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European Bee-eater Merops apiaster Scientific name definitions

Hans-Valentin Bastian and Anita Bastian
Version: 3.0 — Published March 29, 2024

Diet and Foraging

Introduction

Nutritional studies are available from many countries in Europe, Asia, and Africa. The European Bee-eater feeds opportunistically, with Hymenoptera accounting for about half of the diet, but prey composition varies greatly from region to region, depending on the weather conditions and over the course of the breeding season (519, 520, 521). In the oceanic northwest of Europe, as well as on the northern edge of the distribution in the European part of Russia (Vladimir Oblast), the proportion of bumblebees (Bombus spp.) in the diet is very high (439, 522). Dragonflies are a preferred prey near water bodies, and the European Bee-eater can forage intensively on swarming termites, ants, and other insects (523, 524). Adult birds themselves probably feed on all available flying insects between ~5 mm and ~80 mm in length. As the European Bee-eater is a single-prey loader (525, 507), young birds and breeding adults in the nest are selectively fed with particularly large prey. To date, hundreds of insect species have been identified as prey items (317, 526, 527).

Feeding

Main Foods Taken

The species forages almost exclusively on airborne insects caught on the wing in open landscapes; due to its swallow-like flight behavior, it is capable of grabbing almost any flying prey from the air (528).

Microhabitat for Foraging

Foraging during the breeding season takes place at a maximum distance of 2 km from the breeding colony (529), but is usually limited to 1 km from the colony (431). While feeding young birds in the breeding burrow, foraging is limited to a radius of ~400 m around the breeding colony (530, 416, 531, 431). In Hungary, 54% of nesting colonies were between 200–300 m from a feeding area (532), and in Russia, foraging flights were rarely observed further than 200 m from the breeding burrow (439).

When hunting from a stationary perch, the species may choose any elevated perch, including trees, bushes overlooking the vegetation, mounds, but also man-made objects, such as electric fences, telegraph lines, hides, or sprinklers of irrigation systems. A bird will usually sit on a bare perch with a clear outlook and observe the surroundings (528, 273). During foraging search flights, individuals often hunt over all habitats that are frequently overflown by prey that can be easily spotted. These are mainly any kind of open land, fields, and water bodies. For example, in the Camargue (France), birds forage over salt marshes and rice fields (331), in Austria (533) and other locations between grape plants in vineyards or along landmarks (394), and in Ukraine, based on the prey list results, mainly over wheat fields (534).

Food Capture and Consumption

Foraging

The availability of large flying insects, the common prey of the European Bee-eater, increases with air temperatures and duration of daily sunshine (275). Nevertheless, the species can catch prey even during adverse insect flight weather by using different foraging strategies. It uses three main strategies for hunting, which are either carried out close to the ground or high in the open sky. These strategies consist of 1) hunting of flying insects from an elevated perch ("perch foraging") , 2) flights in the open air ("thermal hunting"), or 3) search flights near the ground. The chosen altitude, and thus the form of hunting behavior, depends in part on the habitat (273), the prevailing weather conditions (especially temperature and precipitation), and the stage of the breeding cycle. The different hunting methods are used opportunistically, depending on which promises the highest hunting success. This highly plastic foraging behavior enables the species to survive even long periods of adverse weather, unlike swallows (535). The European Bee-eater often hunts in groups (297). While hunting, prey of the size of a bumblebee is targeted from up to 60 m away (273). When prey is spotted, it is attacked from below: the bird's head is thrown back just before snapping its bill on the insect, which is directly over the bird (4). These flight maneuvers reduce the prey's opportunities for escape (397, 386). If the prey escapes, the bird plunges after it and attempts to catch it again from the air; if it sinks to a water body, the bird retrieves it while flying (297). Prey that has fallen to the ground is rarely picked up (528, 536). However, in captivity, it will pick up beetle larvae and dead cockroaches from the ground (537).

Perch foraging accounted for about one-fifth of all hunting flights in a Czech colony (529). The hunting success of this hunting strategy was about 45% in one study (397). After an unsuccessful attempt, the targeted insect is sometimes attacked again (Bastian and Bastian, unpublished data). After a successful chase, the bird flies back to a perch, often the same perch it flew from; there, the captured prey is first handled before it is eaten or fed to young (see Food Handling in this section).

