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Copying nesting attempts in a new site may be the wrong choice. A case in the European Bee-eater (Merops apiaster)

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Abstract

Insistent nesting attempts by a group of European Beeeaters in a new site, a pebbly bank of the middle course of Trebbia River, northern Italy, mostly failed because of the unmovable pebbles encountered during tunnel excavation. The birds later nested in an artificial sand heap, with full success that time. Various considerations suggest that birds insisted in the unsuitable site because they copied the nesting activity of model conspecifics. Finding social attraction and “public information” from conspecifics in a place where no breeding attempt was previously made would allow disentangling social philopatry from spatial philopatry.
Rivista Italiana di Ornitologia - Research in Ornithology, 90 (2): 91-94, 2020 DOI: 10.4081/rio.2020.478
Short Communication
Copying nesting attempts in a new site may be the wrong choice.
A case in the European Bee-eater (Merops apiaster)
Tiziano Londei
Via San Vincenzo 20, 20123 Milano, Italia.
E-mail: londeit@tin.it
© 2020 Tiziano Londei
Received: 7 October 2020
Accepted for publication: 3 November 2020
Online publication: 27 January 2021
Abstract - Insistent nesting attempts by a group of European Bee-
eaters in a new site, a pebbly bank of the middle course of Trebbia
River, northern Italy, mostly failed because of the unmovable pebbles
encountered during tunnel excavation. The birds later nested in an arti-
cial sand heap, with full success that time. Various considerations
suggest that birds insisted in the unsuitable site because they copied
the nesting activity of model conspecics. Finding social attraction and
“public information” from conspecics in a place where no breeding
attempt was previously made would allow disentangling social philo-
patry from spatial philopatry.
Key words: colonial birds, nesting site, copying behaviour, social
attraction, group delity.
Riassunto - Copiare tentativi di nidicazione in un nuovo sito può
essere la scelta sbagliata. Un caso nel gruccione (Merops apiaster).
I tentativi di nidicazione di un gruppo di gruccioni in una sponda
ghiaiosa del medio corso del ume Trebbia, un sito nuovo alla nidica-
zione di gruccioni, sono quasi tutti falliti a causa di ciottoli inamovibili
incontrati durante lo scavo dei tunnel, inducendo gli uccelli a nidicare
in un cumulo articiale di sabbia, dove invece il successo è apparso
completo. Il numero degli uccelli presenti nel primo sito già prima
dell’inizio del primo scavo era compatibile con il numero delle nidica-
zioni riuscite nel secondo sito. Movimenti tra i due siti indicavano che
gli uccelli avevano esplorato entrambi. Tuttavia, nel primo sito gli scavi
sono avvenuti con più intervalli e più insistenza, producendo tunnel per
lo più troppo corti per la nidicazione. Uno dei due soli nidi riusciti
nel primo sito aveva il tunnel adiacente al tunnel del primo tentativo
in assoluto. Queste osservazioni suggeriscono che l’attività di qualche
membro del gruppo sia servita di modello ai conspecici, portandoli a
un fallimento quasi completo nel primo sito. Trovare attrazione sociale
e “informazione pubblica” da parte dei conspecici in un luogo dove
nessun tentativo di riproduzione sia avvenuto in precedenza permette-
rebbe di svincolare la fedeltà al gruppo dalla fedeltà al luogo di ripro-
duzione.
Parole chiave: uccelli coloniali, sito di nidicazione, comporta-
mento imitativo, attrazione sociale, fedeltà al gruppo.
Several studies suggest that “public information”
(Danchin et al., 2004), i.e. information arising from cues
inadvertently produced by the behaviour of other indi-
viduals with similar requirements, may be important in
bird coloniality. For example, Black-legged Kittiwakes
(Rissa tridactila) appeared to rely on the reproductive
success of locally breeding conspecics, rather than on
their own breeding experience, to choose the nesting cliff
the year after (Danchin et al., 1998). In Lesser Kestrel
(Falco naumanni) colonies, immigration of both adult
and rst-breeding birds was positively related to the pres-
ence of philopatric adults, though not to the conspecic
breeding success the year before (Serrano et al., 2004).
