There is also a small feral population in France, which contains the Blue mutant, and mixes with larger proportion of Fischer's lovebirds - also from aviary origin. Aviaries, and cages need perches in a range of diameters. Natural perches in the form of branches are ideal, especially if they have a variety of forks, angles, and a bit of bounce in them. The reason for this is to give the captives stimuli; it also keeps their feet healthy, and nimble. Research (suitability, and toxicity) must be carried out on all plant material going into any cages. Cherry wood is poisonous, as is broom, kowhai, and avocado (its fruit being surprisingly poisonous to parrots), to mention just a few examples. Nesting boxes are usually utilised throughout the year as sleeping quarters. It is advisable to clean them, but keep them up even after breeding season. The risk of losing a bird to egg laying complications (in the unlikely event they do decide to breed in the winter), is out-weighed by the benefit of keeping the birds content, keeping pair-bonds strong, & the reduced risk of losing a bird to the cold. A supply of willow branches, and roughly slivered corn, or maize husk can be given in the aviary as nesting box lining: It will be ripped up, & carried into the nest box by the female. Lovebirds are reasonably difficult to sex. A "pair" will often be of the same gender, even though they are exhibiting signs of mutual affection. This usually arises when inexperienced bird keepers house two birds alone, & wait on behavioral signs that they're a "true pair", with the intention of swapping one out for another lovebird if they're not; then being excited when they see birds "pair up", even though they may both be of the same sex. These bonds are artificial, & can be broken, or tested if the "pair" are re-housed communally (or split up by the keeper). One or both of a pair of males may go, and breed with lone hens, despite staying connected to their original partner. Or the same sex pair's bond may completely dissolve immediately. The blue mutation was originally found in wild birds in the 1920s and is the oldest colour mutation known in the lovebird genus. The other mutations are a result of selective breeding in aviculture, such as two cobalts which will make a mauve (black). Various color mutations exist, including blue, cobalt, mauve, slate, dilute slate, violet, lutino (ino) and albino.