Avian consumption and seed germination of the hemiparasitic mistletoe Agelanthus natalitius (Loranthaceae)

Abstract

We documented avian consumers of the mistletoe Agelanthus natalitius in the field and quantified their effects on seed germination in captive studies. Seven bird species were frequently observed to feed on mistletoe fruits in the field, namely Cape Batis (Batis capensis), Cape White-eye (Zosterops virens), Red-fronted Tinkerbird (Pogoniulus pusillus), Red-winged Starling (Onychognathus morio), Dark-capped Bulbul (Pycnonotus tricolor), Speckled Mousebird (Colius striatus) and Village Weaver (Ploceus cucullatus). Of the four bird species used in captive feeding trials (Red-winged Starling, Speckled Mousebird, Village Weaver and Cape White-eye), most of the birds removed the pulp-cover of the mistletoe fruits and left the exposed seeds in potentially germinable condition. Red-winged Starlings ingested but then regurgitated the seeds and wiped their bills on a twig to dislodge the sticky viscin-covered seeds individually. Mistletoe seeds obtained from cages of Red-winged Starlings had the highest germination success of all the seeds obtained from the captive bird study. The handling of mistletoe fruits applied by all species of birds both in the field and in captivity showed that mistletoe dispersal is likely to occur over short distances, although the seeds are likely to be dispersed to safe sites due to directed dispersal on parental host trees. Such autoinfection and local aggregation increase the interaction with already parasitised hosts, which through time may enhance local specialisation on the most frequently parasitised host species. Zusammenfassung Wir haben Vogelkonsumenten der Mistel Agelanthus natalitius im Freiland beobachtet und ihre Effekte auf die Samenkeimung in Studien in Gefangenschaft quantifiziert. Sieben Vogelarten wurden häufig dabei beobachtet, wie sie im Freiland Mistelfrüchte fraßen, nämlich Kapbatis (Batis capensis), Kap-Brillenvogel (Zosterops virens), Feuerstirn-Bartvogel (Pogoniulus pusillus), Rotschwingenstar (Onychognathus morio), Graubülbül (Pycnonotus tricolor), Braunflügel-Mausvogel (Colius striatus) und Textor (Ploceus cucullatus). Von den vier Vogelarten, die für Futterversuche in Gefangenschaft verwendet wurden (Rotschwingenstar, Braunflügel-Mausvogel, Textor und Kap-Brilenvogel), entfernten die meisten Vögel die Fruchthülle der Mistelfrüchte und ließen die freiliegenden Samen in potentiell keimfähigem Zustand zurück. Rotschwingenstare nahmen die Samen auf, würg-ten sie dann wieder hervor und wischten ihren Schnabel an einem Zweig ab, um die klebrigen von einer Viscinschicht umschlossenen Samen einzeln zu entfernen. Mistelsamen, die aus den Käfigen von Rotschwingenstaren geholt wurden, hatten den höchsten Keimungserfolg von allem Samen, die aus der Untersuchung der Vögel in Gefangenschaft gewonnen wurden. Die von allen Vögeln im Freiland und in Gefangenschaft angewendete Behandlung der Mistelfrüchte zeigte, dass die Verbreitung von Misteln wahrscheinlich über kurze Entfernungen geschieht, obwohl es wahrscheinlich ist, dass die Samen aufgrund von gerichteter Verbreitung auf den e...

Host specificity in parasitic plants—perspectives from mistletoes

“…have seeds that are dispersed explosively (Hawksworth and Wiens 1996; Kelly et al 2009), while Misodendrum is dispersed by wind (Vidal-Russell and Nickrent 2007) and Tristerix by marsupials (Amico and Aizen 2000). In bird-dispersed mistletoe species, the birds consume mistletoe fruits and subsequently wipe their bills, regurgitate or defaecate the seeds on the branches of host trees (Reid 1991; Aukema 2003; Roxburgh 2007; Green et al 2009; Okubamichael et al 2011 b ). Birds break the physical dormancy of the seed and initiate germination by removing the fruit cover (exocarp), which otherwise inhibits germination (Roxburgh 2007; Okubamichael et al 2011 b ).…”

“…In bird-dispersed mistletoe species, the birds consume mistletoe fruits and subsequently wipe their bills, regurgitate or defaecate the seeds on the branches of host trees (Reid 1991; Aukema 2003; Roxburgh 2007; Green et al 2009; Okubamichael et al 2011 b ). Birds break the physical dormancy of the seed and initiate germination by removing the fruit cover (exocarp), which otherwise inhibits germination (Roxburgh 2007; Okubamichael et al 2011 b ). The behaviours of the birds also expose the sticky viscin, enabling the mistletoe seeds to firmly attach to branches of host trees.…”

“…Birds usually disperse more mistletoe seeds on the parental host tree than elsewhere (Aukema and Martínez del Rio 2002 a , b ; Aukema 2003; Carlo and Aukema 2005). As with many other plant species, there is a negative correlation between mistletoe seed survival and dispersal distance from the parent plant (Okubamichael et al 2011 b ). Birds are responsible for the local aggregation of mistletoes at a locality (infestation patches) or on individual trees (infection intensity) and determine a negative binomial distribution of mistletoes at the population level (Overton 1994; Robinson and Geils 2006).…”

“…Over time, some of these seeds became established and eventually developed reproductive isolation from T. corymbosus (Amico et al 2007). Birds may also enhance the interactions between mistletoes and their prospective hosts, for instance, if the host trees provide a reward such as fleshy fruits that are available at the same time as the mistletoe fruits (Carlo and Aukema 2005; Okubamichael et al 2011 b ). In either case, birds may potentially influence host specificity and host switches in mistletoes.…”

Host specificity, nutrient and water dynamics of the mistletoe Viscum rotundifolium and its potential host species in the Kalahari of South Africa

“…Mistletoe presence and abundance in a given area can be influenced by the distribution of suitable host species (Rödl and Ward, 2002), the degree of host specificity (Okubamichael, 2009), behaviour of avian dispersers (Aukema and Martínez del Rio, 2002), pollinators (Ladley et al, 1997), habitat fragmentation (Lavorel et al, 1999), fire , herbivory (Ehleringer et al, 1986) and parasiteehost chemical interactions (Tomilov et al, 2006;Yoder, 1999). Birds are the primary dispersers of mistletoe seeds, and the influence of bird behaviour on the distribution of mistletoes has been studied more extensively than other determining factors (Aukema and Martínez del Rio, 2002;Lopez de Buen and Ornelas, 1999;Martínez del Rio et al, 1996;Okubamichael et al, 2011). Mistletoe distribution on a landscape scale is not well understood, although some models and predictions have been proposed (Aukema and Martínez del Rio, 2002;Mathiasen et al, 2008;Overton, 1994).…”