Nectar: an energy drink used by European songbirds during spring migration

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Journal of Ornithology ISSN 0021-8375Volume 152Number 4 J Ornithol (2011) 152:923-931DOI 10.1007/s10336-011-0675-4

Nectar: an energy drink used by Europeansongbirds during spring migration

Jacopo G. Cecere, Fernando Spina,Susanne Jenni-Eiermann & Luigi Boitani

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ORIGINAL ARTICLE

Nectar: an energy drink used by European songbirdsduring spring migration

Jacopo G. Cecere • Fernando Spina •

Susanne Jenni-Eiermann • Luigi Boitani

Received: 27 September 2010 / Revised: 15 January 2011 / Accepted: 14 February 2011 / Published online: 4 March 2011

� Dt. Ornithologen-Gesellschaft e.V. 2011

Abstract Nectar exploitation by European birds mainly

refers to passerines feeding on exotic plants, although some

recent studies described nectar-feeding by trans-Saharan

passerines on local plants. We examined which birds and

plants are involved in nectar consumption and investigated

the consequences of nectar use on plasma blood glucose

concentrations during spring migration at a stopover site in

the central Mediterranean. We recorded 12 opportunistic

nectar-feeding species, with Sylvia warblers in particular

showing a marked nectarivory, and 6 plants used by birds,

no one of which shows typical ‘‘bird flowers’’ features.

Moreover, we demonstrated that nectar uptake led to an

increase in plasma glucose concentrations and that nectar

was drunk not only by birds in poor physical condition. The

nectar consumption is fully in accordance with time-min-

imising migration models, allowing birds to obtain water

and energy at stopover sites in a short time: nectar is easy

to find and quick to digest.

Keywords Migrant � Pollen transport � Plasma glucose

concentration � Sylvia � Stop-over site

Zusammenfassung Nektarkonsum durch europaische

Singvogel wurde bisher vornehmlich an exotischen Pflanzen

beobachtet. Jungere Studien beschreiben aber auch Nektar-

konsum durch trans-Sahar Zugvogel an einheimischen

Pflanzen. In dieser Studie untersuchten wir wahrend des

Fruhlingszuges an einem Rastplatz im zentralen Mittelmeer-

raum, welche Vogelarten welche Pflanzen fur den Nektar-

konsum nutzen, und welche Auswirkungen die

Nektaraufnahme auf die Plasmakonzentration der Glukose

hat. 12 Arten, insbesondere Grasmucken, tranken Nektar an

insgesamt sechs Pflanzenarten, von denen keine die typischen

Merkmale der Ornithophilie zeigte. Die Nektaraufnahme

fuhrte zu einem Anstieg der Plasmaglukosekonzentration und

wurde vor allem von Vogeln in schlechter korperlicher Ver-

fassung genutzt. Nektar ist eine Energiequelle mit vielen

Vorteilen: sie ist schnell zu finden, rasch aufzunehmen und

leicht zu verdauen. Dieser Vorteil, Wasser und Energie

in kurzer Zeit aufzunehmen, reduziert die Zeit, die Zugvogel

an einem Rastplatz verbringen mussen und unterstutzt das

Zeit-Minimierungsmodell.

Introduction

Plants pollinated by birds are generally typified by tubular,

red and scentless flowers with a particular nectar compo-

sition and quantity (Johnson and Steiner 2000; Nicolson

2002; Nicolson and Fleming 2003; Rodrıguez-Girones and

Communicated by C. G. Guglielmo.

J. G. Cecere (&) � L. Boitani

Department of Biology and Biotechnology Charles Darwin,

University of Rome ‘‘La Sapienza’’, Viale dell’Universita 32,

00185 Rome, Italy

e-mail: [email protected]

L. Boitani


F. Spina

Istituto Superiore per la Protezione e la Ricerca Ambientale,

Via Ca Forcanetta 9, 40064 Ozzano Emilia, BO, Italy


S. Jenni-Eiermann

Swiss Ornithological Institute, Seerose 1,

6204 Sempach, Switzerland


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J Ornithol (2011) 152:923–931