In sunny, warm weather, when insect flight activity is high, the European Bee-eater often performs foraging flights in the open air ("thermal hunting;" 538, 460, 414). These foraging flights, in which prey is chased in the air with rapid flight maneuvers like those of swallows, is more frequent than short foraging flights from perches (536). Birds may forage individually or in groups; when foraging in groups, individuals stay in contact by calling almost constantly (394, 193). Foraging flights can take place at any time, can reach an altitude of 150 m or more, and last several minutes (397). This foraging strategy is more common after the offspring have fledged and are living in family groups or in large pre-migratory flocks; it is also common during migration, when individuals may forage at high altitudes, sometimes together with swallows. As with perch foraging, large insects captured from thermal hunting foraging flights are carried to a perch, where they are handled before being eaten or fed to young. Smaller insects are eaten in flight, with up to three insects being captured and eaten per flight (457); small insects, which are chased while thermal hunting, are fed less often to young (507).

Foraging search flights start from a perch or directly from the breeding burrow; insects hunted during these flights primarily include dragonflies, butterflies , bumblebees, or other large flying insects. Similar search flights close to the ground, which can last several minutes (397), also occur regularly, and exploit not only flying insects but also other species that move through the herbaceous vegetation in search of flowers. Search flights very close to the ground have the aim of flushing prey; these flights are used in warm, sunny weather (538) and in cold, wet weather (381; Bastian and Bastian, unpublished data). During rainy weather at one colony, several European Bee-eaters were observed carrying a great green bush-cricket (Tettigonia viridissima) in their bills; these were likely flushed during low foraging flights and captured (Bastian and Bastian, unpublished data). Similarly, dragonflies can be flushed and captured by low foraging flights along banks of water courses. In another colony, on a hot day, unfed blue-winged grasshoppers (Oedipoda caerulescens) were found in several breeding burrows; the grasshoppers were apparently caught when they responded with escape flights to birds foraging close above the ground (Bastian and Bastian, unpublished data). This type of foraging can be regarded as a modification of foraging flights in the open sky, and the two strategies are sometimes combined, such that flights just above ground can lead to high altitude foraging (394).

In addition to hunting for flying insects, the European Bee-eater rarely picks up prey from the ground (528) without landing completely (4), including flightless ground beetles (373, 539, 540), earthworms (541), spiders, and gastropods (542, 19, 543), as well as inorganic material like pieces of eggshell, stone materials, sand, and porcelain (377, 279). Caged birds, particularly during the breeding season, learn to pick up food from the floor or a feeder (544, 303, 528). Furthermore, hover gleaning techniques are applied while feeding from treetops and flowers (545, 273). In Africa, birds occasionally follow vehicles or people walking through grasslands, flushing insects (248); they are also attracted to fire and exploit fleeing insects (546).

Birds have also been observed in Zimbabwe plunging into the water like terns (Laridae) or kingfishers (Alcedinidae), and it is assumed they were taking food items from the water (547, 548). On the Bulgarian Black Sea coast, a flock of about 30 individuals was observed diving deep into the water from a low altitude until only the tips of their wings and tails remained above the surface, while they preyed on small fish of the genus Atherina (549). It is assumed that diving in the sea is not a thermoregulatory behavior (478), but that some European Bee-eaters prey on small fish in the sea during spring migration (549). In Hungary, they have also been observed capturing small fish from the Danube River (550).

Feeding Times and Frequency

Feeding frequency of nestlings is influenced by the number of offspring, their age, the contribution of additional cooperative breeding helpers, disturbances by raptors or people, and weather conditions (551, 533). The first feeding in the morning depends on the weather, with warmer temperatures, clear skies, low wind, and low humidity all contributing to greater insect activity and earlier foraging times (551, 275). On windy, rainy days, the activity of flying insects is drastically reduced (551, 275) and birds reduce their foraging activities accordingly or cease feeding temporarily (Bastian and Bastian, unpublished data). In eastern European populations, feeding usually starts between 0600 and 0700 (297, 552, 551), while in western populations, feeding starts later, between 0800 and 0900 (523, 553, 414, 431). The differences in time are just an artifact of time zones, however, as feeding in both regions starts 1–3 hours after sunrise and ends in the evening about 2–3 hours before sunset.

Information transfer within the breeding colony can also affect feeding frequency. When a new profitable food source is found, like beehives, the feeding rate in the colony increases according to the size of the colony and the distance between breeding burrows, as information about food sources is received from neighbors (554). Whether breeding success is also affected has not been studied. The transfer of information optimizes the foraging efficiency in larger colonies, but not in small colonies or solitary nests (554). These results contrast with results from a study in Slovakia, however, where a negative correlation was found between colony size and annual reproductive success, especially the average survival rate of nestlings and the average condition of chicks at 15–20 d of age; several factors were discussed as the possible cause of this pattern (e.g. higher parasite load, food situation, presence of helpers; see Breeding), but the data were not sufficient for a reliable explanation for this unexpected finding (555). In Italy, no correlation was found between colony size and the number of food sources (beehives), but a positive correlation was found between the number of colonies and the number food sources (526). Further research is needed on the relationship between colony size, food sources, and breeding success of the colony or of individual breeding pairs.

