9 Things About Blackbirds

9 Things About Blackbirds

Flight Adversity

Flying demands a lot of energy from fatty acids and high activity from the heart, lungs and muscles because a lot of force is required to maintain altitude. There's a lot of muscle strain regarding wing contraction, as a high fibre length:mass ratio means a considerable and taxing change in length is necessary for it to shorten enough.

Assuming the same straight flight path and environmental conditions - the most arduousness occurs at minimum and maximum flapping speeds. It's akin to human exercises, where slow and controlled movements, as well as explosive exertion are the most intense and strenuous in terms of output. Compared to running, flapping is more costly in the short-term but not long-term since flying is faster for destination arrival.

Hence, flapping is usually intermittent at an intermediate, more steady speed, accompanied by resting glides in-between the flapping intervals, for cessation via wing expansion to increase surface area, utilising an enhanced lift force. Therefore, diminishing accumulated oxygen debt from flapping; whilst making the most of the generated momentum by gliding - optimising energy preservation across prolonged durations.

The energy expenditure is approximately proportional to body mass, except leg and flight muscles seem more efficient in larger birds, perhaps because they have more leverage for power generation with longer wingspans and legs relative to the central body. So fewer flaps are needed, yet more goes into them due to the length of the contraction path, though saving energy overall respecting the lung and heart work ethic (quality>quantity).

So hypothetically, a light mass and small centre - with lanky extremities and a big heart comparatively - would be optimal for flight. The heart would also act as a powerful pump corresponding to the little body, requiring less effort concerning beats for adequate blood flow around the whole structure.

Blackbirds are small compared to birds of prey, which can soar more effectively to scan for prey below and have to work less. Blackbird wing morphology and kinematics - likewise with other light prey birds, are more accustomed to ecological functions of situational, quick movement bursts and agility for predator escape and foraging productivity - resting where they can in between.

Sexual Selection Flight Impact

Flight kinematics such as manoeuvrability, stability and speed get compromised by sexual selection. A desirable trait in a male to a female is an exaggerated tail feather size that hinders fitness for survival/natural selection.

The tail size is crucial for optimising those kinematic factors, yet - in particular, tails that are especially too long relative to the rest of the body incur a high drag:lift force ratio to resist against, detrimenting the ability to maintain flight at all.

It's similar to additional weight they have to carry that's also finicky to deal with. Like how trying to manoeuvre a long bar, with more room for impacting forces, is more awkward and draining to cope with than a shorter one.

Oxygen Adeptness

Birds' incredible cardio/respiratory control has been culminated by genetic, environmental and behavioural elements - gradual progress in individuals over evolutionary generations via exposure. For example, flying is already challenging enough, let alone in high altitude environments, because there is less molecular density concerning pressure (as opposed to sea level), resulting in 'thin' air. At the average helicopter altitude, which birds can choose to fly at, the density gets cut by ~40%.

Naturally, birds have ventilatory adaptations against flight struggle, compensating with a higher cardiac output alongside maximum oxygen consumption per kilogram compared to mammals; and some of the fastest metabolic processes within tissues in the animal kingdom.

For instance, birds have 100% atmospheric oxygen intake inhaling (air is composed of ~20% oxygen, too much would be toxic), whereas - humans have only ~16% - however, this makes them much more susceptible to air pollutants/toxins, amplified with how fast it will get processed.

One reason for these attributes; is that bird haemoglobin possess; either or both a higher oxygen affinity/higher concentration in red blood cells than mammals to oppose hypoxic stress. Another morphological enhancement could include an increased vessel surface area for diffusive efficiency across the membrane interface between the air and blood capillaries in the lungs.

Lung gas exchange is understandably more proficient in migratory birds - especially pertaining to high-altitude flight. They have bigger hearts, both a haemoglobin concentration plus oxygen affinity increase, and more flight muscle capillary density/a high capillary:muscle fibre ratio.

The lungs have more alveolar oxygen tension than in mammals, so there's less space for air to disperse elsewhere, being directed more consistently across vessel walls that are extremely thin - again, to aid gas exchange by means of surface area with less distance to travel. With such thin walls, you'd think the alveoli blood capillaries would burst, but they get support around them from the air capillaries that are densely packed, even though they are super thin too.

Two-cycle Breathing and Air Sacs

For the biggest combatant of flight adversity, birds have their 'Two-cycle' breathing system - the best for gas exchange in animals, supplying oxygen for cell respiration and removing the carbon dioxide waste products at the same time, rather than the alternation of it that's present in mammals.