DOI 10.1007/s10336-011-0675-4

Author's personal copy

Santamarıa 2004), although not all bird-flowers fit this

description (Ollerton 1998). Specialised nectarivores birds,

which regularly visit and pollinate plants, belong to only

seven families (Trochilidae, Drepaninidae, Nectariniidae,

Promeropidae, Meliphagidae, Psittacidae and Fringilli-

dae). But many other species, belonging to several families

(see, for example, Gryj et al. 1990; Symes et al. 2008;

Brown et. 2010a), are occasional nectarivores (Johnson and

Nicolson 2008); this is true of Sylvia warblers in Europe

(Stiles 1981; Ford 1985; Laursen et al. 1997; Pellmyr 2002)

which have representatives that opportunistically visit

flowers and feed on nectar, but do not necessarily pollinate

them.

Nectar exploitation by European birds mainly refers to

passerines feeding on exotic, introduced plants whose

flowers often show features typically used to attract birds

as pollinators (Holm and Laursen 1982; Prinzinger 1988;

Proctor et al. 1996; Harrup 1998; Merino and Nogueras

2003), but it has also been recorded for European plants,

such as Rhamnus alaternus, Ferula communis, Acer, Cra-

taegus, Salix, Ribes (reviewed by Ford 1985; Kay 1985;

Calvario et al. 1989; Schwilch et al. 2001). In 1989, Bur-

quez described the effective pollination of an exotic plant,

Fritillaria imperialis, by Parus caeruleus in Great Britain

(Burquez 1989), and in 2005, there was the first confir-

mation of a native bird-pollinated plant, Anagyris foetida,

in Europe (Ortega-Olivencia et al. 2005). However, the

ecological interactions between flowering plants and

European birds are still poorly studied.

In Europe, nectar consumption has also been observed in

migratory birds landing at stopover sites on Mediterranean

islands (Schwilch et al. 2001; Cecere et al. 2010); nectar-

feeding by trans-Saharan passerines has also recently been

described at oases in the Sahara desert (Salewski et al.

2006). During spring migration, trans-Saharan passerines

must cross two large ecological barriers—the Sahara desert

and the Mediterranean Sea—in order to reach their

breeding areas. Hence, they have to fly for approximately

2,000 km over inhospitable land or water. Some of them,

however, may have the chance to land on suitable areas

such as Mediterranean islands, after non-stop flights of up

to 14–16 h across the sea (Pilastro et al. 1995).

During such endurance flights, migrants have to deal

with a particular physiological situation, since they have to

fast, even up to several days, despite very high energy-

demanding conditions (Jenni and Jenni-Eiermann 1998). It

has been shown that the energy used for flight is mainly

derived from fat stores and to a lesser extent from proteins

(Jenni and Jenni-Eiermann 1998). Proteins not having a

storage form, they are obtained from the breast muscles

and the digestive organs (Schwilch et al. 2002). In a pre-

vious study on Ventotene Island (Italy), it was suggested

that nectar is an ideal first food for birds with a reduced

digestive capacity, because (1) monosaccharides do not

have to be digested as they are absorbed directly, and (2)

protein catabolism is stopped (Schwilch et al. 2001). It has

not yet been established whether nectar consumption

increases the plasma glucose concentration and is therefore

useful for reversing the process of lipolysis and protein

breakdown, whether it would only be a short-term aid for

burst flights or could rather be useful to continue the

migration journey in the longer term.

The aims of this study were to: (1) examine which bird

and plant species are involved in nectar consumption on

Ventotene Island; and (2) test the hypothesis that songbirds

gain physiological benefit in term of plasma glucose con-

centration from nectar consumption. Nectar is normally

characterised by high glucose concentration, and we pre-

dict that its exploitation leads to an increase in bird’s

plasma blood glucose level and that this benefit might

persist after few hours from the last feeding, at the post-

absorptive state. Moreover, we expect that nectar is more

frequently used by Sylvia warblers in poor physical con-

dition after endurance flights, while birds in good condition

target on more usual and nourishing food.

