar. Moreover, Lariviere points out that a single species (e.g. striped skunks and gray foxes) may exhibit several different methods of breaking into an egg; the behaviour varying in accordance with egg size. We know that crows will, for example, carry away small eggs but eat larger ones in situ.
In the case of hedgehogs, some authors have used tooth puncture marks (incisor spacing of less than 5mm, with a tooth diameter just over half the puncture spacing) to identify them as the culprits. Additionally, several authors have noted that hedgehog depredations are usually characterized by a mixture of yolk and shell fragments stomped into the lining of the nest. As we have seen, however, a given predator may have more than one modus operandi when it comes to nests and hedgehogs are no exception. In his 1951 paper, Hugh Cott wrote:
“When it has located a clutch, the hedgehog usually removes the eggs and devours them at some distance from the nest, though he sometimes breaks them and eats them in situ.”
Owing to problems associated with identifying predators from nest remains, subsequent studies have turned to more empirical methods, generally involving video surveillance. Such methods are often expensive to deploy widely, however, and there has recently been some interest in using chemical markers. In an intriguing paper to the New Zealand Journal of Ecology in 2007, Chris Jones at Landcare Research in New Zealand tested a chemical bait marker (called Rhodamine B) to see if it could be used to identify nest predators. Rhodamine B fluoresces under ultraviolet light and is systematically taken up by actively growing keratinous tissues (e.g. claws, hair, feathers, etc.).
During his field trials, Jones injected eggs with Rhodamine B and left them (cracked) in the nest. On 18 occasions the artificial nests either suffered egg predation or disturbance (i.e. movement but not consumption of eggs) – 21 hedgehogs were caught and analysis of their whiskers found Rhodamine B in five, with one female exhibiting two distinct bands (i.e. two depredation events). The technique has its limitations and won’t, for example, tell you which nest was predated or when it happened, but it shows potential.
In his 1935 paper looking at partridge nest loss across the UK, Middleton found that hedgehogs were implicated in 1.3% of cases; by comparison, foxes were responsible for 34%, while badgers (Meles meles) took 28% and accidents involving farm workers or their machinery accounted for 27%. Similarly, Kruuk’s study of the Cumbrian black-headed gull colony found that foxes did more damage (being responsible for about 46% of losses); other gulls accounted for a further 16% of the losses, which is twice the maximum estimate for the hedgehogs. In a 2005 paper to the journal Emu, Rachel Keedwell found that five cats, 11 hedgehogs and a ferret were responsible for 17 predation events involving eggs at a nest site in New Zealand; non-fatal disturbance came in the form of hares (Lepus europaeus), hedgehogs, mice, possums and deer.
Specifically identifying the species involved in nest depredations may be virtually impossible, but some have argued that, on South Uist at least, the fact that nest predation has only been observed since the arrival of hedgehogs implies accountability. During their studies on South Uist, Digger Jackson and Rhys Green found that none of the signs they associated with hedgehog depredation of nests were present prior to 1985-1987 when hedgehogs colonised the area; they also noted there were no depredation signs on the hedgehog-free sites on North Uist or in the exclosure trials on South Uist during 1998. Data from the exclosure trials were published in the Journal of Applied Ecology during 2001 and show that the experimental removal of hedgehogs using fencing led to a small (2.4%) increase in nesting success when compared to breeding birds outside the enclosure. So, what’s the solution?
Keep them hoggies rollin’
The opinion of Scottish National Heritage, who have jurisdiction on South Uist, is that the only sure-fire solution is to the problem of hedgehogs is to remove them altogether – this sounds simple in principle, but is deceptively complex in practice. SNH established an eradication programme that involved capturing and euthanising as many hedgehogs as possible. Pat Morris gives an excellent commentary of this programme and I will not recount it here. Sufficed to say that the scheme proved very expensive and labour-intensive, such that for every hour spent searching one hedgehog was caught (there were 500 caught in 2005, which is only about 12% of the population). One 2005 estimate put the cost at £340 per hog.
