Advances in world poultry production in recent years have driven discussion about the critical points during the productive process that increase losses, with emphasis on poultry mortality. Although research has been conducted towards the raising environment, a lack of studies exists on the pre-slaughter operations, which are the largest sources of losses before arrival at a slaughter plant. Thus, the present study aims to evaluate the effect of the number of birds per cage on the mortality rates, considering time of day. This trial was conducted in a commercial abattoir in the State of Sao Paulo, Brazil, in 2006. The historical data set from more than 13,000 trucks, about broilers mortality during pre-slaughter operations, was achieved through the abattoir. Factors that influence the welfare of the birds were collected, such as the daily thermal condition and the number of birds per cage. Statistical analysis was performed using the Double Generalized Linear Models. As results, there was a sharp reduction in mortality during the night with 7 birds per cage, from which the increase was mild. This similar behavior was recorded during the afternoon, with the lowest mortality associated with the density of 7 birds. During the morning, the increase in mortality occurred up to 5 birds per cage, up to the peak of 9 birds. Therefore, these results suggested an adoption of approximately 7 birds per cage, mainly in the afternoon and night, to reduce the mortality rates.