Methods

The field work was carried out between 1 April and 30 May

2006 and 2007 at Ventotene Island, 50 km off the Tyrrhe-

nian coast of Italy. Ventotene is a small inhabited island

(approx. 1.3 km2) covered by several small fields which

were formerly all cultivated. Many fields are now aban-

doned and are characterised by pioneer vegetation. Natural

Mediterranean maquis is confined to cliffs and small sur-

rounding areas. Several species of spring migrants arrive at

Ventotene Island from North Africa, after crossing at least

500 km of open sea by non-stop flights (Pilastro et al.

1995). Since 1998, the Istituto Superiore per la Protezione e

la Ricerca Ambientale, the Italian Ringing Centre, has co-

ordinated the ‘‘Progetto Piccole Isole’’ (Small Islands Pro-

ject) in order to investigate strategies adopted by spring

migrants when crossing the central Mediterranean (Spina

et al. 1993). During 2006 and 2007, 282 m of mist-net were

checked every hour and every day between 1 h before

sunrise to 1 h after sunset, except in bad weather conditions

(strong wind or driving rain), between 1 March and 31 May.

Nectar-feeding birds and plants census

A 1-km transect across the mosaic of fields in Ventotene

was carried out four times per week between 0900 and

1200 hours in order to collect qualitative data on nectar-

feeding bird species and the plants used by birds. The

transect was walked 48 times during 2006 and 40 times

924 J Ornithol (2011) 152:923–931

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during 2007 for 30 min on average (total approximately

44 h). When an unrecorded bird species was found feeding

on a flower or an already recorded bird species was found

taking nectar from a new plant species, we checked for the

presence of insects or other invertebrates as a potential food

resource on each flower of the individual plant visited. The

absence of invertebrates on the plant and a second visit of

the bird feeding on the checked flowers within 10 min were

regarded as nectar exploitation. We excluded the possibility

that pollen is the target of the birds’ visits to flowers, since

Schwilch et al. (2001) clearly showed by faecal analysis that

birds do not feed on pollen at this site. According to the

authors, in fact, the anthers of Brassica are too large and the

smaller Ferula anthers are ingested just accidentally.

A census of nectar-feeding birds was also carried out

during the ringing activity on Ventotene during 2006 and

2007. For each captured bird, the presence or absence of

pollen on the bill, forehead and breast feathers was asses-

sed and scored. The presence of pollen grains on the

plumage of European migrating songbirds is considered

proof of nectar consumption by different authors (Ash et al.

1959, 1961; Laursen et al. 1997; Schwilch et al. 2001;

Salewski et al. 2006). In particular, birds feeding on nectar

from Brassica sp. usually had yellow and granular pollen

around their bill and on their forehead (Fig. 1), while birds

feeding on nectar from Ferula sp. usually had yellow and

gluey pollen around their bill and on their breast feathers

(Schwilch et al. 2001). We calculated the percentage of

captured individuals carrying pollen per species between

mid-April and mid-May for both study years.

Physical condition

The fat and muscle condition of each bird was also scored

during ringing activity on Ventotene; according to Kaiser

(1993), fat scores were assigned by estimating the visible

amount of subcutaneous fat deposition inside the furcula

and on the abdomen (9 score levels). Muscle scores were

assigned by estimating the thickness of the breast muscles

(4 score levels) (Bairlein 1995 based on Gosler 1991) as an

estimate of breast muscle protein mass (Jenni et al. 2000).

Plasma blood glucose concentration

High glucose concentrations have been found in the nectar

of Brassica sp. and Ferula sp. (Schwilch et al. 2001). To

assess the possible effect of nectar consumption on the

plasma glucose level of the birds we measured plasma

blood glucose concentration in Garden Warblers (Sylvia

borin): (1) which had been feeding on nectar, hence

showing fresh pollen on bill and feathers (n = 55; from

now onwards: ‘‘GW.P.’’); and (2) which probably did not

feed on nectar, showing no signs of pollen (n = 53; from

now onwards ‘‘GW.NO-P.’’). For this part of the study, we

considered only birds with fresh pollen on plumage which

recently fed on nectar, contrary to birds with dried and

agglutinated pollen, which most likely fed on nectar at

stopover sites visited prior to reaching the trapping site

(Cecere et al. 2010). Plasma blood glucose concentration

was measured by a handheld plasma glucose monitor

(Bayer Glucometer Ascensia Elite�) normally used on

humans, but also animals. The reliability of this method to

accurately measure plasma glucose level in birds has been

demonstrated by Lieske et al. (2002), and the same glu-

cometer was also used in recent studies on diel variation in

plasma glucose concentration in birds (Downs et al. 2010;

Lobban et al. 2010). It works with disposable test strips and

requires just one drop of blood. Blood was obtained by

puncturing the brachial vein.