Concerning bee-eaters, a suggestion of public information
is apparent in Supriya et al. (2012): having found no sig-
nicant difference between habitat characteristics at soli-
tary and colonial breeding sites of Chestnut-headed Bee-
eaters (Merops leschenaulti), these authors considered the
various hypotheses of coloniality proposed by Danchin &
Wagner (1997) and discarded habitat-mediated aggrega-
tion, conspecic reproductive success (not found to vary
among breeding sites) and hidden lek (bee-eaters being
mostly monogamous); instead, they accepted the tradi-
tional aggregation hypothesis, which is based on the pres-
ence (i.e., group size) or cues of former presence (i.e., old
nests) of conspecics in traditionally occupied sites. Stud-
ied colonies are usually not new colonies and hypotheses
of social attraction usually imply that some conspecics
have gained experience of the breeding site. Within the
limits that are inherent to studying a single case in a genus
of birds still understudied for public information, the fol-
lowing account suggests that social attraction and conse-
quent information from conspecics may occur in a site
where no nesting attempt has previously been made.
Following is a commented summary of my eld ob-
servations. Since many years, the European Bee-eater has
been known to breed colonially along the lower course
of Trebbia River, northern Italy (Ambrogio, 2001), but
at no point of its middle course. My long birding expe-
rience in the latter area had included many sightings of
these birds during their post-breeding movements, but
no sighting of breeding birds. However, at the begin-
ning of May 2020 I noticed birds ying to and from near
the Trebbia in the southern outskirts of Bobbio, an area
hereafter called Site A, and on 6 May I found the rst
92
bird digging in a riverbank. The day after one other pair
began digging a nest-hole and I did not nd any further
nest-holes along the entire riverbank. Thus, I preferred to
leave the area undisturbed for several days, during which,
observed from a distance, several birds appeared to keep
foraging on the savanna-like ground in the back of the
riverbank, although they sometimes left in groups and
their overall presence decreased with time. On 22 May,
while few birds were foraging in the area and no one was
seen entering the soil, I explored the riverbank closely and
counted 37 nest-holes. They encompassed the riverbank
for about 300 m and formed one row in the seemingly less
coarse upper layer of the alluvium, a conspicuous mix-
ture of pebbles and cobbles for the rest. Subsequent ob-
servations revealed bird activity only at two, well-spaced,
nest-holes (Fig. 1a). As one of them was just in front of
a beach-aimed area backed by the car park of some in-
dustrial buildings, the locally most disturbed part of the
river to an ecologist’s eyes, I ruled out the possibility that
most of the birds had deserted their nesting sites because
of human disturbance. At least some of them seemed still
to be linked to the place by foraging convenience. Espe-
cially in late afternoon I repeatedly saw birds in pairs, or
small groups, ying to and from somewhere downriver.
Thus, I searched for a substitute nesting settlement and
found it 2.5 km straight line downstream in an old, con-
solidated sand heap within a disused gravel processing
plant, a closed area, hereafter called Site B. From various
observation points outside I counted 21 nest-holes there,
at seven of which I saw intense food-bringing activity on
14 July. Having my country house between Site A and
Site B, I could easily observe the to-and-fro movements
of the birds, evident almost daily until the end of July.
On 10 August I discovered a third nesting site 10.5 km
further downstream, with at least 40 nest-holes in a clay
slope bordering the river. Contrary to Site A and Site B,
this site had birds still present in the surroundings, though
not at nest-holes, and being in a less-known area to me, it
might have hosted a colony some time before and be the
origin of the birds settling upstream. Whatever the origin
of these birds, the following considerations led me to as-
sume that they rst tried to nest in Site A and later fell
back to Site B. The rst and second digging attempts in
Site A were well-spaced in time, and, as evident in Tab.
1, the two successful nests did not result from the rst,
or second, excavation. These are suggestions of slow re-
cruitment in nesting activity at Site A. Given the greater
proportion of successful nests relative to the number of
nest-holes in Site B, which meant less repeated attempts,
lesser delay in the production of nestlings after the rst
excavation attempts was expected in Site B. However,
on 18 July, while three edglings were already on trees
in Site A, in Site B only few nestlings were looking out
of nest entrance and only one edgling was seen the day
after. A logical conclusion is that Site B was settled later
than Site A.
