Biology, images, analysis, design...
|"It has long been an axiom of mine that the little things are infinitely the most important" |
Generalist Aphid Predator (Passeriformes: Paridae)
Black-capped chickadeeOn this page: Identification & Distribution Biological Control of Aphids Predation of currant gall aphids Predation of cottonwood gall aphids More cases of aphid predation by chickadees
Identification & Distribution
Adult black-capped chickadees are small rather compact birds with a fairly short dark beak and a long narrow tail. The black-capped chickadee has a black cap that extends down to just below the eyes, a black 'bib' and white cheeks. The underparts are white with rusty brown on the flanks. Its back is grey and the tail is slaty grey with the outer tail feathers edged with white. The wing feathers are grey, also with conspicuous white edges (cf. Carolina chickadee the wing feathers of which which only have narrow faint white edges). Males and females have similar coloration, but males are slightly larger and longer than females. The black-capped Chickadee has complex songs with 16 unique types of vocalizations. The whistled song is typically two clear tones with a higher-pitched 'fee' note followed by a lower-pitched 'bee' note. The Black-capped Chickadee also has a 'chick-a-dee' call that is the namesake of the genus.
The black capped chickadee is a non-migratory bird that lives in deciduous, mixed deciduous-coniferous, and open coniferous forests, shrub thickets, and riparian woodlands. It also visits orchards. It prefers habitats with a rich understory of brush such as riparian woodlands and shrub thickets. It is found throughout Canada and in the northern states of the USA.
Biological Control of Aphids
Smith and Snyder (2017) reviewed studies on the diet of black-capped chickadees. The bird primarily forages on trees by gleaning insects off the bark and leaves, and rarely forages on the ground. Approximately 58% of arthropod prey are taken from bark and 38% are taken from leaves (Robinson and Holmes, 1982).
During the breeding season, the black-capped chickadee's diet is about 80–90% insects and spiders with the rest of the diet comprised of fruit and seeds. During the winter, the diet shifts to about 50% insects and spiders and 50% plant matter - primarily seeds and berries (Smith, 1991). The prey selected included moths, caterpillars, spiders, beetles, flies, wasps, true bugs, aphids (='plant lice'), scale insects, leafhoppers, and tree hoppers. The seeds or fruits of poison-ivies, blueberries and huckleberries, chokecherry and raspberries are also consumed in the summer season. Caterpillars are thought comprise the largest portion of the chickadee arthropod diet, with other insects, spiders, small molluscs and centipedes forming smaller components.
Most researchers have not found (free-living) aphids to be a major part of the diet of the black-capped chickadee. However, the birds are known to frequently predate gall-living insects such as the goldenrod gall fly (Eurosta solidaginis, see picture below).
Image by David Whelan under a Creative Commons CC0 1.0 Universal Public Domain Dedication.
A specialization on gall-dwelling insects would seem very compatible with results of the study of Heinrich and Collins, 1983 who found that chickadees can use leaf damage cues to locate cryptic caterpillars. Caterpillars produce marked damage to leaves, but so do other insects - especially gall-causing aphids. The black-capped chickadee perhaps fills the same gall specialist niche that the great tit (Parus major) occupies in Europe as, like the great tit, it is equipped with a fairly sturdy beak ideally suited for opening galls.
Predation of currant gall aphids
The first picture below shows a black-capped chickadee predating aphids in leaf-nest pseudogalls on a golden currant (Ribes aureum) bush.
Image reproduced by permission, copyright Jeanne Dammarell all rights reserved.
Judging by the lack of red leaf blisters so characteristic of Cryptomyzus ribis, the most likely species are Hyperomyzus lactucae (see picture below) or Aphis mimuli (not pictured), both of which can cause leaf curl.
Predation of cottonwood gall aphids
The leaf-nest pseudogalls on currant are soft, and access to the aphids within is fairly easy. But the woody galls of the sugarbeet root aphid (Pemphigus betae) on their primary host, hybrid cottonwood trees (Populus angustifolia x Populus fremontii), provide much better protection to the enclosed aphids. These galls are usually initiated by founding females on the upper leaf surface, usually along the midrib, and often near the base of the leaf blade (Floate, K. (2010)). Developing galls are green, with mature galls frequently turning reddish (see picture below). Gall densities are highly variable with particularly susceptible trees having five or more galls per leaf.
Image reproduced by permission from Floate, K. (2010), copyright K. Floate, Agriculture and Agri-Food Canada, all rights reserved.
The sugarbeet root aphid is common and widely distributed where cottonwood trees are found. Their secondary hosts include sugar beet (Beta vulgaris), spinach (Spinaceae vulgarisa) and dock (Rumex species).
Dickson & Whitham (1996) looked at bird predation of the leaf-galling aphid, Pemphigus betae in northern Utah, USA. The main avian predators were the black-capped chickadee (Poecile atricapillus) and the black-headed grosbeak (Pheucticus melanocephalus). The grosbeak (see picture below) is even better equipped than the chickadee with a beak that will enable it to break open insect galls.
Image copyright Bill Bouton under a Creative Commons Attribution-Share Alike 2.0 Generic license.
Galls opened by birds were easily identified as they slice open the gall, leaving a distinctive wound as evidence of their removal of aphids. As gall density on individual branches increased, the proportion of galls preyed upon by birds also increased. In other words, bird predation was density-dependent which means it has the potential to regulate the gall (aphid) population. A linear regression showed that gall density explained 42% of the variation of bird predation. The percentage of galls preyed on by birds increased from around 10% at lower densities to 30-40% when gall density exceeded approximately 50, suggesting a threshold effect. Not only did avian predators selectively forage on aphid-susceptible trees, they also foraged at a finer scale by discriminating among different branches of the same tree.
Subsequent work by Bailey et al. (2006) provided further evidence for Pemphigus betae being an important food resource for birds. Martinsen & Whitham (1994) also observed black-capped chickadees opening galls and feeding on aphids, and noted other documented cases of birds preying on aphid galls including gila woodpecker (Melanerpes uropygialis) (Speich and Radke, 1975), and tree sparrow on aphid galls (Tetraneura sp. & Colopha moriokaensis) in Japan (Sunose, 1980). Our photo contributor, Kevin Floate, added the interesting observation that there is also a certain amount of rodent predation of these galls.
More cases of aphid predation by chickadees
Melanie Leeson photographed a chickadee going after aphids in Four Aphids and a Chickadee in October 2013.
Similarly Daniel Glenn photographed a Carolina chickadee eating aphids in Aphids for lunch anyone? at Lake Artemesia in College Park, Maryland in December 2016.