For this part of the study, a sample of 108 Garden

Warblers with muscle score 1 and fat score between 0 and

2 were tested on Ventotene Island during the spring of

2007. The mist-nets were observed continuously from a

hide, and when a bird flew into the net, the exact trapping

time was recorded. Glucose is assimilated very quickly;

therefore, to avoid the possible influence of the last food

intake on glucose concentration, we analysed the blood in

the post-absorptive state, exactly 90 min after capture,

when birds were considered to be in a post-absorptive,

fasting state (Jenni-Eiermann and Jenni 1996). All tests

were carried out between 0900 and 1800 hours.

Statistical analyses

After testing for normal distribution (Shapiro–Wilk test),

we used parametric tests. General Linear Model (GLM)

was used to assess the possible influence of nectar-feeding,

fat reserves, test hour and the possible interactions of theseFig. 1 Common Whitethroat (Sylvia communis) with fresh pollen of

Brassica sp. on beak and forehead. Photo: J.G. Cecere

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variables with plasma glucose concentration; all variables

were entered both forwards and backwards, and selected by

SBC (Schwarz Bayesian information criterion), in order to

choose the model that best balances goodness of fit against

model complexity. In order to test whether nectar is more

frequently used by birds in weaker physical condition we

applied Logistic Regression to analyse the possible influ-

ence of fat and muscle on presence/absence of pollen on

plumage, as an indication of nectar consumption. All

analyses were performed with the statistical program SAS

ver. 9.2 (SAS Institute, Cary, NC, USA).

Results

The birds and plants involved in nectar consumption

Birds

During ringing activity on Ventotene, 68 passerine species

(23,614 individuals) were trapped between 1 April and 30

May 2006, and 72 passerine species (26,864 individuals)

were trapped in the same period in 2007. Pollen was

present on the bill, forehead or breast of birds belonging to

12 species trapped during the 2 years (Table 1). Individu-

als belonging to the other 74 passerine specie were trapped

with no indication of nectar feeding, including Icterine

Warbler (Hippolais icterina) that was recorded with pollen

on plumage, with just one individual, in 1997 (Table 1).

Sylvia species had high frequencies of birds with pollen,

while Phylloscopus species, collected only during spring

2006, had lower percentages (Table 1). Among migrating

Sylvia warblers, the species with the highest degree of

nectarivory was the Blackcap (S. atricapilla), 64.4% of

birds with pollen on plumage (2006 and 2007 data analysed

together), followed by Subalpine Warbler (S. cantillans),

Garden Warbler and Common Whitethroat (S. communis),

with 54.8, 38.7, and 34.3%, respectively. Lesser White-

throat (S. curruca), Spectacled Warbler (S. conspicillata)

and Ruppell’s Warbler (S. rueppelli) were trapped in too

small numbers to calculate percentages, while 50% of the

trapped resident Sardinian Warblers (S. melanocephala)

showed indications of nectar consumption (2006 and 2007

data analysed together). Among Phylloscopus, the species

with the highest frequency of birds with pollen on plumage

was Chiffchaff (P. collybita) with 28.6%, followed by

Willow Warbler (P. trochilus), 6.7% and Wood Warbler

(P. sylatrix), 0.2% (2006 data). The Italian Sparrow (Pas-

ser italiae) was the only species not belonging to Sylvidae

trapped with pollen on plumage. The percentages of birds

with pollen on Ventotene, per species for each year of the

study, are reported in Table 1.

