TROCHILIDAE Hummingbirds FAUNA PARAGUAY

TROCHILIDAE - HUMMINGBIRDS
Nineteen Paraguayan species in two subfamilies described below. This is one of the largest bird families in the traditional sequence of classification and is usually placed in the same order as the Swifts on account of shared morphological features such as the location of the feather tracts, skeletal and musculature characteristics associated with flight and the nerve organisation of the flight muscles. Both groups also share a unique enzyme, malate dehydrogenase. Some argue that these traits are merely due to convergence and place the hummingbirds in their own order Trochiliformes. The first hummingbirds probably appeared during the early Tertiary some 65 million years ago.
Hummingbirds are usually small and have highly specialised flight and feeding methods that necessitate great modifications in morphology and anatomy. They are virtually obligate nectarivores, with insects and pollen making up only a small percentage of their diet. They hover when feeding, the wings tracing a figure of 8 shape. Altering the wing angle enables them to suddenly change direction and even fly backwards. Small species may beat their wings and incredible 80 times per second and this rapid movement is responsible for the humming noise that gives them their common name. Hummingbirds can fly extremely quickly, but require a number of morphological modifications to enable them to do so. They have an elongated, deeply-keeled sternum, eight pairs of ribs to help stabilise the body (most birds have six), and the reinforced coracoids of the pectoral girdle show a flexible, ball and socket joint. Furthermore the flight muscles are connected to the humerus by tendons. The humerus, radius and ulna are much reduced, with two-thirds of the wing being taken up by the elongated carpo-metacarpus. Flight muscles make up 30% of body weight and consist largely of dark red fibres. Unlike most other birds both the upstroke and the downstroke are used in powered flight. In contrast to the highly-developed, elongated wings, the feet and legs are short and useless for walking, serving only for perching.
Because of their small size and active life-style, hummingbirds lose heat rapidly and it has been postulated that they compensate for this by utilising heat produced by contraction of the flight muscles for thermoregulatory purposes. Increased blood flow is required to provide oxygen to the flight muscles, so the heart is correspondingly large, c2.5% of body weight - almost twice that of most passerines. The heart may beat as many as 600 times per minute at rest and 1000 times per minute when excited.
Hummingbirds have 10 primaries, the outermost greatly-elongated and giving pointed wing tips. There are 6 or 7 short secondaries and 10 retrices, the latter playing an important role in manoeuvrability. The contour feathers are connected to the cutaneous muscle and can be erected for thermoregulatory purposes. Adults have no downy feathers and c12-25% the number of contour feathers than the average passerine, presumably an adaptation because of their high energy lifestyle and increased heat production. Members of this family have the highest basal metabolic rate of any bird and a body temperature of c40C. Flight feathers are dull-coloured and lack gloss, glossy feathers have microscopic hooklets between the barbules that weaken their mechanical strength. Iridescent colouration is a physical character, not pigment-based. The feather barbules have platelets, beneath the surface of which are melanin-filled bodies packed with air bubbles. These are layered within the barbules in unequal stacks that create different refractive indices so that the colour appears to change depending on the angle from which it is viewed. Colouration can be correlated to behaviour, males are brightly-coloured for advertisement purposes, females more cryptic for protection whilst incubating. Hermits are dull-coloured because they live in shady forest undergrowth where little light penetrates. There is a single complete moult after breeding. Primaries are replaced from the innermost towards the outermost, with the exception of the outer two being moulted in reverse order so that wing stability is maintained. Tail moult is unusual in that the outermost feathers are moulted early and asymmetry in tail moult is not uncommon.
Hummingbirds sip nectar using the elongated, bifurcate tongue flicked outward by the hyoid apparatus. The basal part of the tongue is cartilaginous, the distal half is split into two curled tubes. In just a few hours a hummingbird may consume its own body weight in nectar. They have a thin crop in which solid food is lubricated with mucus. Nectar on the other hand passes rapidly through the stomach (made up of the large proventriculus and smaller ventriculus) and into the small intestine where it is rapidly absorbed. Solid food particles spend more time in the proventriculus. Unusually hummingbirds lack both caeca and a gall bladder.
The respiratory system is adapted for high-energy demand as a result of hovering flight. Breathing rate at rest may be as high as 300 times a minute and almost doubles when active. At times when energy sources are scarce they may become torpid, failing to respond to stimuli but saving considerable amounts of energy. Around 70% of a hummingbirds day is spent perched. Many species become active before dawn and may continue feeding after dark, suggesting that they have excellent night vision - hummingbirds are mainly visual feeders, one of the reasons that they are attracted to brightly-coloured objects.
Vocalisations are simple and non-musical, foraging calls consisting of dry chirps, and aggressive encounters accompanied by more rapid trills. Nestlings do not call until after fledging. Breeding systems are polygynous, a male mates with several females. The male´s role in the reproductive process ends after mating, females are left to build the nest and rear the young alone. The peak of reproduction is timed to coincide with the peak of nectar availability. The clutch invariably is of two, non-glossy, elliptical, white eggs. Eggs are laid at two-day intervals and incubation generally begins with the laying of the second egg. Incubating females reduce their body temperature by almost 10C when incubating, resulting in a reduction of about 50% in their energy requirement to compensate for their reduced time available for feeding. Hatchlings are altricial and the nestling period is long for the size of the bird.

Subfamily Phaethornithinae: "Hermits"
Two species in the genus Phaethornis. Forest-dwellers, with dull greenish or brownish plumage. Long, often curved bill with brightly-coloured gape. Unique morphology of humeral tendon. Lekking reproductive systems. Nest cone-shaped and pendant, not usually sited close to reliable food source. Slight or no sexual dimorphism.

Subfamily Trochilinae: "Typical Hummingbirds"
All the remaining 17 Paraguayan species belong to this subfamily. They occur in a variety of habitats and have bright, often glossy plumage. They show great heterogeneity of form. Generally territorial, only very few species lek. Cup-shaped nest in most species, usually located close to a reliable food source. Most species with marked sexual dimorphism.

REFERENCES
Campbell B & Lack E 1985 - A Dictionary of Birds - T & AD Poyser.
Schuchman KL P 1999 - Trochilidae Handbook of the Birds of the World Volume 5 - Lynx Ediciones.