Food Handling

Prey handling depends on the type of prey; insects larger than 10–15 mm are carried to a suitable perch and handled before being swallowed. The insects are killed or at least immobilized by repeated strong strikes against the perch (8); the number of strikes increases with the size of the prey. Special skeletal adaptations allow the adult bird to hit and kill large prey by forceful sideways head movements. The considerable forces that occur across the axis of the bill, including upward and sideways forces, are absorbed by a solid morphological adaptation of the bill, with fusion of additional bones in the bill compared to other birds. This jaw adaptation is not yet fully developed in nestlings or subadults (556). Prey items are usually held with the tip of the bill to avoid the danger of being stung by hymenopterans, because the European Bee-eater is not completely immune to bee venom (8, 528, 557); individual stings do not have a noticeable effect, but multiple stings, or even a few in the eye, may kill them (528, 558, 458).

When the European Bee-eater handles hymenopterans, it is often assumed that the stinger is removed (557), but this is incorrect. The stinger of hostile insects is not removed (303, 528), and is often swallowed and found in food remnants (559, 397). Up to 27 stingers have been found in the stomach wall of killed European Bee-eaters, and it is assumed that the dense cuticular wall protects the soft stomach tissues against stings of honeybees (519). Similarly, the bee venom is also not completely removed when the prey is handled; the evidence for this comes from a case of a hunter who suffered a bee venom allergy after eating European Bee-eater meat, which included itching in the mouth mucous membrane, difficulty breathing, and laryngeal edema (559). Further research is needed to investigate whether the most important toxic allergens are detectable in the blood or tissue of the European Bee-eater; if these toxins are indeed incorporated into bird tissues, the colorful plumage of bee-eaters could then be considered aposematic warning coloration.

Stinging Hymenoptera are usually grabbed with the tip of the bill at the base of the abdomen and struck hard against a branch a few times to kill them. They are additionally squeezed with the tip of the bill, sometimes thrown into the air and caught again. Occasionally the European Bee-eater will grab the end of the abdomen with the tip of the bill and rub it vigorously against a branch several times to squeeze the venom out of the venom gland, which removes the venom but not the stinger. Non-stinging insects (e.g., large dragonflies, bee-like hoverflies; 560; Bastian and Bastian, unpublished data), are sometimes also treated with head strikes to kill or immobilize the insects, so that they can be given to the offspring without the risk of the prey escaping. Bumblebees are sometimes handled without killing and are only nibbled strongly before swallowing (561).

Foraging Strategy

The foraging strategy of the European Bee-eater corresponds to models of optimal foraging, where the food is brought to a central place and handled (562). There are two different types of foraging: single-prey loaders, which bring only one piece of prey per feeding flight to the central place, and multiple-prey loaders, which bring more than one piece of prey per feeding trip (562). The European Bee-eater is a single-prey loader.

For single-prey loaders, the probability of catching an attractive prey item is always the same during foraging (525), but a minimum acceptable prey value is proposed, i.e., a rule in the form "try to catch a prey and return to the central place with it if the prey has at least a minimum caloric value; otherwise continue the search" (562). The choice of a minimum acceptable prey value determines the feeding rate because the minimum acceptable prey value determines both the size of the prey and the time needed to catch and deliver a prey item. As the minimum acceptable prey size increases, the total number of acceptable prey items decreases, so that the expected search time increases (525). In addition to prey size and its effect on foraging time, prey choice also depends on distance or flight time to the foraging area; this model follows a probit function, where the point of maximum slope is defined as critical travel time, which depends on the distribution and diversity of prey. Below the critical travel time, foraging is non-selective, such that prey is taken according to its relative abundance, since the short flight time makes it worthwhile to take smaller but more abundant prey (563, 431). Prey becomes larger as the flight time to the nest increases, and the proportion of small items brought to the nest decreases as the critical travel time increases (563). Overall, however, the size of the prey items fed is larger than the size of the prey items eaten by themselves (536, 62, 317, 22, 564). If there is a local surplus of a certain kind of prey, it is exploited in an opportunistic manner, but if the cost of hunting smaller prey items is only slightly less than that of hunting larger prey items, because small prey items are not much more abundant than larger prey items, high-energy, larger prey items are preferred even close to the breeding colony (565) and within the critical travel time determined in an optimal habitat in southern France (563).