Several conservationists took issue with SNH’s extermination policy and proposed that hedgehogs could instead be caught on the islands and translocated on the mainland to help support our apparently declining population. SNH rejected the idea initially, suggesting that the hedgehogs would suffer in an unspecified manner and, in their report published in 2002, the Uist Wader Project wrote:
“Even if serious practical considerations are overlooked ... there is little justification in terms of animal welfare (as assessed by both mortality and suffering) for proceeding with programmes of translocation or long-term captive holding of Uist hedgehogs. … Judged from the perspective of animal welfare, however, trialling translocation would be misguided.”
Many biologists pointed out that this opinion is contrary to the science we already have on the survival of rehabilitated populations carried out by Pat Morris, Nigel Reeve and Hugh Warwick (see QA). Consequently, Uist Hedgehog Rescue was formed with the goals of highlighting the plight of the Uist hedgehogs and assessing whether translocation could work. Working on the premise that hedgehogs are a non-territorial, nocturnal species, a group of biologists from Bristol University fronted by Susie Molony set out to see how well hedgehogs translocated from Uist did on the mainland – their results were published in Biological Conservation during 2006. The biologists captured and released 109 hedgehogs into 20 gardens in suburban Bristol between May and August 2004; 20 were taken from Uist and spent fewer than six days in captivity, another 20 were taken from Uist but spent more than a month in captivity, and the others were controls of various kinds.
The hedgehogs were fitted with radiotransmitters prior to release and by following them the biologists found no evidence of aggressive interactions with resident hogs and saw that translocated individuals used the same sized ranges, travelled the same distances at the same speeds and spent the same amount of time active as resident hogs. Moreover, the resident population showed no signs of having a lower survival rate. One interesting finding of this study was that hedgehogs taken from Uist did better once released if they spent a couple of weeks in captivity – presumably, this allowed them to get over the stress of being handled (or become accustomed to it) and put on weight prior to release.
Morris described a similar study during April 2005, this time in Glasgow, where a small group of female hedgehogs were released and tracked – Morris and his team recorded a 75% survival rate, with no evidence of suffering and an improved chance of survival if the hedgehogs were kept in captivity for a short period prior to release. So, moving hedgehogs from the islands to the mainland is clearly a viable option, although as with extermination it is subject to the same problem of trying to get at all the animals. The last 5-10% of the population is notoriously difficult to catch and, as we know the population has grown from only four individuals, every animal must be removed.
Translocation or extermination are clearly options for reducing hedgehog impact, but elsewhere we have seen fences used to protect breeding bird colonies from predators. SNH suggested that fences would need to be impractically long and would ultimately be too costly, although, as Pat Morris pointed out, they have been used successfully by the Australians to protect their bird colonies. Similarly, in Britain, fences are widely employed to keep deer out of tree plantations and foxes or badgers away from ground-nesting wading birds. Morris accepted that fences would require annual expenditure but, in his opinion, less than a “futile” programme of eradication; hedgehogs can be removed from within the fenced area by trapping at the start of the breeding season.
In conclusion…
Dietary analysis and direct observation confirm that hedgehogs are one of several species that will predate bird nests given the opportunity. Given the hedgehog’s morphology, it appears that only eggs less than 39 x 28 mm (1.5 x 1.1 in.) are particularly vulnerable, unless cracked. How often this happens and how significant they are as a nest predator is contentious, although gamekeepers in Europe have long-considered hedgehogs a predator of gamebird nests and trapped them.
In some locations where they have been introduced, hedgehogs appear to pose a significant threat, although the example of North Ronaldsay reminds us to look at all the data before apportioning blame. In their natural range hedgehogs are not widely considered to be significant nest predators and with a population that appears to be in decline in Britain, it is unlikely this will change in the immediate future. Where hedgehogs are a problem, the science tells us that exclusion with fencing and even translocation are viable management options.