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Associative learning
Visual or auditory signals and their association with food and other rewards have been well studied, and birds have been trained to recognize and distinguish complex shapes. This is probably an important ability that aids their survival..

Associative learning is a method used on animals to assess cognitive abilities. Bebus et al. define associative learning as"acquiring knowledge of a predictive or causal relationship (association) between two stimuli, responses or events."A classic example of associative learning is Pavlovian conditioning. In avian research, performance on simple associative learning tasks is used to assess how cognitive abilities vary with experimental measures.

Associative learning vs. Reversal learning
In this first study, Bebus et al. demonstrate that associative learning in Florida scrub-jays correlates with reversal learning, personality and baseline hormone levels. To measure associative learning abilities, their method consisted of associating coloured rings to food rewards. To test reversal learning, the researchers simply reversed the rewarding and non-rewarding colours to see how quickly the scrub-jays would adapt to the new association. Their results suggest that associative learning is negatively correlated to reversal learning. In other words, birds that learned the first association quickly were slower to learn the new association upon reversal. The authors conclude that there must be a trade-off between learning an association and adapting to a new association.

Neophobia
The researchers also show that reversal learning is correlated with neophobia: birds that were afraid of a novel environment previously set up by the researchers were faster at reversal learning. The researchers also found an inverse correlation, where less neophobic birds performed better on the associative learning task, although this correlation was not statistically significant. Opposite results were found by Guido et al. In their research, Guido et al. show that neophobia in the M. chimango bird of prey negatively correlates to reversal learning ability. According to their results, neophobic birds were slower at reversal learning. The researchers suggest a modern explanation for this discrepancy: since birds living near urban areas benefit from being less neophobic to feed on human resources (such as detritus), but also benefit from being flexible learners (since human activity fluctuates), perhaps low neophobia eventually coevolved with high reversal learning ability due to selection. Therefore, personality alone might be insufficient to predict associative learning due to contextual differences.

Hormones
Finally, researchers found a correlation between baseline hormone levels and associtative learning. According to Bebus et al., low baseline levels of corticosterone (CORT), a hormone involved in stress response, predict better associative learning. In contrast, high baseline levels of CORT predict better reversal learning. In summary, Bebus et al. found that low neophobia (not statistically significant) and low baseline CORT levels predict better associative learning abilities. Inversely, high neophobia and high baseline CORT levels predict better reversal learning abilities.

Diet
In addition to reversal learning, personality and hormonal levels, further research suggests that diet may also correlate with associative learning performance. In this next study, Bonaparte et al. demonstrate that high-protein diets in zebra finches correlates with better associative learning. The researchers show that high-diet treatment was associated with larger head width, tarsus length and body mass in the treated males. In the subsequent testing, researchers show that high-diet and larger head-to-tarsus ratio correlate with better performance on an associative learning task. The researchers used associative learning as a correlate of cognition to support that nutritional stress during development can negatively impact cognitive development which in turn may reduce reproductive success. One such way that poor diet may affect reproductive success is through song learning. According to the developmental stress hypothesis, zebra finches learn songs during a stressful period of development. Their ability to learn complex songs reflects their adequate development.

Contradicting results were found by Kriengwatana et al. In this study, the researchers found that low food diet in zebra finches (before nutritional independence, that is, before the birds are able to feed themselves) enhanced spatial associative learning but impaired memory and had no effect on neophobia. They also failed to find a correlation between growth and associative learning. Though Bonaparte et al focused on protein content whereas Kriengwatana et al. focused on quantity of food, the results seem contradictory. Further research should be conducted to clarify the relationship between diet and associative learning.

Ecology
Furthermore, associative learning may vary across species depending on their ecology. According to Clayton and Krebs, there are differences in associative learning and memory between food-storing and non-storing birds. In their experiment, food-storing jays and marsh tits and non-storing jackdaws and blue tits were introduced to seven sites, one of which contained a food reward. For the first phase of the experiment, the bird randomly searches for the reward under the seven sites until it finds it. All species performed equally well in the first phase of the experiment. For the second phase of the experiment, the sites were hidden again and the birds had to return to the previously rewarding site to obtain the food item. The researchers found that food-storing birds performed better on phase 2 than non-storing birds. While food-storing birds preferentially returned to the rewarding sites, non-storing birds preferentially returned to previously visited sites, regardless of the presence of reward. If the food reward was visible in phase 1, there was no difference in performance between storers and non-storers. Therefore, the results show that memory following associative learning, rather than the learning itself, can vary with ecological lifestyle.

Age
Finally, associative learning correlates with age in Australian magpie according to Mirville et al. In their study, the researchers initially wanted to study the effect of group size on learning. However, they found that group size correlated with the likelihood of interaction with the task, but not with associative learning itself. Instead, they found that age played a role on performance: adults were more successful at completing the associative learning task, but less likely to approach the task initially. Inversely, juveniles were less successful at the task, but more likely to approach it. Therefore, adults in larger groups were the most likely individuals to complete the task due to their increased likelihood to both approach and succeed on the task.

Survival
Though it may seem universally adaptive to be a fast learner, Madden et al. suggest that the weight of individuals and their learning performance both affect survival. The researchers studied common pheasants to show that heavy birds that performed well on associative tasks had an increased probability of survival to 4 months old after being released into the wild, whereas light birds that performed poorly on associative tasks were more likely to survive. The researchers provide two explanations for the effect of weight on the results: perhaps larger individuals are more dominant and benefit from novel resources more than smaller individuals or they simply have a higher survival rate compared to smaller individuals due to bigger food reserves, difficulty for predators to kill them, increased motility, etc. Alternatively, ecological pressures may affect smaller individuals differently. Associative learning might be more costly on smaller individuals, thus reducing their fitness and leading to maladaptive behaviours. Additionally, Madden et al. found that slow reversal learning in both groups correlated with low survival rate. The researchers suggest a trade-off hypothesis where the cost of reversal learning would inhibit the development of other cognitive abilities. According to Bebus et al., there is a negative correlation between associative learning and reversal learning. Perhaps low reversal learning correlates to better survival due to enhanced associative learning. Madden et al. suggest this hypothesis but also note their skepticism since they could not show the same negative correlation between associative and reversal learning found by Bebus et al.

Neural representations
In their research, Veit et al. show that associative learning modified NCL (nidopallium caudolaterale) neuronal activity in crows. To test this, visual cues were presented on a screen for 600ms, followed by a 1000ms delay. Then, the red and blue stimuli were presented simultaneously and the crows had to choose the correct stimulus. Choosing the correct stimulus was rewarded with a food item. During the delay, NCL neurons showed increased selective activity for the rewarding stimulus. In other words, a given NCL neuron that fired when the correct stimulus was the red one increased its firing rate selectively when the crow had to choose the red stimulus. Additionally, its increased activity reflected the crow's increased performance. The researchers suggest that NCL neurons are involved in learning associations as well as making the subsequent behavioural choice for the rewarding stimulus.

Olfactory associative learning
Though most research is concerned with visual associative learning, Slater and Hauber showed that birds of prey are also able to learn associations using olfactory cues. In their study, nine individuals from five species of birds of prey learned to pair a neutral olfactory cue to a food reward.