The European Bee-eater does not hunt selectively (or at least hunts less often) when the travel time to the feeding area is less than about 40–60 s (563). This means that at a flight speed of 10–12 m/s (566, 510, 467), it can be assumed that hunting is more opportunistic in a radius of 400–600 m around the breeding colony. In a telemetry study in southwest Germany, 90% of all positioning data during the breeding period were at a maximum distance of 680 m from the breeding colony; it can be assumed that European Bee-eaters foraged non-selectively and opportunistically during the breeding period at this colony (431). The home range is very small during the nestling period and is focused on the colony location, with the radius of the home range rarely reaching more than ~1 km (529, 416, 431). Adults increasingly took advantage of the opportunity to hunt quickly in the immediate vicinity of the breeding colony to meet the growing food requirements of nestlings (522). Overall, however, the average size of the prey items fed is larger than the average size of the prey items eaten by the adults themselves (536, 397, 62, 317, 22, 564).

Additionally, foraging success depends on how large the colony is, because individuals with numerous nearby neighbors have better access to information about a newly discovered food source. The feeding rate of individual birds correlates positively with the proximity to other breeding pairs, indicating information transfer between colony members (554). This may explain why individuals seek the vicinity of existing colonies, even if the breeding sites still available are suboptimal and breeding burrows are built in substrates that are unsuitable and would not usually be selected (450).

Diet

Major Food Items

Diets have been studied repeatedly in detail in many countries in Europe (Table 6), mainly in relation to food choice, apicultural economics, or to determine seasonal and regional variations. Diet composition, as well as regional differences, are the result of differences in habitat quality, season, and weather, all of which have an effect on insect abundance and accessibility (297, 536, 519, 520, 564, 567, 521). The prey, caught on the wing, should have a certain minimum size (528), typically 14–19 mm, with maximum sizes of 75 mm to over 80 mm sometimes recorded (529, 21, 568). The invasive and economically problematic Asian hornet (Vespa velutina), which has been appearing in Europe for several years, is also captured (569, 570). Prey items smaller than 10 mm are rarely exploited (571, 397, 394). However, there are still knowledge gaps about the prey spectrum of adult birds that hunt for themselves (521).

When not feeding others (juvenile and partner feeding), the European Bee-eater feeds opportunistically and exploits local concentrations of prey, and the abundance of Hymenoptera varies considerably and is not always the most common prey group. The composition of the diet is determined by the availability and the accessibility of suitable prey, with typical prey including bees (especially Bombus sp. bumblebees, Apis mellifera honeybees), wasps (Vespidae; 297, 279, 229), beetles (Coleoptera; 572), dragonflies (Odonata; 563), locusts (Orthoptera; 89), flies (Diptera; 522), and butterflies and moths (Lepidoptera). Locally, the prey list can vary considerably. In addition to prey remains, small stones and occasionally larger quantities of fine sand are found in pellets (377, 536); vegetable matter is almost never found in pellets. However, 5 out of 19 European Bee-eaters that starved to death during a cold spell in Zimbabwe had hard, unripe fruit capsules of the tree Toona ciliata in their throats (573).

Quantitative Analysis

In adult birds, diet studies are based on examination of stomach contents or, more often, pellets collected from under perches. The diet of juveniles is determined from studies of pellets collected from the burrows, by the neck ring method, or by photographic documentation of prey carried into the burrow. The results of 56 studies across Europe, Russia, and Africa, with a total of more than 130,000 prey items (521), show a dominance of Hymenoptera, with shares ranging from 13–100% of the diet; only in five studies did Hymenoptera account for less than a third of the total prey items (321, 523, 572, 534). Within Hymenoptera, the proportion of bumblebees (Bombini) and other Apidae (mainly Apini) in the diet is roughly the same overall, and across all surveys, they each make up about a quarter of the diet, though there are significant differences locally.

The results of dietary studies using pellet and stomach content samples differ significantly from studies based on direct observations. In pellet studies, Hymenoptera sometimes account for over 90% of the prey items (235, 279, 62, 454, 574, 22, 543, 568, 242, 565, 229, 575, 576), Apidae can account for two-thirds to three-quarters of the food items (542, 539, 19, 577, 578, 579, 568), and bumblebees alone can making up over 50% of the prey items (539, 536, 577, 578, 178, 579, 580, 565, 229). The share of Hymenoptera in samples based on direct observation of feeding adults is 20% to 30% lower, however, while direct observations show that Lepidoptera, Odonata, and large Diptera are fed in quantities far different from those found in pellets (e.g., 279, 22, 567, 521). We assume that bee-eaters completely digest weakly sclerotized prey and that these are, therefore, either not detected in pellets and stomachs, or only detected in low numbers (521).