During the qualitative transect census, the same species

were observed feeding on nectar, except for Wood Warbler

and Ruppell’s Warbler (Table 2). In May 2007, some

flocks (5–20 individuals) of Italian Sparrows were

observed several times feeding on nectar from Ferula sp.

just in front of the ringing station and two individuals with

Table 1 Sample size of species recorded with pollen on foreheads, bill or breast during the 2 years of study (2006–2007) and in 1997 (Schwilch

et al. 2001)

Species 1997 (Schwilch et al. 2001) 2006 2007

With

pollen

Birds

checked

% With

pollen

Birds

checked

% With

pollen

Birds

checked

%

Subalpine Warbler (S. cantillans) 250 2,244 11.1 171 309 55.3 159 293 54.3

Garden Warbler (S. borin) 169 852 19.8 856 2,620 32.7 546 998 54.7

Common Whitethroat (S. communis) 70 794 8.8 283 855 33.1 176 483 36.4

Blackcap (S. atricapilla) 31 271 11.4 14 23 60.9 15 22 68.2

Sardinian Warblera (S. melanocephala) 1 74 1.4 1 6 16.7 4 4 100.0

Willow Warbler (P. trochilus) 3 1,485 0.2 12 179 6.7

Chiffchaff (P. collybita) 4 595 0.7 4 14 28.6

Wood Warbler (P. sibilatrix) 1 308 0.3 1 429 0.2

Icterine Warbler (H. icterina) 1 588 0.2 0 1,631 0.0 0 1,205 0.0

Italian Sparrowa (P. Italiae) 0 29 0.0 2 40 5.0

Lesser Whitethroat (S. curruca) 1 2

Spectacled Warblerb (S. conspicillata) 1 2

Ruppell’s Warbler (S. rueppelli) 1 2

Total 530 7,211 7.35 1,344 6,100 22.03 903 3,047 29.64

a Breeding species in Ventoteneb Occasionally breeding species in Ventotene

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pollen on the forehead were also mist-netted. Although this

species has been commonly breeding in several colonies on

the island during the 20 years of the ringing project, it had

never been observed feeding on nectar before.

Plants

Six plant species were used by birds in Ventotene

(Table 2), of which four were local (Brassica montana,

Ferula communis, Fig. 2; Lavatera arborea and probably

Malva sylvestris), and two were introduced species (Pru-

nus avium and Pittosphorum tobira). None of these plants

show any bird-flower characteristics. Their flowers differ

in size and structure: medium size for B. montana, L. arbo-

rea, M. sylvestris and P. avium, and small for P. tobira and

F. communis, the flowers of the last being organised in an

inflorescence, while those of the other plants are single.

F. communis was the plant visited by all 10 bird species

observed during trasects, followed by B. montana (7 spe-

cies; Table 2). Only the Subalpine Warbler, an early pas-

sage long-distance migrant, was observed feeding on the

nectar of Prunus avium that blooms between late March

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Fig. 2 Above Blackcap (Sylvia atricapilla) feeding nectar on

B. montana. Photo: courtesy Paolo Fumagalli. Below Subalpine

Warbler (S. cantillans) feeding nectar on Ferula communis. Photo:

courtesy Marzia Mirabile

J Ornithol (2011) 152:923–931 927

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and early April, when most trans-Saharan migrants have

not yet reached the island. Only Garden Warblers and

Common Whitethroats were observed feeding on nectar of

Pittosphorum tobira which is a plant used for hedges in

several local gardens and blooms in May, when the two

warblers are the most common species on Ventotene. Just

one Garden Warbler was observed nectar feeding on Malva

sylvestris.

Effects of nectar consumption on plasma blood

glucose concentration

The glucose concentration of GW.P was significantly

higher (t = -7.6, n1 = 55, n2 = 53, P \ 0.01) than

GW.NO-P, with a mean difference between the two sam-

ples of 73.63 mg/dl (4.05 mmol/l) (Fig. 3). Concentrations

of plasma blood glucose were significantly influenced only

by nectar consumption both entering variables forwards

and backwards (P \ 0.001, R2 = 0.35) but neither by fat

reserves, hour nor by the interactions between all variables,

Nectar consumption positively influences plasma glucose

concentration in Garden Warbler (glucose concentration

Least Square Means of GW.P. = 328.2 ± 7 mg/dl; LSM

of GW.NO-P. = 254.6 ± 6.8 mg/dl).