Nest-hole photographs (Fig. 1b) and measurements
(Tab. 1) evidenced the reason of the massive nesting fail-
ure occurred in Site A: masked by softer soil on the face
of the upper layer of the riverbank, unmovable stones pre-
vented most of the birds from obtaining tunnels of suf-
cient length. Although the two successful nests made a
small sample, the greater length of their tunnels is self-
evident. The length of the other tunnels was in three cases
(72, 122 and 74 cm) within reported ranges for used nest-
tunnels (e.g., Kerényi & Ivók, 2013), but obstacles might
have persisted for the size of the nest chamber or the beam
of the passage. Some of the shorter tunnels might have
been aimed at “false” nest-holes (to misguide predators,
e.g., Inglisa & Vigna Taglianti, 1987), but the much larger
proportion of unused tunnels compared to Site B strongly
suggests that a large part of the tunnels in Site A had been
aimed at true nests. Like other burrowing birds, European
Bee-eaters avoid banks composed of too compact (as well
too loose) soils and can discriminate not only between
high- and low-quality breeding banks, but also between
different soil layers within banks (Heneberg, 2009). Soil
samples from nest-holes in homogeneous banks never had
grains larger than 10 mm (Heneberg & Šimeček, 2004)
and birds appeared very able to avoid the gravelly layers
of heterogeneous banks (Del Guasta & Marcuzzi, 1993).
In the present case, even if the rst birds that tried digging
were misguided by supercially suitable soil character-
istics, why so many insistent attempts to dig across un-
movable material, beginning at considerable intervals and
mostly ending in failure? The birds moving to and from
Site A before the rst nesting attempt suggest that they
had previous knowledge of other possible nesting sites.
As the European Bee-eater has increased breeding in the
region (Finozzi & Tralongo, 2002), it is quite possible that
breeding sites on the lower part of the river were limited
and less t birds tried breeding upstream, but this cannot
explain their initially massive preference for Site A over
Site B, the latter being in this case a place closer to their
provenance in addition to offering easier soil to dig. What-
ever the reason for the rst digging attempts in Site A, it
seems likely that birds insisted there because they copied
the digging activity of conspecics that served as models.
Tab. 1 - Length (cm) of the 37 tunnels in Site A, obtained from a measuring tape and given in the sequence
of the one-row positions of the tunnels in the riverbank, upstream. The (a) rst and (b) second excavation
attempts. (c) The two successful nests. / Lunghezza (cm) dei 37 tunnel nel Sito A, ottenuta con un metro a
nastro e presentata nella sequenza delle posizioni in la unica dei tunnel nella sponda del ume, risalendo la
corrente. (a) Il primo e (b) il secondo tentativo di scavo. (c) I due nidi riusciti.
72 23 122 13 27 74 37 14 40 13 40 48 27 35 298c 25a 15 31 38 29 47 53 31 190c 43 48 13 15 24 39b 17 23 21 25 28 17 24
SHORT COMMUNICATION
93
Fig. 1 - Site A: a) the extension of nesting attempts of the new colony along a bank of Trebbia River and the position of the only two
successful nests; b) a 40-cm excavation attempt, example of failure because of unmovable pebbles; c) the successful nest close to the
rst excavation attempt (see Tab. 1), with a edgling at the top left of the photograph, 18 July. / Sito A: a) L’estensione dei tentativi di
nidicazione della nuova colonia lungo una sponda del ume Trebbia e la posizione dei due soli nidi riusciti; b) un tentativo di scavo
di 40 cm, esempio del fallimento dovuto a ciottoli inamovibili; c) il nido riuscito adiacente al primo tentativo di scavo (vedi Tab. 1),
con un giovane uscito dal nido in alto a sinistra, il 18 luglio.
