The action or process of fertilizing an egg, female animal, or plant, involving the fusion of male and female gametes to form a zygote. The gametes that participate in fertilisation of plants are the sperm (male), and the egg cell, and in flowering plants a second fertilisation event involves another sperm cell and the central cell which is a second female gamete. In flowering plants there are two sperm from each pollen grain.
        
             
        
        
        
Answer: d. The ecosystem reached its carrying capacity, and was not able to support continued population growth.
Explanation: 
The carrying capacity can be defined as the maximum population size of the species that habitat or ecosystem can support or sustain on the basis of availability of food, water and other necessities. This is an important factor which keeps a balance of the population of a species in a particular region or ecosystem. 
On the basis of the above information, The ecosystem reached its carrying capacity, and was not able to support continued population growth is the correct option to level off the rising populating of the birds. 
 
        
                    
             
        
        
        
Absorbing other cells gave a competitive advantage.
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