Birds are the true creators of the world. Not only they develop interesting and complex structures, but also a variety of materials used, which is absolutely amazing. From the banks of the tree cavities to the Cliff banks, simple birds use any natural substance, they can maneuver with a beak or feet.
They are not above making use of artificial structures, or objects, which is why it is so easy to improve that bird watching activities simply by providing a properly designed bird house or some dog hair and then watch them go work.
The act of nest building is triggered by hormones. Other factors, such as migration, the duration of daylight the presence of food and territoriality also contribute though, that such behaviour in turn are subject to hormonal influences.
Genitic parts of birds have significantly reduced the amount for most of the year which is an adaptive strategy so in flights they have less weight to carry. After the hormones kick in the testes of male and female ovaries increase in size tremendously. At the autopsy, it was easy to say reproductive status of birds by simply checking the size of their bodies.
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