Except for one Algerian study (543), honeybees (Apis mellifera) are always included in prey lists, which is the reason why European Bee-eaters are considered pests by beekeepers and persecuted in many regions of the world (see Conservation and Management). In a colony in the vicinity of beehives, the proportion of honeybees in the diet was about twice as high as in a colony that had no access to hives (571, 526, 564); European Bee-eaters hunt honeybees around hives in particular on cold, cloudy, or rainy days (303, 581, 297, 19, 526, 97, 89, 582), and less frequently during the midday hours than at other times of the day (583). Heavier and more protein rich drones (584) are preferred to lighter worker bees (585, 586, 536, 526, 22), and are usually fed to nestlings (536). The proportion of honeybees in the diet can vary considerably from year to year at the same breeding site depending on the weather conditions (576). In the Czech Republic, the ratio of bee drones to workers in the diet was 53.5% versus 46.5%, although the naturally occurring ratio is about 2 to 100 (529). The proportion of bumblebees in the diet is usually lower in the adult diet than in the nestling diet (e.g., 587, 529, 564), is usually higher in August than in June–July at the same breeding site (536, 529), and increases as the climate tends towards a greater Atlantic influence (522). In northern Europe, the most important single prey type are Bombus spp. (373), because they can be active at temperatures below 12° C (66). In southern Europe (Portugal, Spain, southern France, Italy), Uzbekistan, and South Africa, bumblebees are missing from the diet (539, 588, 589, 578, 579, 567, 89). Only in a few studies do wasps (Vespidae) make up a larger part of the diet, i.e., representing more than 20% of the prey items (297, 279, 540, 590, 22, 591, 439, 229).

Next to Hymenoptera, beetles (Coleoptera) make up the second largest part of the diet; averaged across all evaluated studies, beetles make up 17% of the diet. Based on pellet studies only, the proportion of Coleoptera in the juvenile diet (14.9%) is lower than in adult birds diet (20.4%), while the share of Hymenoptera in the diet of young birds is higher (76.6%) than that of adults (64.3%, 521). In a Portuguese and in several eastern European and Russian populations, Coleoptera made up a quarter to a third of the diet (592, 529, 317, 540, 572, 593, 594, 567, 115). In only seven diet studies, from Germany (216), Netherlands (590, 591), Russia (574, 439), Italy (22), and France (523), were beetles completely missing.

Abundant dragonfly (Odonata) populations are fed to the young birds in the Camargue (France; 523, 563), at lake Neusiedl in eastern Austria (533), in the Danube lowlands of northern Hungary (594), in lake-rich, eastern German post-mining landscapes (564), and in pond areas near Lednice in the south of the Czech Republic (321). In Hungary, 29% of the food items were Odonata; in Sicily, 21%; in a German study, 34%; and in the Camargue, Odonata made up as much as 50% of food items (523, 22, 594, 564). In the Hungarian lowlands of the Danube, the proportion of dragonflies is about the same as that of Hymenoptera (594). In the lower stretches of the Amudarya (Uzbekistan), the diet of juveniles consisted primarily of locusts (89); locusts were also a significant part of the diet in Kenya and Malawi during mass outbreaks (595). On the North Sea coast in Germany, butterflies (Lepidoptera) were the second most important prey item after Hymenoptera, making up 13% of the diet (522). In a Danish study, the diet consisted of 20% Lepidoptera and 26% Odonata (66). In the Camargue, nestlings were also observed to be regularly fed with Lepidoptera, while the analysis of pellets did not show any traces of them (331).

Differences in diet across different prey studies can be explained in most cases by special habitat conditions (e.g., widespread aquatic habitats), plagues of insect pests, or other swarming insects. In a Hungarian study, 38.5% of the food were Coleoptera, most of them pests of cereal crops (572). Given the unusually high proportion of Coleoptera in the diet, it is likely that there was a high abundance due to a pest outbreak, similar to a study from Ukraine, where more than 70% of the diet consisted of a single Coleoptera species (Anisoplia agricola) that eats young cereal grains and can become a pest (534). In a pellet study from Oman, 39% of the diet consisted of swarming Isoptera (524), and in Zimbabwe, almost 60% of the prey found in pellets were ants and termites (596). Diptera, which play a subordinate role in the diet overall, can become locally relevant if, for example, large Tabanidae or Syrphidae are common, possibly only temporarily, as reported in a few studies from Slovakia, Tunisia, Russia, and the Camargue (523, 317, 543, 519, 593, 520).