A
B C
RIVISTA ITALIANA DI ORNITOLOGIA - RESEARCH IN ORNITHOLOGY, 90 (2) 2020
94
No matter if produced by the same pair, or a copying pair,
the presence of a successful nest (Fig. 1c) close to the very
rst excavation (Tab. 1) suggests that the choice of model
conspecics had some ground. If it is true that no bird in
the group had previously bred there, this choice must have
been based on a leadership that did not depend on local
breeding experience. Recent large-scale research on Slen-
der-billed Gulls (Chroicocephalus genei) (Francesiaz et
al., 2017) and Cliff Swallows (Petrochelidon pyrrhonota)
(Hannebaum et al., 2019) has shown that colonial birds
maintain groups with familiar conspecics when they
change settlement and delity is maintained even when
the group splits to several nesting sites in a compromise
with habitat requirements. This might explain the observed
to-and-fro movements between Site A and Site B, as well
as, not far from my study area, the scattered aggregation
of small colonies and “solitary” nests of European Bee-
eaters observed on a provincial scale (Pinoli & Gariboldi,
1987). Forced to leave a saturated colony site, sub-groups
or even single pairs might try nesting in a new site, keep-
ing the balance between seeming habitat suitability and
closeness to the original colony. If successful, they might
repeat breeding there, possibly attracting further familiar
birds from the colony, and contribute to the spreading of
the population. Both Francesiaz et al. (2017) and Hanne-
baum et al. (2019) aimed at disentangling the delity to
a group (social philopatry) from the delity to a breeding
site (spatial philopatry), both being possible causes of the
staying with familiar conspecics. In my opinion, a limit
to this aim in their studies was that a group that changed
breeding site might settle where at least some (older, more
inuential) members had bred previously, even if not the
year before, which would suggest a mixture of social
philopatry and recurrent spatial philopatry. My observa-
tions suggest that familiar conspecics trust each other
even (or even more?) when facing completely unknown
situations. If so, through public information inuential
subjects might lead conspecics to extended failures, as
not rarely occurs in humans.
Acknowledgements
Giuliana Marzi, my wife, was of much help in measu-
ring the nest-holes. An anonymous reviewer contributed
to make my reasoning clearer.
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The European bee-eater (Merops apiaster) is the first known species of the order Coraciiformes and the second bird species whose distribution depends on the granulometrical characteristics of soils constituting suitable banks for breeding. The mean particle size of soil samples from bee-eater nest places was 42.76 ± 13.58 μm (max 66.82 μm, min 20.10 μm). Mean particle size differed significantly between samples from bee-eater and sand martin nest places, and unoccupied cliffs respectively. In total 12 different particle sizes were analysed. There were no bee-eater holes in soils containing particles over 10,000 μm. The number of all psephitic particles (above 2,346 μm) was more than 15 times lower in samples from bee-eater colonies than in those from sand martin holes. However, samples from bee-eater colonies contained 20 times more soil grains between 28.0 and 9.2 μm. These highly significant differences may explain why these two species do not usually breed in mixed colonies.
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We studied the mechanisms that regulate colony dynamics in a Spanish population of Lesser Kestrels, using eight years of data from banded individuals in 494 colony-years. Colony growth was positively related to breeding success at the colony the year before. However, individuals of all dispersal statuses, i.e., adult and first-breeding philopatric and immigrant birds, significantly contributed to changes in colony size, indi-cating an important effect of dispersal on colony dynamics via colony quality. Given that there is strong evidence that Lesser Kestrels base their settlement decisions on conspecifics, we tested whether immigrants used the number of previously settled residents in year t (social or conspecific attraction hypothesis) and/or the breeding performance of conspecifics in year t 1 (performance-based attraction hypothesis) to select their breeding colony. Breeding success of colonies varied both in space and time and was autocorrelated from one year to the next. Moreover, lifetime reproductive success of Lesser Kestrels was pos-itively associated with colony size, and individuals can predict final colony size early in the breeding season, so assumptions of both hypotheses were fulfilled. Our results support the social attraction hypothesis, since immigration was positively related to the number of philopatric adults, but not to conspecific breeding success the year before. Given that departure decisions of adults were based on personal information about breeding success and colony size is related to fitness prospects, previously settled individuals provide easy and reliable information about colony quality, and social attraction could be seen as a particular case of public information in Lesser Kestrels. Consistently, absolute numbers of both philopatric adults and immigrants increased with colony size the year before, although immigrants increased only up to a threshold beyond which this trend disappeared. Therefore, immigrants seem to be prevented from settling in the largest colonies, which could explain why all individuals do not concentrate in a few big colonies, but some settle in suboptimal colonies or colonize unoccupied sites. This opposing effect of conspecifics, together with the low levels of temporal autocorrelation in colony quality between time lags 2 yr, could promote colony size variability and facultative coloniality in this species.