Nectar-feeding and physical condition

In the Garden Warbler, the presence of pollen was negatively

correlated with fat-score: nectar is used most by birds

with lower fat-scores (fat (range 0–5): B = -0.145 ±

0.071, P = 0.041; muscle (range 0–3): B = -0.218 ±

0.120, P = 0.070, n = 996). In the Subalpine Warbler, the

presence of pollen is positively correlated with fat score

(range 0–5) (B = 0.267 ± 0.103, P = 0.010, n = 289) and

negatively with muscle score (range 0–3) (B = -0.513 ±

0.230, P = 0.026, n = 289). No significant correlations

were found in the Common Whitethroat (fat (range 0–5):

B = -0.038 ± 0.080, P = 0.630; muscle (range 0–3):

B = -0.281 ± 0.199, P = 0.158; n = 482).

Discussion

Nectar feeding represents an important resource of sugars,

and thus energy, for several species of animals, such as bats

(Horner et al. 1998), marsupials (Bradshaw and Bradshaw

1999) and birds (Castro and Robertson 1997; Nicolson and

Fleming 2003; Brown et al. 2010a, b). While nectar con-

sumption in birds is widely documented in tropical and

equatorial areas, the extent of nectar consumption and its

physiological effects in European birds during migration

has never been analysed before. With our study, we first

assessed which bird and plant species were involved in

nectar consumption at a Mediterranean stopover site during

spring migration. Secondly, we demonstrated that nectar

feeding may increase plasma glucose concentration in

migrating warblers and that this resource is also exploited

by birds in good condition.

Nectar feeding songbirds

In accordance with previous studies (Schwilch et al. 2001;

Salewski et al. 2006; Cecere et al. 2010), we found that,

among Sylvidae, the genus Sylvia was the one with the

highest frequency of nectar consumption. All species of

Sylvia warblers recorded on Ventotene showed the pres-

ence of pollen on plumage, as an indication of nectar

consumption. In particular, Blackcap and Subalpine War-

bler and Chiffchaff were the species with the highest

degree of nectarivory among Sylvia and Phylloscopus

warblers, respectively. Given that these three warblers are

short- or medium-distance migrants staging on Ventotene

earlier in the season than long-distance species, they may

find a larger availability of resources when landing at this

stopover site (personal observation).

Compared with Schwilch et al. (2001), who analysed the

presence of pollen on bird plumage on Ventotene in 1997,

we found a higher frequency of individuals with pollen in

most nectar feeding species (Table 1). This difference is

surprising, given the same study site, methods, field pro-

tocols and time of year in both studies. A longer time series

will be needed to assess whether these differences may

result from a different resources availability and/or a dif-

ferent blooming season of plants which are exploited or,

rather, from a change in behaviour of migrating warblers.

Innovations in feeding behaviour are common among

animals (Reader and Laland 2003; Boogert et al. 2008),

absent present

pollen on plumage

0

100

200

300

400

500

)ld/gm(

noitartnecnocesoculg

doolba

msalp

Fig. 3 Distribution of plasma glucose concentration values in 55

Garden Warblers (Sylvia borin) with pollen on plumage (mean =

328.23 ± 7.69 mg/dl) and 53 without (mean = 328.23 ± 7.69 mg/dl)

928 J Ornithol (2011) 152:923–931

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where a new behaviour adopted by few individuals can

rapidly spread through social learning (Lefebvre and Pal-

ameta 1988; Reader 2004). This might be the case for the

Italian Sparrows that consumed nectar in our study, despite

it not having been documented previously on Ventotene.

However, nectar feeding is well described for the conge-

nerous House Sparrows (P. domesticus) (Leveau 2008).

Although social learning is easier to hypothesise for a

resident species such as the Italian Sparrow, which can

theoretically observe several individuals of migratory

songbirds feeding on nectar, social learning has also been

demonstrated for migrants at stopover sites during social

foraging activity (Nemeth and Moore 2007).