Opportunism in food choice is also reflected in prey lists that vary over the course of a year, which in turn depends on shifts in the local abundance of insect species (331, 564, 434). In the Camargue, the European Bee-eater hunted mainly beetles and bees in May, while in June it hunted mainly beetles and dragonflies; this variability is likely a reflection of prey availability (331). In Germany, dragonflies decreased in the adult diet as chick rearing started, whereas the consumption of small hymenopterans increased by the end of the provisioning period (564). On one day in early August, silver y (Autographa gamma) moths were very abundant, and consequently accounted for 78% of Lepidoptera prey items, 80.3% of Lepidoptera dry mass, and roughly a quarter of the total prey items in the diet on that day. A few days later, a mass outbreak of dung beetles (Aphodius spp.) occurred, which made Coleoptera the second largest prey order on that day. Similarly, hoverflies (Helophilus spp.) were strongly represented on one day, making Diptera a frequently hunted order with a dry weight percentage of the diet of 14.9% (522). In some cases, the capture of fish has been observed or suspected (see Foraging; 550, 549).

In captivity, some insect groups were rejected, including tiger moths (Arctiinae), sawflies (Tenthredinidae), leaf beetles (Chrysomelidae), lady beetles (Coccinellidae), soldier beetles (Cantharidae), and firefly beetles (Lampyridae), while unexpectedly, burnet moths (Zygaenidae) and Spanish flies (Lytta vescicatoria), both of which have strong chemical defenses, were eaten in small numbers (597).

The investigation method impacts the analysis of the food composition of the diet of the European Bee-eater significantly (see Table 6). The proportion of Hymenoptera in 92 samples based on pellet or stomach content analysis is 72% (Table 6), whereas the proportion in photographic documentation of prey and from the neck ring method (n = 15) is significantly lower, comprising on average 49% of the diet (321, 586, 523, 588, 22, 590, 591, 594, 216, 564, 522, 527, 534, 66). Dragonflies and butterflies are rarely detected in pellets or in stomach contents (3.1% and 0.6% of the diet on average, respectively), however, they are often documented photographically as prey items (19.3% and 11.7% of prey items, respectively). Food analyses based on the evaluation of pellets, as is the case in about 80% of all food studies (521), do not provide a representative picture of the full diet of the European Bee-eater, and can only be used for specific questions, such as the different abundances of Hymenoptera or Coleoptera in the diet (521).

Food Selection and Storage

Food selection is associated with the how different insects fly (397), and will be influenced by season, time of day, abiotic factors such as temperature, humidity, and precipitation, the local habitat, and the supply and composition of potential prey (564), as plant and structural diversity generally increases the number of insects (598, 599), but also affects the species diversity (593). For example, the occurrence of dragonflies is associated with the availability of water bodies, which explains the dominance of Odonata in the diet of populations in some lowland regions of Europe (321). Local concentrations of large insects, and food items that are not limited (600) are often reflected in the European Bee-eater's diet, suggesting opportunistic food choices, which suggests that more food is carried to the nest than the young can eat (557, 601, 600). On the other hand, the composition of prey items found under or in nest burrows does not correspond to a representative sample of juvenile food, as described by direct observation of feeding adults, but deviates considerably from this with a clear over-representation of Hymenoptera (521). Thus, the hypothesis that prey found in or in front of the breeding cavity represents surplus food requires critical examination.

