Patterns of mistletoe infection in four Acacia species in a semi-arid southern African savanna

In a range of systems, studies on mistletoe distribution on the host plant have documented a number of factors that affect their occurrence and spread (Aukema & Martinez del Rio 2002a, Bowie & Ward 2004, Overton 1996, Reid et al. 1995). These patterns can be determined by host specificity, environmental conditions, host plant characteristics (Martinez del Rio et al. 1995) and the movement patterns of dispersal agents (Aukema & Martinez del Rio 2002a, 2002b). In mistletoe plants, host choice can be considerably influenced by the advantages of interacting with relatively abundant hosts (Norton & Carpenter 1998, Norton & De Lange 1999). Besides the relative abundance of host species, characteristics such as branch size, age and height can have a strong effect on mistletoe attachment resulting in size-related mistletoe infection patterns (Overton 1994). Generally positive relationships between mistletoe infection and host size have been demonstrated worldwide (Donohue 1995, Martinez del Rio et al. 1996, Norton et al. 1997, Reid & Stafford Smith 2000) and they have been interpreted in terms of the preferences by dispersing birds to perch and feed in taller trees (Aukema & Martinez del Rio 2002a) and trees accumulating infections as they age (Overton 1994). Aukema & Martinez del Rio (2002a) reported more frequent perching in taller-than-average trees by the phainopepla (Phainopepla nitens), which is the principal disperser of the desert mistletoe Phoradendron californicum. Thus, visits by mistletoe-seed-dispersing birds, and therefore mistletoe seeds received, tend to increase with tree height (Aukema & Martinez del Rio 2002a). Using a simple metapopulation model, Overton (1994) predicted the frequency of parasitized trees to increase with host age. Therefore, assuming that size is a good proxy for age, large trees are likely to be more infected than smaller trees. Reid & Stafford Smith (2000), using experimentally disinfected trees, found that larger trees were disproportionately re-infected with mistletoes. This size–intensity relationship may be used to describe mistletoe infection patterns. However, several previous studies have shown size–intensity relationships to be weak (Aukema & Martinez del Rio 2002a, Donohue 1995, Overton 1994, Reid & Stafford Smith 2000). This indicates that other factors may be important in determining mistletoe infection intensity, including that already parasitized hosts of a specific height are more likely to receive seeds than non-parasitized hosts of the same height or dispersers are likely to be attracted to trees for reasons other than size (Aukema & Martinez del Rio 2002a).

Effectiveness of Dispersal of an Ornithocorous Cactus Myrtillocactus geometrizans (Cactaceae) in a Patchy Environment

Effectiveness of seed dispersal by different species that feed on the fruits of Myrtillocactus geometrizans was evaluated, considering both quantity and quality of dispersal, in a patch of tropical deciduous forest in Zapotitlán de las Salinas. Effectiveness was estimated to be strongly influenced by the post-foraging movements of the frugivores, leading us to suggest that the quality component of seed dispersal occupies a central role in the assessment of disperser effectiveness and to expect that dispersers that stay in the tropical deciduous forest patch after foraging would have highest effectiveness. Birds were the principal dispersers of M. geometrizans. This was particularly true of Phainopepla nitens, since this species showed a high fidelity within the tropical deciduous forest. These observations emphasize that it is important to determine the post-foraging habits of seed dispersers that may move across vegetation patches over the landscape in order to obtain a complete assessment of their role in Neotropical environments.

Social and Genetic Monogamy in Territorial and Loosely Colonial Populations of Phainopepla (Phainopepla Nitens)

We observed courtship and copulatory behavior in two populations of Phainopeplas (Phainopepla nitens) with contrasting social systems and used multilocus DNA fingerprinting to assess rates of extrapair fertilization (EPF). Phainopeplas from both territorial and loosely colonial populations copulated infrequently (≤2 times per day). No extrapair copulations (EPCs) were observed in either population. We found no evidence of EPFs in 48 nestlings from 25 nests. Thus, Phainopeplas appear to be both socially and genetically monogamous. However, Phainopeplas are not monogamous over their lifetimes—they do not retain mates from year to year and appear to switch mates between breeding localities within the same year. Compressed breeding seasons, geographically shifting food resources, and widespread breeding failures may favor monogamy during any given breeding attempt while allowing opportunities to switch mates between breeding localities and years.

Vocal Mimicry in Distress Calls of Phainopeplas

Phainopeplas (Phainopepla nitens) utter contact calls, loud distress screams, and 37 other distinguishable vocalizations when captured, including imitations of at least 12 species. In southern California, Phainopeplas captured in the desert and coastal woodlands imitated species from both regions, suggesting that individuals occupy both habitats during the year and have an extended period of learning that spans the period of time when they move between regions. There was no significant difference in the number of calls imitated based on sex, male age, or habitat. A high percentage of Phainopeplas in both desert and coastal woodlands imitated Red-tailed Hawks (Buteo jamaicensis) and Northern Flickers (Colaptes auratus), whereas the use of other species' calls varied with habitat. Phainopeplas did not simply imitate the calls of the most abundant species or those that were most vocal.

HETEROSPECIFIC RESPONSES TO SCREAM CALLS AND VOCAL MIMICRY BY PHAINOPEPLAS (PHAINOPEPLA NITENS) IN DISTRESS

Using playback experiments, I tested whether distress calls by phainopeplas attract heterospecifics and elicit mobbing of a predator decoy. Phainopeplas use two types of distress calls: loud scream calls and mimicry of other species' vocalisations. Screams and imitations together attracted significantly more heterospecifics than in the absence of distress calls. When broadcast alone, scream calls were more effective at eliciting heterospecific approach and mobbing behaviour than were imitations. Birds were significantly more likely to mob the predator decoy during broadcast of screams and imitations than during broadcast of screams with digitally scrambled imitations, even though there was no significant difference between treatments in the number of birds initially approaching. This suggests that precise mimicry does not play a key role in attracting heterospecifics but may elicit increased mobbing once they have arrived. Mimicry did not appear to be directed at the species whose calls were imitated; only 4 of the 19 species responding to distress calls were model (mimicked) species, and birds mobbed the predator decoy regardless of whether imitations of their own calls were used in playback trials.