As birds have no diaphragm, it involves a system of (usually 9) front and rear air sac structures that act as temporary storage units - providing room by extending out (probably because of elasticity) and then pumping it back out. Meaning air is moved in and out by pressure changes within them corresponding to inhalation and exhalation. The air sacs even connect to pneumatic bones (hollow, lighter ones with air spaces, plus for easier flight) to remove otherwise fatal excess heat production from flight.

The sacs facilitate continuous unidirectional airflow directly through the lungs, unlike in mammals, where the lungs have to expand/contract for air to travel down/back up. Bird lungs don't need to do this and are also smaller because there's less reliance on them regarding the air sacs. However, birds rely on ribcage expansion to make room for air sac expansion, being susceptible to suffocation if there's enough pressure on the chest, even with the nose and mouth uncovered.

In summary, air sac storage and unidirectional lung flow enable cellular oxygen uptake even while exhaling. It virtually doubles a mammal's efficiency and breathing rate because air enters the body - while gas exchange occurs too - so fresh and stale air both get processed simultaneously rather than dealing with each one at a time. Two ventilation cycles for a mammal are equivalent to one bird breath simply due to separate ventilatory structures.

The Repeating Two-cycle Process

1 - after inhaling, oxygen passes through the windpipe, reaching the 'primary' bronchus that bypasses the lungs underneath, arriving at an 'intersection'; with the lungs back and above and the rear air sacs down + further forward. Air gets drawn into said sacs momentarily

2 - exhaling forces air out of the sacs, getting pushed up towards and entering through the lungs via rear bronchi, which branch off into long, slim parabronchi which take up the majority of the lung space for gas exchange time

-this section of the lungs is called the 'paleopulmo', consisting of the string-like parabronchii held together by dorsobronchii above and ventrobronchii below-

3 - the paleopulmo is where cross-current gas exchange across the membrane interface happens, where blood capillary vessels going to the rest of the body tissues cross over the air capillaries. Oxygen from fresh air diffuses from the air capillaries into blood capillaries for cells to respire - while carbon dioxide waste products transfer from blood back to air - being considered stale.

-diffusion costs no energy since the particles can naturally move from high to low concerning concentration gradient movement-

4 - the initial inhale (1) also draws the leftover deoxygenated air up into the front sacs on either side of the windpipe; so as to not interfere with the fresh air coming in, then said exhale (2) drives the air out the front ones, looping back into the windpipe and moving back up to get expelled out the body

So overall, inhaling and exhaling have double functions, transpiring at the same time. Inhaling sucks new air down through the windpipe; whilst drawing the stale air into the separate front sacs post-gas exchange. Whereas; exhaling propels air out of all sacs, only with oxygenated air pre-exchange in the rear - and deoxygenated air post-exchange for exhalation being a main difference.

Another difference is in the pressure changes. The rear sacs are lower, with openings facing more upwards, being easier to draw in the air due to gravity, next needing higher pressure when exhaling/contracting to concentrate the air and bounce it back up and out. Contrarily, the front ones are further up the body and have downward-facing openings, needing higher pressure inhaling/relaxing to pull air upwards and retain it shortly. Gravity makes air going back down and out easier.

In the paleopulmo lung section, the compression induced by exhaling causes higher pressure to promote flow. Enough speed for consistent gas exchange rates between air and blood is vital in how they uptake adequate oxygen despite exhaling.

Songs and Vocalisations

Blackbirds have the most familiar UK song, frequently in the early morning, at dusk and after rain from high vantage points. Others from different countries might sound different, e.g. north america, australia, new zealand, north africa and eurasia.

Environmental factors contribute primarily to any potential song differences, such as ambient noise levels, conspecific presence, learning and/or competition, threat, conspecifics, weather, time of day, and region/environment types - with minute yet unique individual variations. These different aspects have maybe enforced formation of any sub-dialectical differences via environmental, social and genetic acclimation over time.

They have their general song, where the intervals seemingly start with squeaky flute-like whistles, ending with fast and scrambled "chirp" and "cheet" vocalisations. The sounds are so obscure to describe and put into words/onomatopoeia - demonstrating how elaborate and diverse the noise projections are. Blackbirds have another salient song, with each tweet alternating between higher and lower pitches, with relatively consistent durations and intervals.

Reasons for them are ambiguous, perhaps contentment, relief of some form, or an innate 'default setting', but most likely for intersexual communication. The types can change depending on the spatial association between the male and female, assuming they can hear each other and are facing each other's direction. Maybe the more simple song of the two is further reaching, and the complex one is for expression of detail, better recognised in closer ranges.

At longer distances, the female answers the male by following/mimicking songs or song switches, which could associate with initial enticement. At closer distances the female's song can coincide with the male's. It's possibly some kind of status or compatibility assessment of the male, as the female gathers acoustic cues indicative of the perceived survival fitness level.