Nutrition and Energetics

Courtship Feeding

During the courtship period, a male can either provide prey to the female before and during egg-laying (courtship feeding; 602), eat it itself (self-feeding), or both birds hunt separately for themselves. Courtship feeding begins a few days before egg laying, continues during the breeding season, and ends a few days after the young have hatched. In the Caucasus, during the breeding season, the male feeds the female at irregular intervals. If the weather was favorable and insect flight good, the male fed the female 3–4 times in a row, followed by a longer feeding break. The male could feed the female up to 15 times per hour, usually between 2 and 6 times, on average 5.43 times per hour (603). In a detailed study on courtship feeding from Camargue (France), it was found that courtship feeding can meet the females' food requirements almost completely in years with a good food supply, while in years with less, it can still meet about 2/3 of their needs (602). Females receive mostly larger prey items, while smaller prey is eaten by the male himself (602). Although there are no studies from other regions comparable to those carried out in Camargue, a study from Italy on weight fluctuations in males and females during the breeding season may shed light on courtship feeding there (22); in the period from mid-May to June, the average weight of females increased by 18%, but then decreased by 25% during the subsequent nestling feeding period until mid-June (22). Because courtship feeding is mainly done during the egg-laying and incubation period, as well as a few days before and after, the period of weight gain in females likely coincides with the time when they are fed with large, high-calorie prey. During the same period, the weight of the males remained more or less constant and only decreased (-11%) when the hatched chicks were fed (22). Courtship feeding can thus optimize the energy balance of females in preparation for the energy-consuming feeding phase. Feeding females with large insects is also energetically advantageous for the male: it takes longer to pass a small prey item to the female than if the male eats it himself, whereas the opposite is true for large prey (507). Since the time required by a male to transport and handle prey affects the amount of time left for foraging, males must also optimize their own energy budget while maximizing the food/energy provided to the female (602).

Feeding Nestlings

Nestlings are fed by both parents and, if necessary, by additional breeding helpers. The diet of young birds is very diverse, and prey selection diet is driven more by prey availability and less by selection of specific groups. According to the central-place-foraging theory, the share of large prey in the nestling diet increases the further away the foraging area is; then the prey items are usually larger than those caught near the colony or which the adults eat themselves (563). In a feeding experiment, young birds from Camargue received one of three diet treatments: only Hymenoptera, only dragonflies, or a mixed diet of both insect groups. Overall, 40–50% of the dry mass, and 60% of the caloric value of both dragonflies and Hymenoptera, were converted into body mass (588). Similarly, there was no difference in the caloric value of the eliminated pellets and feces from either diet. Despite these energetically similar values for the two insect groups, the weight development of the young birds in the three groups differed: the chicks that received a mixed diet showed a higher daily weight gain than those that received only one type of insect, with experimental growth values being within the range observed in the wild (588). The reason for a more efficient transfer of energy from mixed diets is not known, but it is beneficial to the nestlings to bring multiple types of insects, even when one might be particularly abundant (588).

Feeding Fledged Young

After the young birds have fledged, which takes place asynchronously over several days, the young are still fed by the adults for several days. The young birds sit on dead trees, dry branches, power lines, or other perches, and are fed by both parents. As in courtship feeding, large prey items are preferentially fed to the fledglings, and the remaining small prey items are either fed to the fledglings or eaten by the adults themselves in roughly equal proportions. This pattern of prey delivery deviates from the predictions of optimal foraging theory, which in this case predict that adults should feed all of the small items to fledglings while eating the large prey items themselves, possibly because of the energy needs of adults. The time of feeding fledglings also coincides with molt and preparation for migration; the adults, which become considerably lighter during the nestling feeding period (507, 22), must now seek their own pre-migratory fattening, which helps to explain deviations from the theoretical optimal foraging model during this period (507).

Growth and Feeding Efficiency of Fledglings

Young birds must learn how to hunt and treat their prey. In South Africa, the hunting success of adults was measured at 83%, while in the offspring that had fledged in early January, the success rate was only 23%. The success rate of fledged young doubled within a week, but was still well below the rate of adult birds at the end of January at 62%; it was not until the end of February, i.e., after about eight weeks, did young birds hunt as successfully as adults (506). Not only foraging must be learned, but the whole sequence from hunting and foraging to prey handling and consumption; fledglings not only have a higher rate of unsuccessful hunting attempts, but they also have longer handling times, a greater probability of dropping prey, and a higher failure rate in catching large, fast-moving dragonflies (507).

Metabolism and Temperature Regulation

Metabolism

Few data are available on the metabolism of the European Bee-eater.

The average metabolic cost for a foraging European Bee-eater has been calculated as 1.5 J/s, which is equivalent to 0.071 mg of prey (563). Energy assimilation in the European Bee-eater also matches other Meropidae species, but is low compared to other bird species at 54–62% (588, 604). Following this figure, and calculating the dry mass (563, 564) for bees/wasps (27 mg), bumblebees (87 mg), and medium-sized (131 mg) and large dragonflies (274 mg), we get an insect demand per hour for a foraging bird of nine bees/wasps, three bumblebees, two medium-sized dragonfly, or one large dragonfly. Applying these values, the critical travel time for a flight from the nest to the feeding area (see Feeding) increases from 40 s for smaller prey, to 50 s when medium-sized prey are consumed, and to > 70 s when large flying insects are predominant. The calculation also demonstrates why a mix of small and large insects are preferred, as smaller insects require less energy to hunt (588).

