Hello, that is a good topic,

About your question, it's depend of your goals, because if you want to make the risks assessment on your birds, you don't need the soil samples, you can take the fruits for example.

but if you want to know the transfer from soil to plants, and then to your birds, yeah you need to collect the soil samples, and for the plants you take all parts, and make analysis for each part [( roots, stems, fruits (or grain)]

In addition to the above, we should also consider the method of pesticide application. if the application type is soil drenching / seed treatment, both plant and soil samples need to analyzed.

In this case you have also to know wich kind of pesticides used in your landspace.. 

You can also consider to sample by means of samplers different from part of plants. I mean passive samplers (like Petri dishes filled with liquid, absorbent  wet paper, etc.) or active samplers , as air samplers with membrane filters. As reference standard, you can consider the ISO 22866:2005, that is the standard method to assess the drift of pesticides. 

thank you all so much for your response. Im new to this field. If someone could suggest any useful literature, it would be wonderful

Hi Vijayanti ji, I do not know your objective. For your description sampling of soil and plants. Including grains should be done. It could be helpful if you elobrate the problem. Thanks

 Previous responses are so clear and quantified. lets know your objectives, but at same time you needed to establish the control experiment devoid of the fertilizer application. You can also specify gap in the application of the chemicals i.e rightful timing for the application of pesticides. take the soil and the parts of the plants (various species available) for laboratory assessment. We can as well suggest the method of analysis for the parameters you may wish to test. Regards