-Other examples-

Blackbirds have a generic alarm/vigilance in response to a supposed risk. There are fast "cheets" and/or "twinks" with fairly consistent pitch and interval duration. This call seems to be contagious, becoming amplified in flocks. They may do it as daylight fades since the prospect of darkness entails vulnerability for a diurnal animal.

The alarm can get harsher, with higher amplitude, faster speed and "chooks" mixed in; in the occurrence of elevated danger/threat, competition, aggression and/or territorial defence. If resorting to flee/flight, there's a more spontaneous outburst of "shrieks" + "chuks", with sporadically fluctuating pitch. Furthermore, they have more specialised noises...

Nest-associated female "cheets" = upholds the male's vigilance against an invasion

Nest-associated female scream = triggers the male to try and force an invasive threat away; the young to hide and stop begging for food

"Pok" = a deeper 'bark' for predators underneath - softer as grounded predators like cats usually pose less threat

"Seee" = specified alert for predator overhead, e.g. bird of prey

"Teer" = hierarchical establishment in females of a polygynous male's territory

Vocalisation Directiveness and Competition

Evolution has seemed to favour versatile divalent signalling (its double function associating with direction) in blackbirds, which is adjustable according to the situation.

For close proximity, like in areas of compact vegetation, a more abrupt communicatory structure is directed towards individuals at the greatest risk of predation, for instance. The area density and short range may focus the signal more and heighten its loudness, augmenting the chance of reaching the recipient. Even so, the sound should still reach longer distances to warn others in the locality.

On the other hand, their general melodic motif songs have broader usage, being omnidirectional towards unseen conspecifics. It's essentially one-way 'advertising' to maximise the quantity of potential answerers. They can reorientate whenever to amplify the broadcasts in other particular directions too.

The universal calling gets driven by male-male competition over mate attraction and territorial proclamation. More experience and environmental habituation necessitate faster songs, larger repertoires and higher intensity. Expansive repertoires seem to be accompanied by bigger bodies (size might correlate with a more proficient voice box).

Females prefer more advanced structure/coordination and aggression of male song repertoires, as it could infer traits of competence and territorial protectiveness - suggestive of a more valuable courtship status and 'powerful' individual. Hence, they are less likely to get challenged by rival males, with mostly just predator deterrence to worry about.

Habitat-dependent Noise

In populated urban environments with auditory constraints, blackbirds can sing louder and/or adjust the pitch of their vocalisations to prevent dissipation caused by overlapping, ambient noise pollution.

For example, they can utilise more transmissive, higher frequencies to bypass low-frequency vehicle noise. Or they can learn to vocalise more often during quiet times, saving their energy during rush hours - alike to airports, where sounds are much more powerful, except more distributed across the day.

These environmental pressures on communication to survive may drive divergent evolution in songs, separating urban and rural types more regarding pattern variety and pitch frequencies.

Vision

Sight is the chief sense, especially in diurnal bird species like blackbirds that need to rely on it more. A blackbird's visual projection is mostly a fronto-lateral coverage, with a slight bit of binocular vision (where both eye projections overlap) at the front; and a blind spot behind.

They have rapid head movement for scanning; and usually the capability for independent eye movement, both for alignment of the central retina, for any specific stimuli. In tandem with their visual field, vulnerable areas can get covered for predator motion detection.

The central retina configuration contains the fovea, a small depression full of high-light sensitive cone cells in the very middle. Light gets focused onto it the sharpest - so sensitivity plus spatial resolution quality are the most heightened. Double cones are the most prevalent cells in avian eyes compared to the typical cone cell variety, where their higher acuity play a part in birds' enhanced luminance detection.

Their cones even have different pigmented oil droplet cell organelles - with a lens-like shape helping to filter the light spectrum to improve colour clarification, sensitivity; whilst maintaining its continuity for increased processing time. It applies to the appearance of food, objects and for the judgement of potential mates. Albeit, discernment can become hindered by excessive light intensity and background colour similarity.

Most birds are tetrachromatic, meaning four different cone photoreceptor cell classes are present for high-light interpretation. Presumably, they see more defined shades, hues and saturation of colour that we cannot - signifying a 'narrowed' spectral sensitivity for discriminating the slighter differences in given colours or colour shades.

Although, this could hinder their ability to determine what food is edible. Our more generalised, 'broader' visual spectrum means less detail to process, so differences are more easily and quickly recognised. Despite this, behavioural and genetic evidence has demonstrated they can distinguish taste categories, including a small range of bitter taste cell chemoreceptors for dietary toxin detection.

Their spectral range also encompasses ultraviolet vision, which they are most receptive to. UV plays a part in foraging decisions and plumage signalling. Plumage colouration attunes to bird vision because feathers usually reflect the UV for mate choice appeal.