The calculation of the food requirements of young birds is difficult and only possible by combining different data, some of which are only available from other bee-eater species and thus will be approximations for the European Bee-eater. The mass at hatching is about 5 g, and increases to about 55 g by day 19, which corresponds to an average weight gain of 2.7 g per day (89). For the White-fronted Bee-eater (Merops bullockoides), data on daily food requirements are available (605): nestlings required 55% of their morning weight in food (fresh weight) for the first five days of life, and 40% thereafter. Assuming a fresh to dry weight relationship of 31% (605), and the dry masses of different insects as defined above (563, 564), the daily food requirement of young birds over the first few days would be 80–100 bees/wasps, or 20–25 bumblebees. A 19-day-old juvenile would have a daily requirement of 250 bees/wasps, 80 bumblebees, or 25 large dragonflies. With an assumed daily hunting time of twelve hours, every young bird at an age of 19 d requires either one bee/wasp about every 3 min, one bumblebee every 9 min, or one dragonfly every 30 min. In captivity, a 66-day-old fledgling consumed about 200 wasps a day (528). Even if the basic parameters of the estimate vary locally, this extrapolation illustrates the pressure on adults to provide their young with large prey items.

The average oxygen consumption for a European Bee-eater at rest is 1.09 (± 0.16) ml O2/min at a basal heart rate of 220 beats/min, resulting in a basal metabolism rate (BMR) of 0.360 (± 0.053) W (606). This BMR roughly doubles in soaring flight, although the heart rate remains unchanged. However, when birds are actively flapping, the heart rate increases significantly to 510–570 beats/min, with parallel increases in oxygen consumption and BMR (606).

Thermoregulation

The European Bee-eater is very heat-tolerant and sits freely on dead perches (Bastian and Bastian, unpublished data) even during the hottest parts of the day in tropical nonbreeding grounds. At very high temperatures, it sits high-legged with closely fitted plumage, lifts its wings out of the breast plumage, and pants with its bill open (528). It will bathe on rare occasions for thermoregulation purposes (62). In Tanzania, it has been observed diving into water holes during hot midday hours (Bastian and Bastian, unpublished data). On the hottest days of the season near Eilat (Israel), when temperatures were 43°C or higher, birds were observed bathing in the Red Sea or diving into shallow salt ponds (478). It is possible that bee-eaters submerging in water to cool themselves is the reason for the unusual record of a bird caught by a tiger shark (Galeocerdo cuvier) in the Red Sea (607).

Drinking, Pellet-Casting, and Defecation

Drinking

The European Bee-eater generally does not drink, even during periods of extreme heat. Drinking can occur when the birds occasionally snap with their bill after water-drops during rain-baths (537, 528), or during flight baths they may sometimes drink from the water surface (Bastian and Bastian, unpublished data).

Pellet-Casting

Indigestible parts of the exoskeleton of insects are compressed in the stomach into pellets and then regurgitated. Chitin is excreted completely via pellets and not with the feces (3). Pellets are often used to examine diet, even though the remains of the insects found in pellets does not always reflective of the food supply (see Diet). For example, very few butterflies and dragonflies are found in pellets, but direct observations suggest they are captured in much higher numbers (331). Head plates, venomous spines, and wings of hymenopterans are often detected in pellets, as well as the elytra of beetles. Butterflies and dragonflies sometimes have their wings removed before feeding, which may help to explain their rarity in pellets (394).

Pellets are produced every 1.5–4 h, or about six times a day (528, 62). When a pellet is produced, any offered prey is rejected, but a previously caught prey item is kept in the bill until shortly before the pellet is released (Bastian and Bastian, video documentation). Pellets are loose, glossy black, and have a length of 24–35 mm, a width of 8–15 mm (377, 553, 536, 543, 608, 609), and a dry weight of 0.10–0.55 g, although they are typically 0.15–0.30 g (539).

Defecation

Nestlings in the burrow back up until the tail touches the burrow wall before defecating; this keeps the middle of the cavity clean. The excrement is rather fluid and has no mucous membranes. After defecation, there is often a short, repeated scratching back, which over time forms a flat hollow in the center (528, 273).

Recommended Citation

Bastian, H.-V. and A. Bastian (2024). European Bee-eater (Merops apiaster), version 3.0. In Birds of the World (S. M. Billerman and B. K. Keeney, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.eubeat1.03
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