Plants involved in nectar exploitation

Six plant species, belonging to five different families, were

used by birds for nectar feeding on Ventotene island. None

of them have clear bird-flower features, such as tubular, red

and scentless flowers (Johnson and Steiner 2000; Rodrı-

guez-Girones and Santamarıa 2004). F. communis and

B. montana were the plants used by the highest number of

bird species (Table 2), which is consistent with the

importance of these plant species for nectar-feeding song-

birds on Ventotene, as shown by previously studies

(Schwilch et al. 2001; Cecere et al. 2010). The fact that six

plant species used by birds are characterised by different

floral structure and size suggests a strong opportunism by

warblers. The use of Ferula and Brassica was already

described in Schwilch et al. (2001), although B. montana,

with leaves lacking a petiole, was incorrectly identified as

B. fruticulosa, which has leaves with a petiole (Pignatti

1982). To our knowledge, the use of nectar of the other

four plants (Lavatera arborea, Prunus avium, Pittospho-

rum tobira and probably Malva sylvestris) by birds has

never been previously described in Europe. The plasticity

of some bird species to nectar feeding, also supported by

their opportunistic behaviour, can be an important selective

pressure driving plants to adapt towards the use of birds as

pollinators. Johnson and Nicolson (2008) showed that, both

in Africa and in America, plants pollinated by occasional

nectarivores birds are characterised by specific nectar

properties which differ from those of plants pollinated by

specialised nectarivores birds. However, this model still

has to be properly investigated in the Mediterranean region.

Consequences of nectar consumption

In our study, Garden Warblers with pollen on their plum-

age had higher levels of plasma glucose concentration

compared with birds with no pollen. All birds were tested

in a short-term fasting state, characterised by a significant

decrease of triglycerides and a significant increase of

ketone bodies in plasma, indicating fat catabolism (Jenni-

Eiermann and Jenni 1996). It is known that, during this

state, glucose is regulated within narrow limits in Garden

Warblers: no significant change in plasma glucose con-

centration was observed between the resorptive state, a

short-term fasting state (90–120 min without food) and a

long-term fasting state (10–12 h) (Jenni-Eiermann and

Jenni 1991). In our study, the difference in plasma glucose

levels between birds with and without pollen on plumage is

remarkable. Considering the normally narrow range of

variation (see above), this difference could be explained

only by the food ingested, hence by nectar feeding (Fig. 2).

Since only birds with fresh pollen on plumage were ana-

lysed, we can maintain that these birds fed on nectar in situ,

as suggested by Cecere et al. (2010). During spring

migration, birds with fresh pollen on plumage are present

on Ventotene Island, but they are virtually absent from

other two nearby islands, Ponza and Zannone, located in

the same archipelago and, therefore, within the same

migratory flyway (Cecere et al. 2010). This observation

indicates that fresh pollen came directly from Ventotene as

also confirmed by the time budget analysis reported in the

same study.

The increase in plasma glucose, as suggested by

Schwilch et al. (2001), may stop protein catabolism and

play a significant role in helping birds with a reduced

digestive capacity. It is possible that nectar, as a source of

energy, could be an important aid for migrating birds after

endurance flights across challenging ecological barriers.

Nectar feeding and physical condition

Nectar was fed by birds disposing of sufficient fat reserves

(Common Whitethroat, Subalpine Warbler) as well as by

birds disposing of small fat reserves (Garden Warbler).

Muscle score had a significant effect only in Subalpine

Warblers and not in the other species. These results suggest

that all birds, independent of their physical condition, used

nectar as a quickly absorbed energy drink. Moreover,

nectar may also be a valued water source (Symes et al.

2011) on dry Mediterranean islands. It is therefore rea-

sonable to think that its use could reduce the time spent by

migrants at the stopover site. Future studies should confirm

this hypothesis and clarify whether there is a real long

term-advantage, for nectar feeding birds, in the ensuing

completion of migration and the settlement at breeding

territories, verses migrants which only ate proteins en

route.

Acknowledgments We are extremely grateful to Andrea Ferri and

all other ringers who participated in the field activities at Ventotene

ringing station. We thank Carlo Catoni and Diego Rubolini for

important and useful suggestions and Simona Imperio for her

enthusiastic and constructive discussions during the preparation of the

J Ornithol (2011) 152:923–931 929

123


manuscript. We also thank Fabrizio Bartolucci and Laura Cornara for

the identification of Brassica montana and Anna McCann for the first

proofreading. Result from Progetto Piccole Isole (ISPRA) paper no. 45.

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Nectar: an energy drink used by European songbirds during spring migration

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